Journal of Applied Biomechanics, 2014, 30, 514-520 http://dx.doi.org/10.1123/jab.2013-0259 © 2014 Human Kinetics, Inc.
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Offensive and Defensive Agility: A Sex Comparison of Lower Body Kinematics and Ground Reaction Forces Tania Spiteri, Nicolas H. Hart, and Sophia Nimphius Edith Cowan University The aim of this study was to compare biomechanical and perceptual-cognitive variables between sexes during an offensive and defensive agility protocol. Twelve male and female (n = 24) recreational team sport athletes participated in this study, each performing 12 offensive and defensive agility trials (6 left, 6 right) changing direction in response to movements of a human stimulus. Three-dimensional motion, ground reaction force (GRF), and impulse data were recorded across plant phase for dominant leg change of direction (COD) movements, while timing gates and high-speed video captured decision time, total running time, and post COD stride velocity. Subjects also performed a unilateral isometric squat to determine lower body strength and limb dominance. Group (sex) by condition (2 × 2) MANOVAs with follow-up ANOVAs were conducted to examine differences between groups (P ≤ .05). Male athletes demonstrated significantly greater lower body strength, vertical braking force and impulse application, knee and spine flexion, and hip abduction, as well as faster decision time and post COD stride velocity during both agility conditions compared with females. Differences between offensive and defensive movements appear to be attributed to differences in decision time between sexes. This study demonstrates that biomechanical and perceptual-cognitive differences exist between sexes and within offensive and defensive agility movements. Keywords: change of direction, force production, impulse, decision-making, performance, human stimulus Many team sports require athletes to make appropriate decisions as well as possess the physical and technical ability to execute the chosen response with a successful outcome. Possibly the most common athletic maneuvers requiring a combination of physical capacity, technical ability, and tactical awareness are agility movements, performed by athletes to evade and pursue opponents during competition.1,2 These agility movements occur during both offensive and defensive orientations, requiring athletes to be equally efficient at producing a fast physical and perceptual-cognitive performance through constantly changing spatial and temporal conditions.3 The ability to rapidly identify and extract cues to predict upcoming movements from an opposition during a compatible (defensive) stimulus response condition has been assessed by several studies.1,4,5 While these findings provide insight into the multidimensional physical and perceptual-cognitive components of a defensive agility performance, the biomechanical mechanisms associated with faster changes of direction during an offensive and defensive agility performance have yet to be investigated. Currently, the investigation of lower body biomechanics for agility performance is minimal,6 with research focusing on sex differences in lower body kinematics from an injury prevention perspective.7–9 A critical finding of this research is the demonstration of a significant difference between preplanned change of direction (COD) tasks and a defensive agility movement. Increased medial ground reaction forces (GRFs), larger hip abduction, and knee valgus angles have been reported for both sexes during defensive agility movement,3 suggesting reactive environments elicit a comparable
Tania Spiteri, Nicolas H. Hart, and Sophia Nimphius are with the School of Exercise and Health Science at Edith Cowan University in Joondalup, Australia. Address author correspondence to Tania Spiteri at t.spiteri@ ecu.edu.au. 514
kinematic response between sexes. Altered lower body biomechanics during agility movements appear to result from increased spatial uncertainty and perceptual-cognitive stress experienced by athletes, reducing the time to implement appropriate postural strategies,3,10 increasing injury risk and decreasing performance outcomes. When the goal is to produce a faster agility performance, research has observed differences in both total running time and decision time between sexes during defensive and offensive agility movements.2 This suggests greater differences in biomechanical strategies may be observed when a faster decision-making time and agility performance is achieved. When examining performance enhancement strategies, the magnitude of GRF, impulse, and lower body kinematics have been identified as critical factors during straight-line sprinting and preplanned COD movements to detect differences between sexes11 and faster and slower sprinters.12,13 However, the major difference between sprinting and agility movements is the application of GRF and greater knee flexion and hip abduction angle to promote rapid deceleration and reacceleration in response to a sensory perturbation. This may result in a unique kinetic and kinematic pattern that has yet to be investigated between sexes from a performance perspective. Therefore, to understand the kinetic and kinematic strategies required during agility performance, the primary purpose of this study is to quantify vertical force and impulse and lower body kinematic variables during an offensive and defensive agility movement between sexes. The second purpose of this study is to compare the differences in the magnitude of the GRF and kinematic variables between offensive and defensive agility conditions. As a result of physical characteristics and strength capacity, it was hypothesized that males would produce greater force and impulse and perform the agility movement in a more advantageous lower body position (ie, greater knee flexion, hip abduction, spine flexion) compared with females, resulting in a faster agility movement (exit velocity).
Kinematics and Ground Reaction Forces During Agility 515
It was also hypothesized that differences in kinetics and kinematics between offensive and defensive agility movements would be evident, as a result of differences in decision-making and information processing strategies employed between the two conditions.
Methods
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Subjects Twelve male (n = 12) and 12 female (n = 12) semiprofessional team sport (soccer: n = 8; basketball: n = 8; and netball: n = 8) athletes (Table 1) with no previous history of major lower limb injury (ie, ligament sprains, muscle strains, or bone fractures) and no significant vision problems participated in this study. Subjects were required to partake in a minimum of two competitive games and one structured skills training session for their chosen sport each week to be included in the study. All testing and familiarization sessions occurred during their particular team sport season. The Human Research Ethics Committee at Edith Cowan University approved all test procedures, with written informed consent obtained from all subjects before the commencement of the study.
Experimental Design The current study used a cross-sectional design to compare GRF, impulse, lower body kinematics, strength, decision time, post COD stride velocity, and total running time between sexes during offensive and defensive agility tests, and compared performance between agility conditions. All testing procedures took place during 1 session with anthropometrical measurements (height, body mass) being recorded for each subject, followed by the completion of three unilateral left leg and right leg isometric back squats to ascertain peak force values as a measure of strength. Participants then completed a total of 24 agility trials; 12 offensive and 12 defensive trials, changing direction in response to movements of a human stimulus. Specific kinematic, vertical GRF, and impulse variables were measured for the COD step during each agility trial. Subjects completed a 10 minute dynamic warm up before any testing. Each testing procedure was separated with a minimum recovery period of 10 minute to ensure any fatigue did not influence the results.
Isometric Strength Testing Lower body strength was assessed with a unilateral maximal isometric back squat performed on a portable force plate (400 Series Performance Plate; Fitness Technology, Adelaide, Australia) sampling at 600 Hz. The subject pushed against an immovable bar position across the shoulders with both the knee and hip angle set at approximately 140°.14 Subjects were instructed to position the leg to be assessed under their center of mass while the other limb was unsupported and flexed at an angle of 90°. Subjects were required Table 1 Subject characteristics Male
Female
P
ES
Age (years)
22.50 ± 3.61
20.58 ± 2.15
.13
0.65
Height (cm)
178.30 ± 8.67
170.57 ± 9.72
.05
0.84
Body mass (kg)
77.14 ± 13.69
65.85 ± 13.99
.05
0.82
UL strength dominant (N⋅kg–1)
18.52 ± 3.93
14.21 ± 5.80
.04
0.87
Abbreviations: UL, unilateral strength; ES, effect size.
to perform a total of three trials for each limb, for 5 seconds in duration.15 Each trial was separated by a 2 minute recovery period. Intraclass correlation coefficient (ICC) and coefficient of variation (CV) for strength were as follows: left leg (ICC = 0.95; CV = 7.0%) and right leg (ICC = 0.95; CV = 5.5%). The highest peak force of the three trials was used and presented as a value relative to body mass (N·kg–1). These results are presented in Table 1.
Reactive Agility Protocol Each subject performed 12 sidestepping maneuvers under reactive offensive and defensive conditions, changing direction in response to movements performed by a human stimulus. The four movement patterns performed by the human stimulus have previously been used to create a reliable stimulus during reactive agility protocols (ICC = 0.71–0.99; CV = 1.11%–4.77%; TE = 0.15–0.59).4,5,16 Each movement pattern was presented in a randomized order and consisted of three offensive and three defensive trials of the following: 1. Two-step left: Step forward with the left leg, then right leg, then change direction to the left. 2. One-step left: Step forward with the right leg, then change direction to the left. 3. Two-step right: Step forward with the right leg, then left leg, then change direction to the right. 4. One-step right: Step forward with the left leg, then change direction to the right. Before the commencement of each trial, the human stimulus was informed of the movement pattern and direction to move (left or right), while the subjects were instructed to move either in the same direction as the human stimulus during a defensive trial or in the opposite direction during an offensive trial. Subjects began on a marked line 9 m opposite the human stimulus facing them and were instructed to run in a straight line toward the stimulus. Once the subject reached a marked line placed 3 m from the starting position, the human stimulus initiated one of the four movement patterns. After changing direction in response to the human stimulus, subjects were then required to sprint 2 m to the left or right completing the trial. Subjects were instructed to run toward and respond to movements of the human stimulus at a speed of 4.5 ± 0.5 m·s–1, which was monitored using a dual beam infrared timing light system (Speedlight Timing System; Swift Performance Equipment, Old, Australia), over a 3 m distance to determine approach velocity (displacement divided by time) for each trial. Approach velocity was controlled to ensure any differences observed in the data could not be attributed to velocity variations between subjects. Subjects were also required to change direction at an angle of 45° ± 5°, which was monitored by tape markings placed 45° from the center of all three force plates (Kistler Quattro, Type 9290AD; Victoria, Australia) and visual inspection from high-speed video recording of each trial (Sony HDD Camcorder HDRXR550V; Sony, Australia). Three force plates were used in the study to account for variations in decision-making time, and therefore, the initiation of the change of direction movement between subjects. For a trial to be deemed successful, subjects were required to contact their whole foot on one of the three force plates without targeting the plate, run at the set approach speed, cut at the required angle, have performed a side-step cut only, and responded in the correct movement direction as indicated at the beginning of each trial. If the subject did not correctly meet one of these conditions, the trial was reattempted. A minimum 1 minute recovery period was enforced between trials.
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516 Spiteri, Hart, and Nimphius
Ground reaction forces were recorded at 1000 Hz using a 600 × 900 mm triaxial force plate with Bioware software (Ver. 1.08; Kistler, Victoria, Australia). Raw mediolateral, anteroposterior, and vertical GRF data were exported to MATLAB programing software (R2010a; The Mathworks Inc., Chatswood, Australia) to examine specific vertical force variables during the COD step for the dominant limb only. Limb dominance was defined as the limb that produced the greater amount of force during the unilateral isometric strength assessment. Variables of interest include peak braking and propulsive force (N·kg–1), time to peak braking and propulsive force (s), contact time (s), time between peak braking and propulsive peaks (s), and relative braking impulse, propulsive impulse, and total impulse (m·s–1). All force and impulse variables were calculated relative to body mass (kg) and analyzed over the stance phase, with heel strike defined as the instance the vertical GRF data exceeds 10 N, and toe off defined as the instance the vertical GRF data are below 10 N.3,13 Braking impulse was calculated from heel strike to the minimum of the midsupport phase, and propulsive impulse was calculated from minimum of midsupport phase to toe off. To record three-dimensional movements, 37 retroreflective markers were fixed to anatomical landmarks of the athlete in accordance with the previously validated Vicon Plug-In Gait model, with movements captured using a 10 camera, 250 Hz Vicon motion analysis system (Oxford Metrics Ltd., Oxford, United Kingdom). Before data collection, a static calibration for each athlete was performed to locate anatomical landmarks and define segment and joint coordinate systems to the kinematic model.17 All trials were processed in Vicon Nexus (Ver. 1.6.1; Oxford Metrics Ltd., Oxford, United Kingdom) through a custom pipeline to obtain filtered marker trajectories using a zero-lag fourth order 18 Hz, low-pass Butterworth filter.17 Joint kinematic data for each trial was interpolated and time normalized to 100% of stance (heel strike to toe off). Variables of interest include maximum and minimum spine flexion, hip flexion, hip abduction and knee flexion (degrees), and post COD stride velocity (m·s–1). Post COD stride velocity was determined as the time from toe off of the COD step to heel strike of the first step after changing direction of the athlete’s center of mass.13,18
Results
Statistical Analysis
Males demonstrated an overall faster decision time (P = .001; ES = 1.29–1.37) and post stride velocity (P = .001; ES = 0.75–0.83) when compared with females during both offensive and defensive conditions (Figure 1). Males also demonstrated a significantly faster post stride velocity during the offensive condition compared with the defensive condition (P = .004; ES = 0.48). As approach speed was controlled during the testing protocol, no significant differences were observed between sexes (males: 4.33 ± 0.4 m⋅s–1; females: 4.08 ± 0.8 m⋅s–1, P = .12; ES = 0.38) and conditions (offensive: 4.26 ± 0.32 m⋅s–1; defensive: 4.25 ± 0.51 m⋅s–1, P = 1.12; ES = 0.01). MANOVA revealed that significant differences occurred between sexes for vertical (Pillai’s trace ≤ 0.01) GRF and impulse variables (Table 2). Males demonstrated significantly greater maximum braking force, propulsive force, braking impulse, propulsive impulse, and total impulse compared with females during one-step and two-step offensive trials and one-step defensive trials. During two-step defensive trials, significantly greater braking impulse was observed for male subjects, while females produced greater propulsive impulse and total impulse; however, this difference was nonsignificant. No significant differences were observed in timing variables between sexes during both defensive and offensive trials. Differences between offensive and defensive conditions were only observed for between two-step trials (Table 2). Females produced significantly greater braking force (P = .02; ES = 0.98) and propulsive force during defensive trials. Males produced significantly greater propulsive force (P = .01; ES = 1.13), braking impulse (P = .04; ES = 0.86), propulsive impulse (P = .01; ES = 1.48), and total impulse (P = .01; ES = 1.52) during offensive trials. Time to maximum propulsive force and time between maximum braking and propulsive peaks were significantly faster for males during the offensive condition (P = .01–0.03; ES = 0.63–1.00). Mean joint rotations presented as a function of stance (Figure 2) reveal males demonstrated significantly greater knee flexion (P = .03–0.05; ES = 0.97) and spine flexion (P = .01; ES = 1.47) compared with females during one- and two-step offensive and defensive trials. In addition, males produced greater hip abduction (P = .01; ES = 1.14) during two-step offensive and defensive trials compared with females. No significant differences were observed between offensive and defensive conditions within sexes.
All results are represented as means ± SD. Independent t tests were conducted to examine differences in subject demographics (height, body weight, age, and unilateral strength) and response times (approach speed, decision time, post stride velocity, and total running time) between sexes. A 2 × 2 multivariate analysis (MANOVA) was conducted to examine differences between sexes and condition (offensive: one-step, two-step; defensive: one-step, two-step) across all variables. A follow up 1-way analysis of variance (ANOVA) was conducted on each dependent variable to determine precisely where significant differences occurred, with sequential Bonferroni corrections19 made to reduce type I errors. A significance level of P ≤ .05 was employed throughout all statistical analysis unless otherwise stated. Effect sizes (ES) were calculated for group comparisons by dividing the difference between groups by the pooled standard deviation.20 The magnitude of ES calculations were interpreted following Hopkins’21 guidelines, with trivial = ≤ 0; small = 0 to 0.2; moderate = 0.2 to 0.6; large = 0.6 to 1.2; very large = 1.2 to 2.0; nearly perfect = 2.0 to 4.0; perfect = ≥ 4.0. All statistical computations were performed using a statistical analysis program (SPSS, Version 17.0; Chicago, Illinois).
Figure 1 — Sex differences in reaction time (RT) and post stride velocity (PSV) between offensive and defensive conditions. *Significant difference in means between males and females (P ≤ .05). ^Significant difference in means between offensive and defensive conditions (P ≤ .05).
Kinematics and Ground Reaction Forces During Agility 517
Table 2 Mean stance phase ground kinetics for male and female subjects during offensive and defensive agility trials Defensive
Offensive
Male
Female
P
ES
Male
Female
P
ES
Braking force (N⋅kg–1)
48.85 ± 13.05
25.08 ± 2.85
.01
2.53
46.73 ± 13.05
22.47 ± 3.52
.01
2.54
Propulsive force (N⋅kg–1)
26.81 ± 5.20
19.78 ± 1.51
.01
1.84
26.68 ± 7.09
18.72 ± 1.97
.01
1.53
1.42 ± 0.39
0.88 ± 0.17
.01
1.80
1.25 ± 0.43
0.85 ± 0.20
.01
1.19
3.25 ± 0.63
2.39 ± 0.27
.01
1.77
3.34 ± 0.57
2.34 ± 0.24
.01
2.28
4.67 ± 0.98
3.27 ± 0.37
.01
1.89
4.58 ± 0.89
3.17 ± 0.38
.01
2.06
Time to peak braking force (ms)
0.03 ± 0.01
0.04 ± 0.01
.08
1.00
0.03 ± 0.01
0.04 ± 0.01
.28
1.00
Time to peak propulsive force (ms)
0.10 ± 0.01
0.11 ± 0.01
.59
1.00
0.11 ± 0.02
0.11 ± 0.02
.91
0.00
Time between B:P force (ms)
0.07 ± 0.01
0.07 ± 0.01
.11
0.00
0.08 ± 0.02
0.08 ± 0.01
.61
0.00
Contact time (s)
0.25 ± 0.02
0.25 ± 0.02
1.00
0.00
0.24 ± 0.02
0.26 ± 0.02
.08
1.00
35.00 ± 8.52
22.52 ± 4.17
.01
1.86
42.32 ± 12.32
19.12 ± 2.60
.01
2.61
18.93 ± 3.42
19.11 ± 1.68
.87
0.06
25.20 ± 7.09
16.36 ± 1.43
.01
1.73
1.05 ± 0.15
0.78 ± 0.14
.01
1.86
1.26 ± 0.31
0.80 ± 0.09
.07
2.01
2.06 ± 0.67
2.43 ± 0.14
.06
0.76
2.93 ± 0.49
2.31 ± 0.19
.01
1.67
3.10 ± 0.69
3.19 ± 0.24
.69
0.17
4.19 ± 0.74
3.11 ± 0.20
.01
1.99
0.03 ± 0.01
0.04 ± 0.01
.06
1.00
0.03 ± 0.01
0.04 ± 0.01
.34
1.00
Variable One-step Trials
Braking impulse
(m⋅s–1)
Propulsive impulse
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Total impulse
(m⋅s–1)
(m⋅s–1)
Two-step Trials Braking force (N⋅kg–1)a Propulsive force
(N⋅kg–1)a, b
Braking impulse (m⋅s–1)b Propulsive impulse Total impulse
(m⋅s–1)b
(m⋅s–1)b
Time to peak braking force (ms)
0.09 ± 0.01
0.10 ± 0.02
.06
0.63
0.10 ± 0.01
0.11 ± 0.02
.29
0.63
Time between B:P force (ms)b
0.06 ± 0.01
0.07 ± 0.01
.55
1.00
0.07 ± 0.01
0.08 ± 0.01
.59
1.00
Contact time (s)
0.23 ± 0.03
0.25 ± 0.02
.16
0.78
0.24 ± 0.02
0.25 ± 0.01
.16
1.00
Time to peak propulsive force
(ms)b
Note. Significant differences (P ≤ .05) between defensive and offensive conditions for a females and bmales. B:P, braking and propulsive.
Discussion The current study is the first to quantify and examine the differences in lower body kinematics, vertical GRF, and impulse variables between sexes during offensive and defensive agility conditions and compare differences in kinematic and kinetic variables between agility conditions. The findings from this study demonstrated that males produced significantly faster defensive and offensive agility movements as characterized by a faster decision time and post stride velocity; producing greater force, impulse, and trunk and knee flexion angles when compared with females. Differences between offensive and defensive movements were only observed during two-step movement trials. Significantly greater force and impulse were produced during the agility condition for each sex where a faster decision time was recorded. These observed differences in kinematics and kinetics between sexes and agility conditions might be attributed to numerous factors, including decision-making ability, strength, and neuromuscular variables, which have been shown to cause variations in rate of force development capabilities, force application, and postural strategies employed when changing direction.3,22,23 Sex differences associated with the production of force during high-velocity athletic movements have been attributed to differences in perceptual-cognitive speed,24 neuromuscular properties of the lower limbs,8,25 and lower body strength characteristics.13 The combination of a faster decision time to identify specific kine-
matic cues available from the human stimulus and greater lower body strength, as observed by male athletes in the current study, increases the time available to apply more force and impulse during the braking phase of the movement,13 enabling a faster exit velocity to be achieved.12,13,18 This may be due to a “faster” or “more refined” neuromuscular system, whereby the interaction between higher cognitive function and muscle (neural and morphological) characteristics is required during high-velocity movements involving a temporal sensory perturbation such as agility for a faster post COD stride velocity to be achieved. Differences in muscle fiber type, size, and elasticity23,26 have been reported to influence rate of force development capabilities and produce variations in lower body kinematics and kinetics between sexes during eccentric and concentric phases of a jump.22,27 Specifically, a greater percentage of slow twitch muscle fibers and reduced tendon stiffness have been found to increase electromechanical delay and time to activation, resulting in a longer movement time in females during a lower body reaction time test.24 This apparent delay in neuromuscular activation between sexes, coupled with longer decision-making time evident in female athletes, may explain the differences observed in maximal braking (eccentric) and propulsive (concentric) force and impulse production between sexes in the current study. An important component of agility performance is an athlete’s ability to rapidly decelerate and reaccelerate, which requires sufficient lower body strength to both stop and then increase the momentum and velocity of the center of mass to successfully avoid
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518 Spiteri, Hart, and Nimphius
Figure 2 — Mean stance phase joint rotations for male (n = 12) and female (n = 12) subjects during defensive and offensive agility trials. Data are represented for: one-step trials including (a) knee flexion-extension, (b) hip flexion-extension, (c) hip abduction-adduction, and (d) spine flexion-extension; and two-step trials including (e) knee flexion-extension, (f) hip flexion-extension, (g) hip abduction-adduction, and (h) spine flexion-extension.
or pursue opponents. Increased GRF production is a commonly observed feature of faster running speeds28 in athletes who have greater lower body strength to rapidly apply force throughout the duration of the movement. In particular, increasing GRF during the braking phases of sprinting or COD movements,13 as observed by males during offensive and defensive movements, has been identified as contributing to the storage and utilization of elastic energy,12 enabling an increased propulsive force output improving acceleration ability. Many studies have reported mixed findings regarding improvements in strength and subsequent translation to COD performance.29,30 Recent research has reported strong and sig-
nificant correlations between propulsive force, propulsive impulse, and post COD stride velocity during COD movements for stronger athletes.31 These findings, in conjunction with the current study, indicate that a higher level of strength capacity enables athletes to rapidly and systematically coordinate force application throughout the movement. This is required to increase impulse production, which involves a time component in which to apply force, in order to increase an athlete’s momentum out of directional changes. Differences in lower body kinematics between sexes during COD movements have been well documented.3,7,9,10 While a majority of kinematic differences are attributed to anthropometrical
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Kinematics and Ground Reaction Forces During Agility 519
characteristics,32 findings of the current study are similar to previous performance-based research,6,13 reporting significantly greater knee flexion and spine flexion angle in starter or male athletes who demonstrated significantly greater lower body strength and subsequently produced faster COD and agility performance. While no significant differences were observed for hip flexion angle between sexes, the upright body position and unfavorable kinematic chain adopted by female athletes not only increases the risk of lower limb injury,3,7 but also reduces the ability to produce and direct force application to improve post COD stride velocity. For example, during a movement such as a sprint start, an athlete is permitted to adjust their hip, knee, and ankle angle out of the start blocks; enabling triple extension of the lower limbs and lengthening of the muscle-tendon complex to optimize the force-length relationship, resulting in a greater force output and acceleration.33 Agility movements however, require athletes to possess greater lower body strength to enter a lower body position across stance phase13,30 to improve mechanical functioning of the lower limbs to increase acceleration. When changing direction to opposition movements, an athlete’s perception regarding the amount of time and space available may compromise postural preparation, increasing injury risk and compromising performance outcomes. During two-step movement trials, the human stimulus initiated movement closer to the COD, resulting in a significantly greater hip abduction angle observed in male athletes. Previous research has also observed a greater lateral foot plant during agility movements3,34 due to an athlete’s perceived need to rapidly decelerate and enable rapid reacceleration in the new direction. Executing the COD movement with a prominent lateral foot plant increases the acceleration of the center of mass to the contralateral side,35 as a greater approach angle and lower body position creates an optimal kinematic chain to better direct force application. As no significant difference in hip abduction angle was observed during one-step movement trials between sexes, we can conclude that males had a greater ability to overcome the increased spatial demand by demonstrating a faster decision time in response to the human stimulus, enabling a more optimal body position to be adopted during stance phase. Reactive movements during team sports are often in response to opponent’s actions, either in offensive (incompatible) or defensive (compatible) situations. Stimulus-response compatibility, determined by the spatial correlation between the stimulus and the appropriate response,36 is known to affect the rate of information processing and the speed of the upcoming motor response depending on the tactical situation. While previous research has observed faster reaction times during compatible conditions, as the required response is processed within intrahemispheric nervous circuits,36,37 results of the current study partly contradict pervious findings, with males producing a faster decision time during incompatible (offensive) agility movements. It can be assumed that during defensive movements, greater attentional demand and tracking of kinematic cues is constantly required to defend the opposition, whereas offensive movements require athletes to locate and identify relevant cues at that one point in time to successfully evade the opposition. This may explain why males produced a faster agility performance during the offensive condition, as they displayed a greater ability to anticipate the upcoming action by eliciting a higher percentage of negative decision time results. Despite males and females responding differently, it is clear that differences between offensive and defensive movements appear to be attributed to perceptual-cognitive factors, with both sexes producing greater force and impulse during the condition where a faster decision time was observed.
Previous research has also identified that preactivation of the neuromuscular system is advantageous during competition by enabling a quicker motor response.22 Faster movement times, specifically a significantly faster time between braking and propulsive peaks and time to peak propulsive force, coupled with greater force and impulse produced by males, resulted in a significantly faster post COD stride velocity during offensive agility movements. This could be due to their faster decision time, where preactivation and preparation enables a reduction in the natural electromechanical delay (EMD) of the body that occurs before heel strike, which has been demonstrated previously in other high-velocity movements.22,38 While the current study did not measure muscle activation, previous research has demonstrated that a shorter EMD is associated with a higher magnitude of initial muscle activation and force production.39,40 This enables a higher aggregate of force to be produced in an equivalent time period.39,40 Improved timing and greater force production as a result of increased preparation time may explain the faster exit velocity observed in males during offensive movements and females during defensive movements. While there were kinematic differences between sexes, no differences were observed between offensive and defensive maneuvers within each sex. This could further implicate decision time and strength as trainable factors to improve the kinetics profile of offensive and defensive agility movements. While the findings of the current study provide understanding of the significant biomechanical variables contributing to a faster agility performance, the current study did not directly measure lower body muscle activation, EMD, or visual gaze strategies that may have been employed throughout the movement. These contributing factors may further distinguish agility performance between sexes. In addition, these factors may provide a greater understanding of the perceptual-cognitive and neuromuscular factors required to achieve faster offensive and defensive agility movements. To address these limitations and expand on the findings of the current study, future research should directly quantify these variables between male and female athletes and offensive and defensive agility movements in order to advance the understanding of the limiting factors of agility to improve performance. While differences were observed between male and female athletes, which can be attributed to the various anthropometrical, neuromuscular, and neurophysiological differences between sexes, it appears differences between offensive and defensive movements are largely a result of neuromuscular and neurophysiological qualities. Therefore, it appears based upon the present data that testing, training, and developing agility should be specifically designed for both offensive and defensive movements, as distinctive biomechanical and perceptualcognitive differences are evident between movement scenarios.
References 1. Serpell BG, Ford M, Young WB. The development of a new test of agility for rugby league. J Strength Cond Res. 2010;24(12):3270–3277. PubMed 2. Spiteri T, Nimphius S, Wilkie J. Comparision of running times during reactive offensive and defensive agility protocols. J Aus Strength Cond. 2012;20(Suppl 1):73–78. 3. McLean SG, Lipfert SW, van den Bogert AJ. Effect of gender and defensive opponent on the biomechanics of sidestep cutting. Med Sci Sports Exerc. 2004;36(6):1008–1016. PubMed doi:10.1249/01. MSS.0000128180.51443.83 4. Sheppard JM, Young WB, Doyle TL, Sheppard TA, Newton RU. An evaluation of a new test of reactive agility and its relationship to sprint speed and change of direction speed. J Sci Med Sport. 2006;9(4):342–349. PubMed doi:10.1016/j.jsams.2006.05.019
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520 Spiteri, Hart, and Nimphius 5. Young WB, Willey B. Analysis of a reactive agility field test. J Sci Med Sport. 2010;13(3):376–378. PubMed doi:10.1016/j.jsams.2009.05.006 6. Green BS, Blake C, Caulfield BM. A comparison of cutting technique performance in rugby union players. J Strength Cond Res. 2011;25(10):2668–2680. PubMed doi:10.1519/ JSC.0b013e318207ed2a 7. Ford KR, Myer GD, Toms HE, Hewett TE. Gender differences in the kinematics of unanticipated cutting in young athletes. Med Sci Sports Exerc. 2005;37(1):124–129. PubMed doi:10.1249/01. MSS.0000150087.95953.C3 8. Landry SC, McKean KS, Hubley-Kozey CL, Stanish WD, Deluzio KJ. Gender differences exist in neuromuscular control patterns during the pre-contact and early stance phase of an unanticipated side-cut and cross-cut maneuver in 15–18 years old adolescent soccer players. J Electromyogr Kinesiol. 2009;19(5):e370–379. PubMed doi:10.1016/j. jelekin.2008.08.004 9. Sigward SM, Powers CM. The influence of gender on knee kinematics, kinetics and muscle activation patterns during side-step cutting. Clin Biomech (Bristol, Avon). 2006;21(1):41–48. PubMed doi:10.1016/j. clinbiomech.2005.08.001 10. Pollard CD, Davis IM, Hamill J. Influence of gender on hip and knee mechanics during a randomly cued cutting maneuver. Clin Biomech (Bristol, Avon). 2004;19(10):1022–1031. PubMed doi:10.1016/j. clinbiomech.2004.07.007 11. Cheuvront SN, Carter R, DeRuisseau KC, Moffatt RJ. Running performance differences between men and women: an update. Sports Med. 2005;35(12):1017–1024. PubMed doi:10.2165/00007256-20053512000002 12. Hunter JP, Marshall RN, McNair PJ. Relationships between ground reaction force impulse and kinematics of sprint-running acceleration. J Appl Biomech. 2005;21(1):31–43. PubMed 13. Spiteri T, Cochrane JL, Hart NH, Haff GG, Nimphius S. Effect of strength on plant foot kinetics and kinematics during a change of direction task. Eur J Sport Sci. 2013;13(6):646–652. PubMed 14. Requena B, Gonzalez-Badillo JJ, de Villareal ES, Ereline J, Garcia I, Gapeyeva H, Paasuke M. Functional performance, maximal strength, and power characteristics in isometric and dynamic actions of lower extremities in soccer players. J Strength Cond Res. 2009;23(5):1391– 1401. PubMed doi:10.1519/JSC.0b013e3181a4e88e 15. Nuzzo JL, McBride JM, Cormie P, McCaulley GO. Relationship between countermovement jump performance and multijoint isometric and dynamic tests of strength. J Strength Cond Res. 2008;22(3):699– 707. PubMed doi:10.1519/JSC.0b013e31816d5eda 16. Spiteri T, Cochrane JL, Nimphius S. Human stimulus reliability during an offensive and defensive agility protocol. J Aus Strength Cond. 2012;20(4):20–27. 17. Besier TF, LLoyd DG, Cochrane JL, Ackland TR. External loading of the knee joint during running and cutting maneuvers. Med Sci Sports Exerc. 2001;33(7):1168–1175. PubMed doi:10.1097/00005768200107000-00014 18. Glaister BC, Orendurff MS, Schoen JA, Bernatz GC, Klute GK. Ground reaction forces and impulses during a transient maneuver. J Biomech. 2008;41(14):3090–3093. PubMed doi:10.1016/j.jbiomech.2008.07.022 19. Holm S. A simple sequentially rejective multiple test procedure. Scand J Stat. 1979;6(2):65–70. 20. Cohen J. Statistical power analysis for the behavioural sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum Associates; 1988. 21. Hopkins WG. A new view of statistics. Internet Society for Sport Science; 2002. Available: http://www.sportsci.org/resource/stats 22. McBride JM, McCaulley GO, Cormie P. Influence of preactivity and eccentric muscle activity on concentric performance during vertical jumping. J Strength Cond Res. 2008;22(3):750–757. PubMed doi:10.1519/JSC.0b013e31816a83ef
23. Perez-Gomez J, Rodriduez GV, Ara I, et al. Role of muscle mass on sprint performance: gender differences? Eur J Appl Physiol. 2008;102(6):685–694. PubMed doi:10.1007/s00421-007-0648-8 24. Spiteri T, Cochrane JL, Nimphius S. The evaluation of a new lowerbody reaction time test. J Strength Cond Res. 2013;27(1):174–180. PubMed doi:10.1519/JSC.0b013e318250381f 25. Bencke J, Zebis MK. The influence of gender on neuromuscular preactivity during side-cutting. J Electromyogr Kinesiol. 2011;21(2):371– 375. PubMed doi:10.1016/j.jelekin.2010.10.008 26. Staron RS, Hagerman FC, Hikida RS, et al. Fiber type composition of the vastus lateralis muscle of young men and women. J Histochem Cytochem. 2000;48(5):623–629. PubMed doi:10.1177/002215540004800506 27. Kubo K, Kanehisa H, Fukunaga T. Gender differences in the viscoelastic properties of tendon structures. Eur J Appl Physiol. 2003;88(6):520–526. PubMed doi:10.1007/s00421-002-0744-8 28. Brughelli M, Cronin J, Levin G, Chaouachi A. Understanding change of direction ability in sport: a review of resistance training studies. Sports Med. 2008;38(12):1045–1063. PubMed doi:10.2165/00007256200838120-00007 29. Negrete R, Brophy J. The relationship between isokinetic open and closed kinetic chain lower extremity strength and functional performance. J Sport Rehabil. 2000;9(1):46–61. 30. Young WB, Hawken M, McDonald L. Relationship between speed, agility, and strength qualities in Australian rules football. Strength Cond Coach. 1996;4(4):3–6. 31. Baker DG, Newton R. Comparison of lower body strength, power, acceleration, speed, agility, and sprint momentum to describe and compare playing rank among professional rugby league players. J Strength Cond Res. 2008;22(1):153–158. PubMed doi:10.1519/ JSC.0b013e31815f9519 32. Malinzak RA, Colby SM, Kirkendall DT, Yu B, Garrett WE. A comparison of knee joint motion patterns between men and women in selected athletic tasks. Clin Biomech (Bristol, Avon). 2001;16(5):438–445. PubMed doi:10.1016/S0268-0033(01)00019-5 33. Mero A, Kuitunen S, Harland M, Kyrolainen H, Komi PV. Effects of muscle–tendon length on joint moment and power during sprint starts. J Sports Sci. 2006;24:165–173. PubMed doi:10.1080/02640410500131753 34. Sigward SM, Powers CM. Loading characteristics of females exhibiting excessive valgus moments during cutting. Clin Biomech (Bristol, Avon). 2007;22(7):827–833. PubMed doi:10.1016/j.clinbiomech.2007.04.003 35. Patla AE, Adkin A, Ballard T. Online steering: coordination and control of body center of mass, head and body reorientation. Exp Brain Res. 1999;129(4):629–634. PubMed doi:10.1007/s002210050932 36. Kato Y, Endo H, Kizuka T, Asami T. Automatic and imperative motor activations in stimulus-response compatibility: magnetoencephalographic analysis of upper and lower limbs. Exp Brain Res. 2006;168(12):51–61. PubMed doi:10.1007/s00221-005-0090-2 37. Adam JJ. The additivity of stimulus-response compatibility with perceptual and motor factors in a visual choice reaction time task. Acta Psychol (Amst). 2000;105(1):1–7. PubMed doi:10.1016/S00016918(00)00042-1 38. Gabriel DA, Boucher JP. Effects of repetitive dynamic contractions upon electromechanical delay. Eur J Appl Physiol Occup Physiol. 1998;79(1):37–40. PubMed doi:10.1007/s004210050470 39. Cavanagh PR, Komi PV. Electromechanical delay in human skeletal muscle under concentric and eccentric contractions. Eur J Appl Physiol Occup Physiol. 1979;42(3):159–163. PubMed doi:10.1007/ BF00431022 40. Winter EM, Brookes FB. Electromechanical response times and muscle elasticity in men and women. Eur J Appl Physiol Occup Physiol. 1991;63(2):124–128. PubMed doi:10.1007/BF00235181
An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH
Offensive and Defensive Agility: A Sex Comparison of Lower Body Kinematics and Ground Reaction Forces Tania Spiteri, Nicolas H. Hart, and Sophia Nimphius Edith Cowan University The aim of this study was to compare biomechanical and perceptual-cognitive variables between sexes during an offensive and defensive agility protocol. Twelve male and female (n = 24) recreational team sport athletes participated in this study, each performing 12 offensive and defensive agility trials (6 left, 6 right) changing direction in response to movements of a human stimulus. Three-dimensional motion, ground reaction force (GRF), and impulse data were recorded across plant phase for dominant leg change of direction (COD) movements, while timing gates and high-speed video captured decision time, total running time, and post COD stride velocity. Subjects also performed a unilateral isometric squat to determine lower body strength and limb dominance. Group (sex) by condition (2 × 2) MANOVAs with follow-up ANOVAs were conducted to examine differences between groups (P ≤ .05). Male athletes demonstrated significantly greater lower body strength, vertical braking force and impulse application, knee and spine flexion, and hip abduction, as well as faster decision time and post COD stride velocity during both agility conditions compared with females. Differences between offensive and defensive movements appear to be attributed to differences in decision time between sexes. This study demonstrates that biomechanical and perceptual-cognitive differences exist between sexes and within offensive and defensive agility movements. Keywords: change of direction, force production, impulse, decision-making, performance, human stimulus Many team sports require athletes to make appropriate decisions as well as possess the physical and technical ability to execute the chosen response with a successful outcome. Possibly the most common athletic maneuvers requiring a combination of physical capacity, technical ability, and tactical awareness are agility movements, performed by athletes to evade and pursue opponents during competition.1,2 These agility movements occur during both offensive and defensive orientations, requiring athletes to be equally efficient at producing a fast physical and perceptual-cognitive performance through constantly changing spatial and temporal conditions.3 The ability to rapidly identify and extract cues to predict upcoming movements from an opposition during a compatible (defensive) stimulus response condition has been assessed by several studies.1,4,5 While these findings provide insight into the multidimensional physical and perceptual-cognitive components of a defensive agility performance, the biomechanical mechanisms associated with faster changes of direction during an offensive and defensive agility performance have yet to be investigated. Currently, the investigation of lower body biomechanics for agility performance is minimal,6 with research focusing on sex differences in lower body kinematics from an injury prevention perspective.7–9 A critical finding of this research is the demonstration of a significant difference between preplanned change of direction (COD) tasks and a defensive agility movement. Increased medial ground reaction forces (GRFs), larger hip abduction, and knee valgus angles have been reported for both sexes during defensive agility movement,3 suggesting reactive environments elicit a comparable
Tania Spiteri, Nicolas H. Hart, and Sophia Nimphius are with the School of Exercise and Health Science at Edith Cowan University in Joondalup, Australia. Address author correspondence to Tania Spiteri at t.spiteri@ ecu.edu.au. 514
kinematic response between sexes. Altered lower body biomechanics during agility movements appear to result from increased spatial uncertainty and perceptual-cognitive stress experienced by athletes, reducing the time to implement appropriate postural strategies,3,10 increasing injury risk and decreasing performance outcomes. When the goal is to produce a faster agility performance, research has observed differences in both total running time and decision time between sexes during defensive and offensive agility movements.2 This suggests greater differences in biomechanical strategies may be observed when a faster decision-making time and agility performance is achieved. When examining performance enhancement strategies, the magnitude of GRF, impulse, and lower body kinematics have been identified as critical factors during straight-line sprinting and preplanned COD movements to detect differences between sexes11 and faster and slower sprinters.12,13 However, the major difference between sprinting and agility movements is the application of GRF and greater knee flexion and hip abduction angle to promote rapid deceleration and reacceleration in response to a sensory perturbation. This may result in a unique kinetic and kinematic pattern that has yet to be investigated between sexes from a performance perspective. Therefore, to understand the kinetic and kinematic strategies required during agility performance, the primary purpose of this study is to quantify vertical force and impulse and lower body kinematic variables during an offensive and defensive agility movement between sexes. The second purpose of this study is to compare the differences in the magnitude of the GRF and kinematic variables between offensive and defensive agility conditions. As a result of physical characteristics and strength capacity, it was hypothesized that males would produce greater force and impulse and perform the agility movement in a more advantageous lower body position (ie, greater knee flexion, hip abduction, spine flexion) compared with females, resulting in a faster agility movement (exit velocity).
Kinematics and Ground Reaction Forces During Agility 515
It was also hypothesized that differences in kinetics and kinematics between offensive and defensive agility movements would be evident, as a result of differences in decision-making and information processing strategies employed between the two conditions.
Methods
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Subjects Twelve male (n = 12) and 12 female (n = 12) semiprofessional team sport (soccer: n = 8; basketball: n = 8; and netball: n = 8) athletes (Table 1) with no previous history of major lower limb injury (ie, ligament sprains, muscle strains, or bone fractures) and no significant vision problems participated in this study. Subjects were required to partake in a minimum of two competitive games and one structured skills training session for their chosen sport each week to be included in the study. All testing and familiarization sessions occurred during their particular team sport season. The Human Research Ethics Committee at Edith Cowan University approved all test procedures, with written informed consent obtained from all subjects before the commencement of the study.
Experimental Design The current study used a cross-sectional design to compare GRF, impulse, lower body kinematics, strength, decision time, post COD stride velocity, and total running time between sexes during offensive and defensive agility tests, and compared performance between agility conditions. All testing procedures took place during 1 session with anthropometrical measurements (height, body mass) being recorded for each subject, followed by the completion of three unilateral left leg and right leg isometric back squats to ascertain peak force values as a measure of strength. Participants then completed a total of 24 agility trials; 12 offensive and 12 defensive trials, changing direction in response to movements of a human stimulus. Specific kinematic, vertical GRF, and impulse variables were measured for the COD step during each agility trial. Subjects completed a 10 minute dynamic warm up before any testing. Each testing procedure was separated with a minimum recovery period of 10 minute to ensure any fatigue did not influence the results.
Isometric Strength Testing Lower body strength was assessed with a unilateral maximal isometric back squat performed on a portable force plate (400 Series Performance Plate; Fitness Technology, Adelaide, Australia) sampling at 600 Hz. The subject pushed against an immovable bar position across the shoulders with both the knee and hip angle set at approximately 140°.14 Subjects were instructed to position the leg to be assessed under their center of mass while the other limb was unsupported and flexed at an angle of 90°. Subjects were required Table 1 Subject characteristics Male
Female
P
ES
Age (years)
22.50 ± 3.61
20.58 ± 2.15
.13
0.65
Height (cm)
178.30 ± 8.67
170.57 ± 9.72
.05
0.84
Body mass (kg)
77.14 ± 13.69
65.85 ± 13.99
.05
0.82
UL strength dominant (N⋅kg–1)
18.52 ± 3.93
14.21 ± 5.80
.04
0.87
Abbreviations: UL, unilateral strength; ES, effect size.
to perform a total of three trials for each limb, for 5 seconds in duration.15 Each trial was separated by a 2 minute recovery period. Intraclass correlation coefficient (ICC) and coefficient of variation (CV) for strength were as follows: left leg (ICC = 0.95; CV = 7.0%) and right leg (ICC = 0.95; CV = 5.5%). The highest peak force of the three trials was used and presented as a value relative to body mass (N·kg–1). These results are presented in Table 1.
Reactive Agility Protocol Each subject performed 12 sidestepping maneuvers under reactive offensive and defensive conditions, changing direction in response to movements performed by a human stimulus. The four movement patterns performed by the human stimulus have previously been used to create a reliable stimulus during reactive agility protocols (ICC = 0.71–0.99; CV = 1.11%–4.77%; TE = 0.15–0.59).4,5,16 Each movement pattern was presented in a randomized order and consisted of three offensive and three defensive trials of the following: 1. Two-step left: Step forward with the left leg, then right leg, then change direction to the left. 2. One-step left: Step forward with the right leg, then change direction to the left. 3. Two-step right: Step forward with the right leg, then left leg, then change direction to the right. 4. One-step right: Step forward with the left leg, then change direction to the right. Before the commencement of each trial, the human stimulus was informed of the movement pattern and direction to move (left or right), while the subjects were instructed to move either in the same direction as the human stimulus during a defensive trial or in the opposite direction during an offensive trial. Subjects began on a marked line 9 m opposite the human stimulus facing them and were instructed to run in a straight line toward the stimulus. Once the subject reached a marked line placed 3 m from the starting position, the human stimulus initiated one of the four movement patterns. After changing direction in response to the human stimulus, subjects were then required to sprint 2 m to the left or right completing the trial. Subjects were instructed to run toward and respond to movements of the human stimulus at a speed of 4.5 ± 0.5 m·s–1, which was monitored using a dual beam infrared timing light system (Speedlight Timing System; Swift Performance Equipment, Old, Australia), over a 3 m distance to determine approach velocity (displacement divided by time) for each trial. Approach velocity was controlled to ensure any differences observed in the data could not be attributed to velocity variations between subjects. Subjects were also required to change direction at an angle of 45° ± 5°, which was monitored by tape markings placed 45° from the center of all three force plates (Kistler Quattro, Type 9290AD; Victoria, Australia) and visual inspection from high-speed video recording of each trial (Sony HDD Camcorder HDRXR550V; Sony, Australia). Three force plates were used in the study to account for variations in decision-making time, and therefore, the initiation of the change of direction movement between subjects. For a trial to be deemed successful, subjects were required to contact their whole foot on one of the three force plates without targeting the plate, run at the set approach speed, cut at the required angle, have performed a side-step cut only, and responded in the correct movement direction as indicated at the beginning of each trial. If the subject did not correctly meet one of these conditions, the trial was reattempted. A minimum 1 minute recovery period was enforced between trials.
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516 Spiteri, Hart, and Nimphius
Ground reaction forces were recorded at 1000 Hz using a 600 × 900 mm triaxial force plate with Bioware software (Ver. 1.08; Kistler, Victoria, Australia). Raw mediolateral, anteroposterior, and vertical GRF data were exported to MATLAB programing software (R2010a; The Mathworks Inc., Chatswood, Australia) to examine specific vertical force variables during the COD step for the dominant limb only. Limb dominance was defined as the limb that produced the greater amount of force during the unilateral isometric strength assessment. Variables of interest include peak braking and propulsive force (N·kg–1), time to peak braking and propulsive force (s), contact time (s), time between peak braking and propulsive peaks (s), and relative braking impulse, propulsive impulse, and total impulse (m·s–1). All force and impulse variables were calculated relative to body mass (kg) and analyzed over the stance phase, with heel strike defined as the instance the vertical GRF data exceeds 10 N, and toe off defined as the instance the vertical GRF data are below 10 N.3,13 Braking impulse was calculated from heel strike to the minimum of the midsupport phase, and propulsive impulse was calculated from minimum of midsupport phase to toe off. To record three-dimensional movements, 37 retroreflective markers were fixed to anatomical landmarks of the athlete in accordance with the previously validated Vicon Plug-In Gait model, with movements captured using a 10 camera, 250 Hz Vicon motion analysis system (Oxford Metrics Ltd., Oxford, United Kingdom). Before data collection, a static calibration for each athlete was performed to locate anatomical landmarks and define segment and joint coordinate systems to the kinematic model.17 All trials were processed in Vicon Nexus (Ver. 1.6.1; Oxford Metrics Ltd., Oxford, United Kingdom) through a custom pipeline to obtain filtered marker trajectories using a zero-lag fourth order 18 Hz, low-pass Butterworth filter.17 Joint kinematic data for each trial was interpolated and time normalized to 100% of stance (heel strike to toe off). Variables of interest include maximum and minimum spine flexion, hip flexion, hip abduction and knee flexion (degrees), and post COD stride velocity (m·s–1). Post COD stride velocity was determined as the time from toe off of the COD step to heel strike of the first step after changing direction of the athlete’s center of mass.13,18
Results
Statistical Analysis
Males demonstrated an overall faster decision time (P = .001; ES = 1.29–1.37) and post stride velocity (P = .001; ES = 0.75–0.83) when compared with females during both offensive and defensive conditions (Figure 1). Males also demonstrated a significantly faster post stride velocity during the offensive condition compared with the defensive condition (P = .004; ES = 0.48). As approach speed was controlled during the testing protocol, no significant differences were observed between sexes (males: 4.33 ± 0.4 m⋅s–1; females: 4.08 ± 0.8 m⋅s–1, P = .12; ES = 0.38) and conditions (offensive: 4.26 ± 0.32 m⋅s–1; defensive: 4.25 ± 0.51 m⋅s–1, P = 1.12; ES = 0.01). MANOVA revealed that significant differences occurred between sexes for vertical (Pillai’s trace ≤ 0.01) GRF and impulse variables (Table 2). Males demonstrated significantly greater maximum braking force, propulsive force, braking impulse, propulsive impulse, and total impulse compared with females during one-step and two-step offensive trials and one-step defensive trials. During two-step defensive trials, significantly greater braking impulse was observed for male subjects, while females produced greater propulsive impulse and total impulse; however, this difference was nonsignificant. No significant differences were observed in timing variables between sexes during both defensive and offensive trials. Differences between offensive and defensive conditions were only observed for between two-step trials (Table 2). Females produced significantly greater braking force (P = .02; ES = 0.98) and propulsive force during defensive trials. Males produced significantly greater propulsive force (P = .01; ES = 1.13), braking impulse (P = .04; ES = 0.86), propulsive impulse (P = .01; ES = 1.48), and total impulse (P = .01; ES = 1.52) during offensive trials. Time to maximum propulsive force and time between maximum braking and propulsive peaks were significantly faster for males during the offensive condition (P = .01–0.03; ES = 0.63–1.00). Mean joint rotations presented as a function of stance (Figure 2) reveal males demonstrated significantly greater knee flexion (P = .03–0.05; ES = 0.97) and spine flexion (P = .01; ES = 1.47) compared with females during one- and two-step offensive and defensive trials. In addition, males produced greater hip abduction (P = .01; ES = 1.14) during two-step offensive and defensive trials compared with females. No significant differences were observed between offensive and defensive conditions within sexes.
All results are represented as means ± SD. Independent t tests were conducted to examine differences in subject demographics (height, body weight, age, and unilateral strength) and response times (approach speed, decision time, post stride velocity, and total running time) between sexes. A 2 × 2 multivariate analysis (MANOVA) was conducted to examine differences between sexes and condition (offensive: one-step, two-step; defensive: one-step, two-step) across all variables. A follow up 1-way analysis of variance (ANOVA) was conducted on each dependent variable to determine precisely where significant differences occurred, with sequential Bonferroni corrections19 made to reduce type I errors. A significance level of P ≤ .05 was employed throughout all statistical analysis unless otherwise stated. Effect sizes (ES) were calculated for group comparisons by dividing the difference between groups by the pooled standard deviation.20 The magnitude of ES calculations were interpreted following Hopkins’21 guidelines, with trivial = ≤ 0; small = 0 to 0.2; moderate = 0.2 to 0.6; large = 0.6 to 1.2; very large = 1.2 to 2.0; nearly perfect = 2.0 to 4.0; perfect = ≥ 4.0. All statistical computations were performed using a statistical analysis program (SPSS, Version 17.0; Chicago, Illinois).
Figure 1 — Sex differences in reaction time (RT) and post stride velocity (PSV) between offensive and defensive conditions. *Significant difference in means between males and females (P ≤ .05). ^Significant difference in means between offensive and defensive conditions (P ≤ .05).
Kinematics and Ground Reaction Forces During Agility 517
Table 2 Mean stance phase ground kinetics for male and female subjects during offensive and defensive agility trials Defensive
Offensive
Male
Female
P
ES
Male
Female
P
ES
Braking force (N⋅kg–1)
48.85 ± 13.05
25.08 ± 2.85
.01
2.53
46.73 ± 13.05
22.47 ± 3.52
.01
2.54
Propulsive force (N⋅kg–1)
26.81 ± 5.20
19.78 ± 1.51
.01
1.84
26.68 ± 7.09
18.72 ± 1.97
.01
1.53
1.42 ± 0.39
0.88 ± 0.17
.01
1.80
1.25 ± 0.43
0.85 ± 0.20
.01
1.19
3.25 ± 0.63
2.39 ± 0.27
.01
1.77
3.34 ± 0.57
2.34 ± 0.24
.01
2.28
4.67 ± 0.98
3.27 ± 0.37
.01
1.89
4.58 ± 0.89
3.17 ± 0.38
.01
2.06
Time to peak braking force (ms)
0.03 ± 0.01
0.04 ± 0.01
.08
1.00
0.03 ± 0.01
0.04 ± 0.01
.28
1.00
Time to peak propulsive force (ms)
0.10 ± 0.01
0.11 ± 0.01
.59
1.00
0.11 ± 0.02
0.11 ± 0.02
.91
0.00
Time between B:P force (ms)
0.07 ± 0.01
0.07 ± 0.01
.11
0.00
0.08 ± 0.02
0.08 ± 0.01
.61
0.00
Contact time (s)
0.25 ± 0.02
0.25 ± 0.02
1.00
0.00
0.24 ± 0.02
0.26 ± 0.02
.08
1.00
35.00 ± 8.52
22.52 ± 4.17
.01
1.86
42.32 ± 12.32
19.12 ± 2.60
.01
2.61
18.93 ± 3.42
19.11 ± 1.68
.87
0.06
25.20 ± 7.09
16.36 ± 1.43
.01
1.73
1.05 ± 0.15
0.78 ± 0.14
.01
1.86
1.26 ± 0.31
0.80 ± 0.09
.07
2.01
2.06 ± 0.67
2.43 ± 0.14
.06
0.76
2.93 ± 0.49
2.31 ± 0.19
.01
1.67
3.10 ± 0.69
3.19 ± 0.24
.69
0.17
4.19 ± 0.74
3.11 ± 0.20
.01
1.99
0.03 ± 0.01
0.04 ± 0.01
.06
1.00
0.03 ± 0.01
0.04 ± 0.01
.34
1.00
Variable One-step Trials
Braking impulse
(m⋅s–1)
Propulsive impulse
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Total impulse
(m⋅s–1)
(m⋅s–1)
Two-step Trials Braking force (N⋅kg–1)a Propulsive force
(N⋅kg–1)a, b
Braking impulse (m⋅s–1)b Propulsive impulse Total impulse
(m⋅s–1)b
(m⋅s–1)b
Time to peak braking force (ms)
0.09 ± 0.01
0.10 ± 0.02
.06
0.63
0.10 ± 0.01
0.11 ± 0.02
.29
0.63
Time between B:P force (ms)b
0.06 ± 0.01
0.07 ± 0.01
.55
1.00
0.07 ± 0.01
0.08 ± 0.01
.59
1.00
Contact time (s)
0.23 ± 0.03
0.25 ± 0.02
.16
0.78
0.24 ± 0.02
0.25 ± 0.01
.16
1.00
Time to peak propulsive force
(ms)b
Note. Significant differences (P ≤ .05) between defensive and offensive conditions for a females and bmales. B:P, braking and propulsive.
Discussion The current study is the first to quantify and examine the differences in lower body kinematics, vertical GRF, and impulse variables between sexes during offensive and defensive agility conditions and compare differences in kinematic and kinetic variables between agility conditions. The findings from this study demonstrated that males produced significantly faster defensive and offensive agility movements as characterized by a faster decision time and post stride velocity; producing greater force, impulse, and trunk and knee flexion angles when compared with females. Differences between offensive and defensive movements were only observed during two-step movement trials. Significantly greater force and impulse were produced during the agility condition for each sex where a faster decision time was recorded. These observed differences in kinematics and kinetics between sexes and agility conditions might be attributed to numerous factors, including decision-making ability, strength, and neuromuscular variables, which have been shown to cause variations in rate of force development capabilities, force application, and postural strategies employed when changing direction.3,22,23 Sex differences associated with the production of force during high-velocity athletic movements have been attributed to differences in perceptual-cognitive speed,24 neuromuscular properties of the lower limbs,8,25 and lower body strength characteristics.13 The combination of a faster decision time to identify specific kine-
matic cues available from the human stimulus and greater lower body strength, as observed by male athletes in the current study, increases the time available to apply more force and impulse during the braking phase of the movement,13 enabling a faster exit velocity to be achieved.12,13,18 This may be due to a “faster” or “more refined” neuromuscular system, whereby the interaction between higher cognitive function and muscle (neural and morphological) characteristics is required during high-velocity movements involving a temporal sensory perturbation such as agility for a faster post COD stride velocity to be achieved. Differences in muscle fiber type, size, and elasticity23,26 have been reported to influence rate of force development capabilities and produce variations in lower body kinematics and kinetics between sexes during eccentric and concentric phases of a jump.22,27 Specifically, a greater percentage of slow twitch muscle fibers and reduced tendon stiffness have been found to increase electromechanical delay and time to activation, resulting in a longer movement time in females during a lower body reaction time test.24 This apparent delay in neuromuscular activation between sexes, coupled with longer decision-making time evident in female athletes, may explain the differences observed in maximal braking (eccentric) and propulsive (concentric) force and impulse production between sexes in the current study. An important component of agility performance is an athlete’s ability to rapidly decelerate and reaccelerate, which requires sufficient lower body strength to both stop and then increase the momentum and velocity of the center of mass to successfully avoid
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518 Spiteri, Hart, and Nimphius
Figure 2 — Mean stance phase joint rotations for male (n = 12) and female (n = 12) subjects during defensive and offensive agility trials. Data are represented for: one-step trials including (a) knee flexion-extension, (b) hip flexion-extension, (c) hip abduction-adduction, and (d) spine flexion-extension; and two-step trials including (e) knee flexion-extension, (f) hip flexion-extension, (g) hip abduction-adduction, and (h) spine flexion-extension.
or pursue opponents. Increased GRF production is a commonly observed feature of faster running speeds28 in athletes who have greater lower body strength to rapidly apply force throughout the duration of the movement. In particular, increasing GRF during the braking phases of sprinting or COD movements,13 as observed by males during offensive and defensive movements, has been identified as contributing to the storage and utilization of elastic energy,12 enabling an increased propulsive force output improving acceleration ability. Many studies have reported mixed findings regarding improvements in strength and subsequent translation to COD performance.29,30 Recent research has reported strong and sig-
nificant correlations between propulsive force, propulsive impulse, and post COD stride velocity during COD movements for stronger athletes.31 These findings, in conjunction with the current study, indicate that a higher level of strength capacity enables athletes to rapidly and systematically coordinate force application throughout the movement. This is required to increase impulse production, which involves a time component in which to apply force, in order to increase an athlete’s momentum out of directional changes. Differences in lower body kinematics between sexes during COD movements have been well documented.3,7,9,10 While a majority of kinematic differences are attributed to anthropometrical
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Kinematics and Ground Reaction Forces During Agility 519
characteristics,32 findings of the current study are similar to previous performance-based research,6,13 reporting significantly greater knee flexion and spine flexion angle in starter or male athletes who demonstrated significantly greater lower body strength and subsequently produced faster COD and agility performance. While no significant differences were observed for hip flexion angle between sexes, the upright body position and unfavorable kinematic chain adopted by female athletes not only increases the risk of lower limb injury,3,7 but also reduces the ability to produce and direct force application to improve post COD stride velocity. For example, during a movement such as a sprint start, an athlete is permitted to adjust their hip, knee, and ankle angle out of the start blocks; enabling triple extension of the lower limbs and lengthening of the muscle-tendon complex to optimize the force-length relationship, resulting in a greater force output and acceleration.33 Agility movements however, require athletes to possess greater lower body strength to enter a lower body position across stance phase13,30 to improve mechanical functioning of the lower limbs to increase acceleration. When changing direction to opposition movements, an athlete’s perception regarding the amount of time and space available may compromise postural preparation, increasing injury risk and compromising performance outcomes. During two-step movement trials, the human stimulus initiated movement closer to the COD, resulting in a significantly greater hip abduction angle observed in male athletes. Previous research has also observed a greater lateral foot plant during agility movements3,34 due to an athlete’s perceived need to rapidly decelerate and enable rapid reacceleration in the new direction. Executing the COD movement with a prominent lateral foot plant increases the acceleration of the center of mass to the contralateral side,35 as a greater approach angle and lower body position creates an optimal kinematic chain to better direct force application. As no significant difference in hip abduction angle was observed during one-step movement trials between sexes, we can conclude that males had a greater ability to overcome the increased spatial demand by demonstrating a faster decision time in response to the human stimulus, enabling a more optimal body position to be adopted during stance phase. Reactive movements during team sports are often in response to opponent’s actions, either in offensive (incompatible) or defensive (compatible) situations. Stimulus-response compatibility, determined by the spatial correlation between the stimulus and the appropriate response,36 is known to affect the rate of information processing and the speed of the upcoming motor response depending on the tactical situation. While previous research has observed faster reaction times during compatible conditions, as the required response is processed within intrahemispheric nervous circuits,36,37 results of the current study partly contradict pervious findings, with males producing a faster decision time during incompatible (offensive) agility movements. It can be assumed that during defensive movements, greater attentional demand and tracking of kinematic cues is constantly required to defend the opposition, whereas offensive movements require athletes to locate and identify relevant cues at that one point in time to successfully evade the opposition. This may explain why males produced a faster agility performance during the offensive condition, as they displayed a greater ability to anticipate the upcoming action by eliciting a higher percentage of negative decision time results. Despite males and females responding differently, it is clear that differences between offensive and defensive movements appear to be attributed to perceptual-cognitive factors, with both sexes producing greater force and impulse during the condition where a faster decision time was observed.
Previous research has also identified that preactivation of the neuromuscular system is advantageous during competition by enabling a quicker motor response.22 Faster movement times, specifically a significantly faster time between braking and propulsive peaks and time to peak propulsive force, coupled with greater force and impulse produced by males, resulted in a significantly faster post COD stride velocity during offensive agility movements. This could be due to their faster decision time, where preactivation and preparation enables a reduction in the natural electromechanical delay (EMD) of the body that occurs before heel strike, which has been demonstrated previously in other high-velocity movements.22,38 While the current study did not measure muscle activation, previous research has demonstrated that a shorter EMD is associated with a higher magnitude of initial muscle activation and force production.39,40 This enables a higher aggregate of force to be produced in an equivalent time period.39,40 Improved timing and greater force production as a result of increased preparation time may explain the faster exit velocity observed in males during offensive movements and females during defensive movements. While there were kinematic differences between sexes, no differences were observed between offensive and defensive maneuvers within each sex. This could further implicate decision time and strength as trainable factors to improve the kinetics profile of offensive and defensive agility movements. While the findings of the current study provide understanding of the significant biomechanical variables contributing to a faster agility performance, the current study did not directly measure lower body muscle activation, EMD, or visual gaze strategies that may have been employed throughout the movement. These contributing factors may further distinguish agility performance between sexes. In addition, these factors may provide a greater understanding of the perceptual-cognitive and neuromuscular factors required to achieve faster offensive and defensive agility movements. To address these limitations and expand on the findings of the current study, future research should directly quantify these variables between male and female athletes and offensive and defensive agility movements in order to advance the understanding of the limiting factors of agility to improve performance. While differences were observed between male and female athletes, which can be attributed to the various anthropometrical, neuromuscular, and neurophysiological differences between sexes, it appears differences between offensive and defensive movements are largely a result of neuromuscular and neurophysiological qualities. Therefore, it appears based upon the present data that testing, training, and developing agility should be specifically designed for both offensive and defensive movements, as distinctive biomechanical and perceptualcognitive differences are evident between movement scenarios.
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