Journal of Sports Sciences
ISSN: 0264-0414 (Print) 1466-447X (Online) Journal homepage: http://www.tandfonline.com/loi/rjsp20
The relationships of eccentric strength and power with dynamic balance in male footballers Marc Jon Booysen, Philippe Jean-Luc Gradidge & Estelle Watson To cite this article: Marc Jon Booysen, Philippe Jean-Luc Gradidge & Estelle Watson (2015): The relationships of eccentric strength and power with dynamic balance in male footballers, Journal of Sports Sciences, DOI: 10.1080/02640414.2015.1064152 To link to this article: http://dx.doi.org/10.1080/02640414.2015.1064152
Published online: 08 Jul 2015.
Submit your article to this journal
Article views: 107
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=rjsp20 Download by: [Universite Laval]
Date: 05 October 2015, At: 19:10
Journal of Sports Sciences, 2015 http://dx.doi.org/10.1080/02640414.2015.1064152
The relationships of eccentric strength and power with dynamic balance in male footballers
MARC JON BOOYSEN
, PHILIPPE JEAN-LUC GRADIDGE & ESTELLE WATSON
Centre for Exercise Science and Sports Medicine, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa
Downloaded by [Universite Laval] at 19:10 05 October 2015
(Accepted 5 April 2015)
Abstract Unilateral balance is critical to kicking accuracy in football. In order to design interventions to improve dynamic balance, knowledge of the relationships between dynamic balance and specific neuromuscular factors such as eccentric strength and power is essential. Therefore, the aim was to determine the relationships of eccentric strength and power with dynamic balance in male footballers. The Y-balance test, eccentric isokinetic strength testing (knee extensors and flexors) and the countermovement jump were assessed in fifty male footballers (university (n = 27, mean age = 20.7 ± 1.84 years) and professional (n = 23, mean age = 23.0 ± 3.08 years). Spearman Rank Order correlations were used to determine the relationships of eccentric strength and power with dynamic balance. Multiple linear regression, adjusting for age, mass, stature, playing experience and competitive level was performed on significant relationships. Normalised reach score in the Y-balance test using the non-dominant leg for stance correlated with (1) eccentric strength of the non-dominant leg knee extensors in the university group (r = 0.50, P = 0.008) and (2) countermovement jump height in the university (r = 0.40, P = 0.04) and professional (r = 0.56, P = 0.006) football groups, respectively. No relationships were observed between eccentric strength (knee flexors) and normalised reach scores. Despite the addition of potential confounders, the relationship of power and dynamic balance was significant (r = 0.52, P < 0.0002). The ability to generate power correlates moderately with dynamic balance on the non-dominant leg in male footballers. Keywords: Y-balance test, isokinetic testing, countermovement jump height, non-dominant leg, football
Introduction Dynamic balance can be defined as the ability to maintain the centre of gravity within the body’s base of support whilst performing an intended movement (Butler, Southers, Gorman, Kiesel, & Plisky, 2012) and is a vital component of performance in the game of football (Hrysomallis, 2011). Football activity is characterised by intense, explosive movements (Gerbino, Griffin, & Zurakowski, 2007; Stolen, Chamari, Castagna, & Wisloff, 2005), many of which are executed from a single leg (Bressel, Yonker, Kras, & Heath, 2007; Paillard et al., 2006), suggesting that the stability of the stance foot in the execution of successful football related movement is crucial (Chew-Bullock et al., 2012; Paillard et al., 2006). An inability to remain balanced can disturb force production, attenuate performance (Flanagan, 2012) and potentially increase the risk of injury (Butler, Lehr, Fink, Kiesel, & Plisky, 2013; Plisky, Rauh, Kaminski, & Underwood, 2006). Elite football players demonstrate better balance
capability than their non-elite peers (Butler et al., 2012; Paillard et al., 2006) and when compared with other sporting populations, footballers have performed better on either leg in unilateral balance tests (Bressel et al., 2007; Matsuda, Demura, & Uchiyama, 2008). In light of the importance of balance in football, awareness of the relationship between dynamic balance and certain neuromuscular factors such as power and eccentric strength could justify the development of more effective balance interventions. Investigations into the relationship of power and balance in athletic populations remain inconclusive. Erkmen, Taşkin, Sanioğlu, Kaplan, and Baştürk (2010) demonstrated that there was a significant relationship between power and non-dominant leg balance in football players, whilst Johnson and Woollacott (2011) observed that power-trained athletes responded faster to external perturbations in a bilateral stance than endurance athletes. However, in comparison, Muehlbauer, Gollhofer, and Granacher
Correspondence: Marc Jon Booysen, Centre for Exercise Science and Sports Medicine, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, 27 St Andrews Rd, Parktown, Johannesburg 2050, South Africa. E-mail: [email protected] © 2015 Taylor & Francis
Downloaded by [Universite Laval] at 19:10 05 October 2015
2
M.J. Booysen et al.
(2013) found a non-significant association between dynamic balance using the dominant leg for stance and countermovement jump height in both healthy young and middle-aged adults, concluding that these abilities may be independent of each other. Studies investigating the relationship between strength and dynamic balance have focused on the concentric (Paterno, Schmitt, Ford, & Hewett, 2009; Thorpe & Ebersole, 2008) and isometric (Muehlbauer et al., 2013) components of force production, concluding that strength and dynamic balance were independent of each other. However, evidence suggests that eccentric strength of the lower extremity may play a more important role in dynamic balance. For example, Norris and Trudelle-Jackson (2011) observed high activation levels of the vastus medialis during the lowering phase of the star excursion balance test (SEBT). This could possibly be a result of eccentric contractions absorbing, decelerating and stabilising the forces imposed on the body’s centre of mass (Earl & Hertel, 2001; Lindstedt, Lastayo, & Reich, 2001). Recently, Lockie, Schultz, Callaghan, and Jeffriess (2013) found that male team sport athletes who had a greater relative reach using their left leg for stance in the SEBT also had stronger left knee extensors (at an eccentric contraction mode of 30° s–1). Superior dynamic balance is critical to the football player (Gerbino et al., 2007; Hrysomallis, 2011) where stability of the stance foot is related to kicking accuracy (Chew-Bullock et al., 2012). However, the relationship between eccentric strength and power with dynamic balance in male football players remains unresolved. A better understanding of these neuromuscular factors may help advance current balance-training interventions in footballers, possibly improving performance. Therefore, the aim of this study was to determine the relationships of eccentric strength and power of the lower extremity with dynamic balance in male footballers. Methods Study design The relationships of eccentric strength and power with dynamic balance were examined using a crosssectional, observational study design. Participants Fifty male footballers from two football teams volunteered to participate in the study. Twenty seven participants (mean age = 20.7 ± 1.84 (SD) years; body mass = 64.4 ± 7.16 kg; stature = 1.71 ± 0.05 m; body mass index (BMI) = 22.1 ± 2.37 kg · m−2) were from the same university senior first team squad. Twenty three participants (mean age = 23.0 ± 3.08 (SD)
years; body mass = 66.6 ± 6.81 kg; stature = 1.70 ± 0.06 m; BMI = 22.9 ± 1.65 kg · m−2) were from a top-ranked South African second division football team. The university group had a mean football playing experience of 11.2 ± 2.27 years and had at least two training sessions and one match per week. The professional group had a mean football playing experience of 13.8 ± 2.94 years, trained twice daily, six days per week, and played two matches per week. Both groups had not been exposed to balance/perturbation type training prior to this study. Furthermore, both groups had no experience in performing the Y-balance test and isokinetic testing. Testing was performed during the pre-season for both teams which was April and July of the same year for the university and professional teams, respectively. The investigation was approved by the Human Research Ethics Committee (Medical) of the University of the Witwatersrand. Before testing was initiated, all participants completed a self-reported medical screening questionnaire and signed a written informed consent. Participants were excluded if they reported the presence of any lower limb injury within the last six months, a current upper respiratory tract infection, any bone or joint abnormalities, any uncorrected visual and vestibular problems and/or a concussion within the last three months. Procedures and instrumentation Prior to testing, participants were given an 8 min standardised warm up which included light jogging and dynamic stretching. Individual participants completed all performance tests on one occasion. The test order was (1) dynamic balance testing, (2) power testing and (3) strength testing. Extraneous factors were reduced by asking all participants to refrain from exercising on the test day (Gribble, Hertel, Denegar, & Buckley, 2004), perform all tests barefoot (Robbins, Waked, Gouw, & Mcclaran, 1994) and in the afternoon (from 12–5 pm) to eliminate diurnal variation in postural control (Gribble, Tucker, & White, 2007). A standardised explanation and demonstration were given to all participants prior to commencement of each test. Balance and power data were collected by the main researcher who was experienced in administering the Y-balance and countermovement jump tests. Further assistance was received during eccentric strength testing from trained isokinetic clinicians. Anthropometric measurements Body mass was measured using a digital scale (Seca, Medical Scales and Measuring Systems, Birmingham, United Kingdom), and stature was measured using a stadiometer (Seca, Birmingham, United Kingdom).
The relationships of strength, power and balance Leg length was measured using a standard tape measure (Seca, Birmingham, United Kingdom) with the participant lying in the supine position on a plinth. Both legs were straightened to equalise the pelvis, and the measurement taken from the anterior superior iliac spine to the centre of the medial malleolus of the ipsilateral leg (Gribble & Hertel, 2003).
Downloaded by [Universite Laval] at 19:10 05 October 2015
Dynamic balance Dynamic balance was measured using an instrumented version of the SEBT, namely the Y-balance test (Y-Balance Test™, Move2Perform, Evanville, IN), which has demonstrated good reliability in the literature (Plisky et al., 2009). The Y-balance test requires the participant to maintain a unilateral stance, while reaching maximally with the contra lateral limb in a specified direction (anterior, post-medial and postlateral) without losing their balance (Gribble & Hertel, 2003; Kinzey & Armstrong, 1998). The further the distance reached by the participant, the greater the challenge to the neuromuscular and balance control systems (Earl & Hertel, 2001; Norris & Trudelle-Jackson, 2011; Thorpe & Ebersole, 2008). Performances in the Y-balance test were recorded by one experienced tester. The participant performed the Y-balance test with four practice trials in each direction on both legs prior to testing (Robinson & Gribble, 2008b). A standard test procedure was followed to improve the reliability and consistency of performance (Plisky et al., 2009). The testing included three trials standing on the right foot reaching in the anterior direction followed by three trials on the left foot in the same direction (Plisky et al., 2009). This procedure was repeated for the postmedial direction and lastly, the post-lateral direction (Plisky et al., 2009). For each trial, the participant positioned the most distal aspect of his stance foot at the centre of the platform on the anterior red line. The participant was then instructed to make a maximal reach by pushing the reach indicator as far as possible with the most distal aspect of the free leg. During the reaching movement, the participant was required to maintain a balanced position, with hands placed on their hips and the stance heel in contact with the platform (Robinson & Gribble, 2008a, 2008b; Thorpe & Ebersole, 2008). A trial was discarded and repeated if the participant (1) failed to maintain a unilateral stance on the stance platform and touched the floor outside the starting zone with the reach foot (Plisky et al., 2009), (2) placed his toes on top of the reach indicator for support (Plisky et al., 2009), (3) did not return his reach foot to the designated starting area under control (Plisky et al., 2009), (4) failed to maintain the reach foot in contact with reach indicator on the red target area while it was in motion (Plisky et al., 2009) and (5) lifted the
3
stance leg heel off the platform or removed hands from the hips (Robinson & Gribble, 2008a, 2008b; Thorpe & Ebersole, 2008). To reduce the effects of fatigue on performance in the test, a 15-s rest period after each trial (Norris & Trudelle-Jackson, 2011) and 3-min rest breaks between each direction were adhered to (Norris & Trudelle-Jackson, 2011). Reach distances (cm) in each direction were averaged over the three trials and normalised to leg length (Gribble & Hertel, 2003; Plisky et al., 2009). Balance performance for each leg was defined as a normalised reach score and was quantified by calculating the average of the normalised reach distances of each direction (Gorman, Butler, Rauh, Kiesel, & Plisky, 2012). Countermovement jump The countermovement jump (CMJ) is a non-invasive and valid method of assessing jumping ability and explosive power of the lower extremity (Markovic, Dizdar, Jukic, & Cardinale, 2004; Moir, Button, Glaister, & Stone, 2004; Slinde, Suber, Suber, Edwén, & Svantesson, 2008). Maximal vertical jump displacement was measured on a contact mat (Fusion Sport Smart Jump mat, Fusion Sport, 2 Henley ST, Coopers Plains, QLD, 4108, Australia) using a flight-time-based method. Flight-time-based measurements on contact mats are considered to be the most reliable and valid of all field tests for measuring height in vertical jump testing with an intraclass correlation coefficient value of 0.98 (Markovic et al., 2004). At the start of each trial, the participant stood barefoot and motionless at the centre of the contact mat. The participant then made a quick countermovement to a self-selected depth, flexing at the hips, knees and ankles before jumping for maximal vertical height (Mclellan, Lovell, & Gass, 2011). Participants were requested not to flex their knees whilst in flight during the vertical jump (Bosco et al., 1983; Markovic et al., 2004) and to maintain the hands on their hips position throughout the countermovement jump (Castagna & Castellini, 2013). A 2min standing rest was given between each jump (Castagna & Castellini, 2013). Three maximal trials (Castagna & Castellini, 2013) were performed with the highest jump (cm) selected for data analysis. Eccentric strength testing Eccentric strength testing was performed on an isokinetic dynamometer (Biodex System 3, Biodex Medical System, INC, 20 Ramsay Road, Shirley, NY, 11967, USA) to determine the eccentric peak torque of the knee flexors and extensors at an angular velocity of 90° s–1. Procedures for isokinetic
Downloaded by [Universite Laval] at 19:10 05 October 2015
4
M.J. Booysen et al.
assessment of the knee joint followed the protocol as set out by Perrin (1994). After the dynamometer was calibrated, the rotational axis of the dynamometer was visually aligned with the lateral femoral condyle of the knee being tested. The participant was then secured in a seated position with the hip angle at 90° with the body maximally strapped and arms crossed at the chest (Perrin, 1994). The range of motion at the knee joint that was comfortable for the athlete was set for both extension and flexion and approximated 80° at the knee joint in both study groups. Gravity correction was done by weighing the limb at 30° of extension. Consistent verbal encouragement was given to ensure maximal effort was given during maximal trials. Four practice trials were performed at 25, 50, 75 and 100 per cent of maximal effort, respectively. On completion of the practice trials, the participant rested for 90 s prior to the maximal eccentric trials. Two sets consisting of five continuous repetitions were performed at 90° s–1 with the starting leg always being the right. A rest period of 90 s was given between sets. Eccentric peak torque (newton metres) values were automatically normalised to body mass by the Biodex 3 for each leg in both flexion and extension and expressed as peak torque to body weight (Brown, 2000). The set with the highest peak torque to body weight values was selected for further analysis.
applied to calculate significant between-group differences in the performance and physical measurements. The Spearman Rank Order correlation was used to determine the relationships between eccentric peak torque to body weight values of the knee extensors/flexors and countermovement jump height (CMJ height) with normalised reach distance in the Y-balance test within each football group. Multiple linear regression were performed over the full sample to determine whether eccentric strength and power variables were associated with dynamic balance in male footballers. Potential confounding variables (age, mass, stature, playing experience and competitive level) were included in the regression model. Significance level was set at a level of P < 0.05.
Results The performance characteristics of the university and professional groups No significant bilateral deficits were found for dynamic balance and knee extensor eccentric strength testing in either group (Table I). Significant bilateral strength deficits were observed in the knee flexors muscles, where the dominant leg in both groups was significantly stronger (university group; P = 0.0008 and professional group; P = 0.01) than the non-dominant. No significant differences were observed between the groups for dynamic balance, eccentric strength and power values (P < 0.05) (Table I).
Statistical analysis Statistica Version 12.0 (Series 0313b, StatSoft) was used for statistical analyses. Numerical values were expressed as a mean with 95% confidence limits. Normality was tested using the Shapiro–Wilk’s W test. Due to data being not normally distributed, the Wilcoxon Matched Pairs Test was used to identify bilateral deficits for performance variables in both groups. The Mann–Whitney U Test was
Relationships between eccentric strength of the knee extensors and dynamic balance Using the non-dominant leg for stance in the Y-balance test, a significant correlation was observed between normalised reach score and eccentric peak
Table I. Performance values of the university and professional groups.
Y-balance test Dominant leg (%) Non-dominant leg (%) Eccentric strength testing Knee Extensor – dominant leg (%) Knee Extensor – non-dominant leg (%) Knee Flexor – dominant leg (%) Knee Flexor – non-dominant leg (%) Power Countermovement jump height (cm)
University Mean (95% CL)
Professional Mean (95% CL)
89.7 (87.3–92.1) 88.8 (86.2–91.5)
91.0 (89.1–92.8) 91.1 (89.1–93.1)
375 363 259 228
406 387 266 247
(346–404) (332–394) (240–277)* (207–249)
38.6 (36.5–40.8)
(374–438) (352–422) (249–283)** (230–264)
36.1 (34.0–38.1)
Note: *Dominant leg of the university group significantly stronger than the non-dominant leg (P < 0.05). **Dominant leg of the professional group significantly stronger than the non-dominant leg (P < 0.05). Y-balance values expressed as a percentage of leg length; Eccentric strength values expressed as a percentage of body mass.
The relationships of strength, power and balance Table II. Correlations of eccentric strength and power with normalised reach score using the dominant leg for stance.
Knee extensor strength of the dominant leg Knee flexor strength of the dominant leg CMJ height
University r (P-value)
Professional r (P-value)
Y-balance
Y-balance
0.32 (0.10)
0.06 (0.79)
0.15 (0.46)
−0.29 (0.19)
0.35 (0.08)
0.36 (0.10)
normalised reach score on the non-dominant leg across the full sample population remained significant (r = 0.52, P = 0.0002). Reliability of tests produced coefficient of variance (CV) values of
ISSN: 0264-0414 (Print) 1466-447X (Online) Journal homepage: http://www.tandfonline.com/loi/rjsp20
The relationships of eccentric strength and power with dynamic balance in male footballers Marc Jon Booysen, Philippe Jean-Luc Gradidge & Estelle Watson To cite this article: Marc Jon Booysen, Philippe Jean-Luc Gradidge & Estelle Watson (2015): The relationships of eccentric strength and power with dynamic balance in male footballers, Journal of Sports Sciences, DOI: 10.1080/02640414.2015.1064152 To link to this article: http://dx.doi.org/10.1080/02640414.2015.1064152
Published online: 08 Jul 2015.
Submit your article to this journal
Article views: 107
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=rjsp20 Download by: [Universite Laval]
Date: 05 October 2015, At: 19:10
Journal of Sports Sciences, 2015 http://dx.doi.org/10.1080/02640414.2015.1064152
The relationships of eccentric strength and power with dynamic balance in male footballers
MARC JON BOOYSEN
, PHILIPPE JEAN-LUC GRADIDGE & ESTELLE WATSON
Centre for Exercise Science and Sports Medicine, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa
Downloaded by [Universite Laval] at 19:10 05 October 2015
(Accepted 5 April 2015)
Abstract Unilateral balance is critical to kicking accuracy in football. In order to design interventions to improve dynamic balance, knowledge of the relationships between dynamic balance and specific neuromuscular factors such as eccentric strength and power is essential. Therefore, the aim was to determine the relationships of eccentric strength and power with dynamic balance in male footballers. The Y-balance test, eccentric isokinetic strength testing (knee extensors and flexors) and the countermovement jump were assessed in fifty male footballers (university (n = 27, mean age = 20.7 ± 1.84 years) and professional (n = 23, mean age = 23.0 ± 3.08 years). Spearman Rank Order correlations were used to determine the relationships of eccentric strength and power with dynamic balance. Multiple linear regression, adjusting for age, mass, stature, playing experience and competitive level was performed on significant relationships. Normalised reach score in the Y-balance test using the non-dominant leg for stance correlated with (1) eccentric strength of the non-dominant leg knee extensors in the university group (r = 0.50, P = 0.008) and (2) countermovement jump height in the university (r = 0.40, P = 0.04) and professional (r = 0.56, P = 0.006) football groups, respectively. No relationships were observed between eccentric strength (knee flexors) and normalised reach scores. Despite the addition of potential confounders, the relationship of power and dynamic balance was significant (r = 0.52, P < 0.0002). The ability to generate power correlates moderately with dynamic balance on the non-dominant leg in male footballers. Keywords: Y-balance test, isokinetic testing, countermovement jump height, non-dominant leg, football
Introduction Dynamic balance can be defined as the ability to maintain the centre of gravity within the body’s base of support whilst performing an intended movement (Butler, Southers, Gorman, Kiesel, & Plisky, 2012) and is a vital component of performance in the game of football (Hrysomallis, 2011). Football activity is characterised by intense, explosive movements (Gerbino, Griffin, & Zurakowski, 2007; Stolen, Chamari, Castagna, & Wisloff, 2005), many of which are executed from a single leg (Bressel, Yonker, Kras, & Heath, 2007; Paillard et al., 2006), suggesting that the stability of the stance foot in the execution of successful football related movement is crucial (Chew-Bullock et al., 2012; Paillard et al., 2006). An inability to remain balanced can disturb force production, attenuate performance (Flanagan, 2012) and potentially increase the risk of injury (Butler, Lehr, Fink, Kiesel, & Plisky, 2013; Plisky, Rauh, Kaminski, & Underwood, 2006). Elite football players demonstrate better balance
capability than their non-elite peers (Butler et al., 2012; Paillard et al., 2006) and when compared with other sporting populations, footballers have performed better on either leg in unilateral balance tests (Bressel et al., 2007; Matsuda, Demura, & Uchiyama, 2008). In light of the importance of balance in football, awareness of the relationship between dynamic balance and certain neuromuscular factors such as power and eccentric strength could justify the development of more effective balance interventions. Investigations into the relationship of power and balance in athletic populations remain inconclusive. Erkmen, Taşkin, Sanioğlu, Kaplan, and Baştürk (2010) demonstrated that there was a significant relationship between power and non-dominant leg balance in football players, whilst Johnson and Woollacott (2011) observed that power-trained athletes responded faster to external perturbations in a bilateral stance than endurance athletes. However, in comparison, Muehlbauer, Gollhofer, and Granacher
Correspondence: Marc Jon Booysen, Centre for Exercise Science and Sports Medicine, School of Therapeutic Sciences, Faculty of Health Sciences, University of the Witwatersrand, 27 St Andrews Rd, Parktown, Johannesburg 2050, South Africa. E-mail: [email protected] © 2015 Taylor & Francis
Downloaded by [Universite Laval] at 19:10 05 October 2015
2
M.J. Booysen et al.
(2013) found a non-significant association between dynamic balance using the dominant leg for stance and countermovement jump height in both healthy young and middle-aged adults, concluding that these abilities may be independent of each other. Studies investigating the relationship between strength and dynamic balance have focused on the concentric (Paterno, Schmitt, Ford, & Hewett, 2009; Thorpe & Ebersole, 2008) and isometric (Muehlbauer et al., 2013) components of force production, concluding that strength and dynamic balance were independent of each other. However, evidence suggests that eccentric strength of the lower extremity may play a more important role in dynamic balance. For example, Norris and Trudelle-Jackson (2011) observed high activation levels of the vastus medialis during the lowering phase of the star excursion balance test (SEBT). This could possibly be a result of eccentric contractions absorbing, decelerating and stabilising the forces imposed on the body’s centre of mass (Earl & Hertel, 2001; Lindstedt, Lastayo, & Reich, 2001). Recently, Lockie, Schultz, Callaghan, and Jeffriess (2013) found that male team sport athletes who had a greater relative reach using their left leg for stance in the SEBT also had stronger left knee extensors (at an eccentric contraction mode of 30° s–1). Superior dynamic balance is critical to the football player (Gerbino et al., 2007; Hrysomallis, 2011) where stability of the stance foot is related to kicking accuracy (Chew-Bullock et al., 2012). However, the relationship between eccentric strength and power with dynamic balance in male football players remains unresolved. A better understanding of these neuromuscular factors may help advance current balance-training interventions in footballers, possibly improving performance. Therefore, the aim of this study was to determine the relationships of eccentric strength and power of the lower extremity with dynamic balance in male footballers. Methods Study design The relationships of eccentric strength and power with dynamic balance were examined using a crosssectional, observational study design. Participants Fifty male footballers from two football teams volunteered to participate in the study. Twenty seven participants (mean age = 20.7 ± 1.84 (SD) years; body mass = 64.4 ± 7.16 kg; stature = 1.71 ± 0.05 m; body mass index (BMI) = 22.1 ± 2.37 kg · m−2) were from the same university senior first team squad. Twenty three participants (mean age = 23.0 ± 3.08 (SD)
years; body mass = 66.6 ± 6.81 kg; stature = 1.70 ± 0.06 m; BMI = 22.9 ± 1.65 kg · m−2) were from a top-ranked South African second division football team. The university group had a mean football playing experience of 11.2 ± 2.27 years and had at least two training sessions and one match per week. The professional group had a mean football playing experience of 13.8 ± 2.94 years, trained twice daily, six days per week, and played two matches per week. Both groups had not been exposed to balance/perturbation type training prior to this study. Furthermore, both groups had no experience in performing the Y-balance test and isokinetic testing. Testing was performed during the pre-season for both teams which was April and July of the same year for the university and professional teams, respectively. The investigation was approved by the Human Research Ethics Committee (Medical) of the University of the Witwatersrand. Before testing was initiated, all participants completed a self-reported medical screening questionnaire and signed a written informed consent. Participants were excluded if they reported the presence of any lower limb injury within the last six months, a current upper respiratory tract infection, any bone or joint abnormalities, any uncorrected visual and vestibular problems and/or a concussion within the last three months. Procedures and instrumentation Prior to testing, participants were given an 8 min standardised warm up which included light jogging and dynamic stretching. Individual participants completed all performance tests on one occasion. The test order was (1) dynamic balance testing, (2) power testing and (3) strength testing. Extraneous factors were reduced by asking all participants to refrain from exercising on the test day (Gribble, Hertel, Denegar, & Buckley, 2004), perform all tests barefoot (Robbins, Waked, Gouw, & Mcclaran, 1994) and in the afternoon (from 12–5 pm) to eliminate diurnal variation in postural control (Gribble, Tucker, & White, 2007). A standardised explanation and demonstration were given to all participants prior to commencement of each test. Balance and power data were collected by the main researcher who was experienced in administering the Y-balance and countermovement jump tests. Further assistance was received during eccentric strength testing from trained isokinetic clinicians. Anthropometric measurements Body mass was measured using a digital scale (Seca, Medical Scales and Measuring Systems, Birmingham, United Kingdom), and stature was measured using a stadiometer (Seca, Birmingham, United Kingdom).
The relationships of strength, power and balance Leg length was measured using a standard tape measure (Seca, Birmingham, United Kingdom) with the participant lying in the supine position on a plinth. Both legs were straightened to equalise the pelvis, and the measurement taken from the anterior superior iliac spine to the centre of the medial malleolus of the ipsilateral leg (Gribble & Hertel, 2003).
Downloaded by [Universite Laval] at 19:10 05 October 2015
Dynamic balance Dynamic balance was measured using an instrumented version of the SEBT, namely the Y-balance test (Y-Balance Test™, Move2Perform, Evanville, IN), which has demonstrated good reliability in the literature (Plisky et al., 2009). The Y-balance test requires the participant to maintain a unilateral stance, while reaching maximally with the contra lateral limb in a specified direction (anterior, post-medial and postlateral) without losing their balance (Gribble & Hertel, 2003; Kinzey & Armstrong, 1998). The further the distance reached by the participant, the greater the challenge to the neuromuscular and balance control systems (Earl & Hertel, 2001; Norris & Trudelle-Jackson, 2011; Thorpe & Ebersole, 2008). Performances in the Y-balance test were recorded by one experienced tester. The participant performed the Y-balance test with four practice trials in each direction on both legs prior to testing (Robinson & Gribble, 2008b). A standard test procedure was followed to improve the reliability and consistency of performance (Plisky et al., 2009). The testing included three trials standing on the right foot reaching in the anterior direction followed by three trials on the left foot in the same direction (Plisky et al., 2009). This procedure was repeated for the postmedial direction and lastly, the post-lateral direction (Plisky et al., 2009). For each trial, the participant positioned the most distal aspect of his stance foot at the centre of the platform on the anterior red line. The participant was then instructed to make a maximal reach by pushing the reach indicator as far as possible with the most distal aspect of the free leg. During the reaching movement, the participant was required to maintain a balanced position, with hands placed on their hips and the stance heel in contact with the platform (Robinson & Gribble, 2008a, 2008b; Thorpe & Ebersole, 2008). A trial was discarded and repeated if the participant (1) failed to maintain a unilateral stance on the stance platform and touched the floor outside the starting zone with the reach foot (Plisky et al., 2009), (2) placed his toes on top of the reach indicator for support (Plisky et al., 2009), (3) did not return his reach foot to the designated starting area under control (Plisky et al., 2009), (4) failed to maintain the reach foot in contact with reach indicator on the red target area while it was in motion (Plisky et al., 2009) and (5) lifted the
3
stance leg heel off the platform or removed hands from the hips (Robinson & Gribble, 2008a, 2008b; Thorpe & Ebersole, 2008). To reduce the effects of fatigue on performance in the test, a 15-s rest period after each trial (Norris & Trudelle-Jackson, 2011) and 3-min rest breaks between each direction were adhered to (Norris & Trudelle-Jackson, 2011). Reach distances (cm) in each direction were averaged over the three trials and normalised to leg length (Gribble & Hertel, 2003; Plisky et al., 2009). Balance performance for each leg was defined as a normalised reach score and was quantified by calculating the average of the normalised reach distances of each direction (Gorman, Butler, Rauh, Kiesel, & Plisky, 2012). Countermovement jump The countermovement jump (CMJ) is a non-invasive and valid method of assessing jumping ability and explosive power of the lower extremity (Markovic, Dizdar, Jukic, & Cardinale, 2004; Moir, Button, Glaister, & Stone, 2004; Slinde, Suber, Suber, Edwén, & Svantesson, 2008). Maximal vertical jump displacement was measured on a contact mat (Fusion Sport Smart Jump mat, Fusion Sport, 2 Henley ST, Coopers Plains, QLD, 4108, Australia) using a flight-time-based method. Flight-time-based measurements on contact mats are considered to be the most reliable and valid of all field tests for measuring height in vertical jump testing with an intraclass correlation coefficient value of 0.98 (Markovic et al., 2004). At the start of each trial, the participant stood barefoot and motionless at the centre of the contact mat. The participant then made a quick countermovement to a self-selected depth, flexing at the hips, knees and ankles before jumping for maximal vertical height (Mclellan, Lovell, & Gass, 2011). Participants were requested not to flex their knees whilst in flight during the vertical jump (Bosco et al., 1983; Markovic et al., 2004) and to maintain the hands on their hips position throughout the countermovement jump (Castagna & Castellini, 2013). A 2min standing rest was given between each jump (Castagna & Castellini, 2013). Three maximal trials (Castagna & Castellini, 2013) were performed with the highest jump (cm) selected for data analysis. Eccentric strength testing Eccentric strength testing was performed on an isokinetic dynamometer (Biodex System 3, Biodex Medical System, INC, 20 Ramsay Road, Shirley, NY, 11967, USA) to determine the eccentric peak torque of the knee flexors and extensors at an angular velocity of 90° s–1. Procedures for isokinetic
Downloaded by [Universite Laval] at 19:10 05 October 2015
4
M.J. Booysen et al.
assessment of the knee joint followed the protocol as set out by Perrin (1994). After the dynamometer was calibrated, the rotational axis of the dynamometer was visually aligned with the lateral femoral condyle of the knee being tested. The participant was then secured in a seated position with the hip angle at 90° with the body maximally strapped and arms crossed at the chest (Perrin, 1994). The range of motion at the knee joint that was comfortable for the athlete was set for both extension and flexion and approximated 80° at the knee joint in both study groups. Gravity correction was done by weighing the limb at 30° of extension. Consistent verbal encouragement was given to ensure maximal effort was given during maximal trials. Four practice trials were performed at 25, 50, 75 and 100 per cent of maximal effort, respectively. On completion of the practice trials, the participant rested for 90 s prior to the maximal eccentric trials. Two sets consisting of five continuous repetitions were performed at 90° s–1 with the starting leg always being the right. A rest period of 90 s was given between sets. Eccentric peak torque (newton metres) values were automatically normalised to body mass by the Biodex 3 for each leg in both flexion and extension and expressed as peak torque to body weight (Brown, 2000). The set with the highest peak torque to body weight values was selected for further analysis.
applied to calculate significant between-group differences in the performance and physical measurements. The Spearman Rank Order correlation was used to determine the relationships between eccentric peak torque to body weight values of the knee extensors/flexors and countermovement jump height (CMJ height) with normalised reach distance in the Y-balance test within each football group. Multiple linear regression were performed over the full sample to determine whether eccentric strength and power variables were associated with dynamic balance in male footballers. Potential confounding variables (age, mass, stature, playing experience and competitive level) were included in the regression model. Significance level was set at a level of P < 0.05.
Results The performance characteristics of the university and professional groups No significant bilateral deficits were found for dynamic balance and knee extensor eccentric strength testing in either group (Table I). Significant bilateral strength deficits were observed in the knee flexors muscles, where the dominant leg in both groups was significantly stronger (university group; P = 0.0008 and professional group; P = 0.01) than the non-dominant. No significant differences were observed between the groups for dynamic balance, eccentric strength and power values (P < 0.05) (Table I).
Statistical analysis Statistica Version 12.0 (Series 0313b, StatSoft) was used for statistical analyses. Numerical values were expressed as a mean with 95% confidence limits. Normality was tested using the Shapiro–Wilk’s W test. Due to data being not normally distributed, the Wilcoxon Matched Pairs Test was used to identify bilateral deficits for performance variables in both groups. The Mann–Whitney U Test was
Relationships between eccentric strength of the knee extensors and dynamic balance Using the non-dominant leg for stance in the Y-balance test, a significant correlation was observed between normalised reach score and eccentric peak
Table I. Performance values of the university and professional groups.
Y-balance test Dominant leg (%) Non-dominant leg (%) Eccentric strength testing Knee Extensor – dominant leg (%) Knee Extensor – non-dominant leg (%) Knee Flexor – dominant leg (%) Knee Flexor – non-dominant leg (%) Power Countermovement jump height (cm)
University Mean (95% CL)
Professional Mean (95% CL)
89.7 (87.3–92.1) 88.8 (86.2–91.5)
91.0 (89.1–92.8) 91.1 (89.1–93.1)
375 363 259 228
406 387 266 247
(346–404) (332–394) (240–277)* (207–249)
38.6 (36.5–40.8)
(374–438) (352–422) (249–283)** (230–264)
36.1 (34.0–38.1)
Note: *Dominant leg of the university group significantly stronger than the non-dominant leg (P < 0.05). **Dominant leg of the professional group significantly stronger than the non-dominant leg (P < 0.05). Y-balance values expressed as a percentage of leg length; Eccentric strength values expressed as a percentage of body mass.
The relationships of strength, power and balance Table II. Correlations of eccentric strength and power with normalised reach score using the dominant leg for stance.
Knee extensor strength of the dominant leg Knee flexor strength of the dominant leg CMJ height
University r (P-value)
Professional r (P-value)
Y-balance
Y-balance
0.32 (0.10)
0.06 (0.79)
0.15 (0.46)
−0.29 (0.19)
0.35 (0.08)
0.36 (0.10)
normalised reach score on the non-dominant leg across the full sample population remained significant (r = 0.52, P = 0.0002). Reliability of tests produced coefficient of variance (CV) values of
