Journal of Biomechanics 47 (2014) 3674–3680

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A resistance band increased internal hip abduction moments and gluteus medius activation during pre-landing and early-landing Boyi Dai a,n, Erika M. Heinbaugh a, Xiaopeng Ning b, Qin Zhu a a b

Division of Kinesiology and Health, University of Wyoming, Laramie, USA Department of Industrial and Management Systems Engineering, West Virginia University, Morgantown, WV, USA

art ic l e i nf o

a b s t r a c t

Article history: Accepted 25 September 2014

An increased knee abduction angle during jump-landing has been identified as a risk factor for anterior cruciate ligament injuries. Activation of the hip abductors may decrease the knee abduction angle during jump-landing. The purpose of this study was to examine the effects of a resistance band on the internal hip abduction moment and gluteus medius activation during the pre-landing (100 ms before initial contact) and early-landing (100 ms after initial contact) phases of a jump–landing–jump task. Thirteen male and 15 female recreational athletes (age: 21.1 7 2.4 yr; mass: 73.8 714.6 kg; height: 1.76 70.1 m) participated in the study. Subjects performed jump–landing–jump tasks with or without a resistance band applied to their lower shanks. During the with-band condition, subjects were instructed to maintain their movement patterns as performing the jump-landing task without a resistance band. Lower extremity kinematics, kinetics, and gluteus medius electromyography (EMG) were collected. Applying the band increased the average hip abduction moment during pre-landing (po 0.001, Cohen's d (d)¼ 2.8) and early-landing (p o0.001, d¼1.5), and the average gluteus medius EMG during prelanding (p o0.001, d¼ 1.0) and early-landing (p ¼0.003, d ¼0.55). Applying the band decreased the initial hip flexion angle (p¼0.028, d ¼0.25), initial hip abduction angle (po 0.001, d¼ 0.91), maximum knee flexion angle (p¼ 0.046, d ¼0.17), and jump height (p ¼0.004, d¼ 0.16). Applying a resistance band provides a potential strategy to train the strength and muscle activation for the gluteus medius during jump-landing. Additional instructions and feedback regarding hip abduction, hip flexion, and knee flexion may be required to minimize negative changes to other kinematic variables. & 2014 Elsevier Ltd. All rights reserved.

Keywords: ACL injuries Hip abductors EMG Jumping Kinematics Kinetics

1. Introduction Anterior cruciate ligament (ACL) injuries typically occur in the early phase of landing when individuals demonstrate a decreased knee flexion angle and an increased knee abduction angle (Agel et al., 2005; Boden et al., 2000; Koga et al., 2010; Krosshaug et al., 2007). A decreased knee flexion angle and an increased external knee abduction moment are associated with an increased ACL loading (Berns et al., 1992; Markolf et al., 1995). An increased peak knee abduction angle and an increased peak external knee abduction moment during jump-landing have been identified as risk factors for ACL injuries (Hewett et al., 2005). Therefore, landing with a small knee abduction angle may decrease ACL loading and the risk of ACL injuries. The lower extremity acts as a kinetic chain during dynamic tasks (Powers, 2003, 2010). Excessive hip adduction during landing can n Correspondence to: Division of Kinesiology and Health, Dept. 31961000 E, University avenue, Laramie, WY 82071, USA. Tel.: þ1 307 766 5423; fax: þ 1 307 766 4098. E-mail address: [email protected] (B. Dai).

http://dx.doi.org/10.1016/j.jbiomech.2014.09.032 0021-9290/& 2014 Elsevier Ltd. All rights reserved.

cause the knee joint to move medially, and increase the knee abduction angle (Powers, 2003, 2010). Activation of the hip abductors can generate an internal hip abduction moment to decrease the medial displacement of the knee and knee abduction angle. Investigators have studied the relationships between the hip abductor strength, hip abductor activation, and knee abduction angle during landing and squatting tasks (Homan et al., 2013; Jacobs and Mattacola, 2005; Jacobs et al., 2007; Patrek et al., 2011; Russell et al., 2006; Wallace et al., 2008; Zeller et al., 2003), but limited intervention strategies are available to train the hip abductors during jump-landing. Several investigators have described hip abductor activation during functional tasks (Boudreau et al., 2009; Distefano et al., 2009; Dwyer et al., 2010), however, muscles demonstrate specificity between training and testing tasks for strength gains (Hortobagyi et al., 1996; Rutherford and Jones, 1986; Wilson et al., 1996). The transfer effect from functional tasks to athletic tasks may be limited. In addition, because ACL injuries usually occur in the early phase of landing (Koga et al., 2010; Krosshaug et al., 2007), activation of the hip abductors during early landing or even before landing should be encouraged. Functional tasks may have limited training effects on specific muscle activation patterns during athletic tasks.

B. Dai et al. / Journal of Biomechanics 47 (2014) 3674–3680

The ACL research Retreat VI group has stated that it is needed to “optimize the transfer of learned movement patterns to sportsspecific movements performed on the field” (Shultz et al., 2012). Strategies to train the strength and muscle activation for the hip abductors during athletic tasks may be more applicable to the sports field. One way to train the hip abductors is to apply a resistance band between an individual's lower extremities (Cambridge et al., 2012; Distefano et al., 2009). Applying a resistance band to the lower extremities can produce a medial force and an external hip adduction moment, which may increase the demands of the hip abductors to generate a counteracting internal hip abduction moment. Cambridge et al. (2012) placed a resistance band on individuals' knees, ankles, and feet during Sumo Walking and Monster Walking. These researchers found that the electromyography (EMG) of the gluteus medius increased as a result of the band placement moving distally. Gooyers et al. (2012) evaluated the effects of a resistance band on knee width indices and peak external knee abduction moments during squatting and jumping tasks. However, the effects of a resistance band on hip joint moments and hip abductor activation during jump-landing are still unknown. The primary purpose of this study was to quantify the effects of a resistance band on internal hip abduction moments and gluteus medius activation during the pre-landing and early-landing phases of a jump–landing–jump task. A secondary purpose was to assess the effects of a resistance band on knee and hip biomechanics and task performance during the same phases of landing. It was hypothesized that a resistance band would increase internal hip abduction moments and gluteus medius activation without negatively affecting knee and hip biomechanics and task performance.

2. Methods 2.1. Subjects Based on a pilot study with two subjects and a previous study (Cambridge et al., 2012), a medium to large effect size was expected for the internal hip abduction moment and gluteus medius EMG between the without-band and with-band conditions. Assuming an effect size of 0.6 for a paired test, a total sample size of 24 was needed for a type I error at the level of 0.05 to achieve a power of 0.8. A total of 28 recreational athletes (gender: 13 males, 15 females; age: 21.172.4 yr; mass: 73.8714.6 kg; height: 1.7670.1 m) participated in the study. The subjects were required to have experience in playing sports that involved jump-landing tasks. Sports experience was defined as currently playing sports at least one time per week or having previously played at high school, college, or club levels. Subjects were also required to be currently physically active, which was defined as participating in sports / exercise at least two times per week for a total of 2–3 hours per week. A subject was excluded if he/she previously had one of the following conditions: (1) an ACL injury or other major lower extremity injuries; (2) a lower extremity injury that prevented participation in physical activities for more than two weeks over the previous six months; and (3) a condition that prevent him/her from participating in sporting activities. Permission to conduct this study was obtained from the University of Wyoming Institutional Review Board.

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Fig. 1. Mark placement and EMG transmitter. reaction forces were collected using a force plate at a sampling frequency of 1600 Hz (4060-10, Bertec, Columbus, OH). Subjects performed a standing trial and multiple trials of a jump–landing–jump task (Padua et al., 2012b) (Fig. 2) with or without a resistance band (LifeLineUSA, Madison, WI; Fig. 3). The cuffs of the band were placed right above the ankle joints. The rest length of the band was set in such way that the distance between the centers of the two ankle cuffs was 15 cm, which was generally shorter than an adult's stance width. Two markers were placed on the cable pockets to define the length of the band and the direction of the force generated by the band. The length–force relationship of the band was calibrated during each data collection by slowly lifting a constant weight from a force plate (Fig. 4). During the calibration, the length of the band was defined by the two markers. The force generated by the band was calculated by subtracting the vertical ground reaction force from the constant weight. Subjects performed 2 or 3 practice trials without the resistance band to ensure correct jump-landing forms. Additional practice trials were allowed before the official trials if required. Subjects then performed 3 official trials of the jump–landing–jump task with or without a resistance band (randomized order). In the with-band condition, subjects were instructed to maintain their movement patterns as performing the jump-landing without a resistance band. Subjects had a 30-second break between two trials to avoid fatigue. After completing jumplanding trials, subjects performed a 10-second hip abductor maximum voluntary isometric contraction (MVIC) against a stationary weight in a standing posture, with hip joint angles at 0 degree in all three anatomical planes. EMG data were collected during the 10-second MVIC trial to ensure a true 1-second MVIC was achieved. 2.3. Data reduction

2.2. Procedures Subjects wore spandex shorts, shirts, and standard running shoes (Ghost 5, Brooks Sports, Bothell, Washington). Subjects performed 5-min of self-selected running and stretching exercises for warm-up. The testing side (left or right) was randomly selected. On the testing side, a surface electrode was placed on the muscle belly of the gluteus medius, defined as 50% on the line from the iliac crest to the greater trochanter (Bolgla and Uhl, 2007). A reference electrode was placed on the tibial tuberosity. A proper electrode placement was confirmed by visual inspection of the EMG when subjects abducted their hips against a manual resistance. A harness (Fig. 1) was placed above the subjects' pelvis to hold the EMG transmitter (Myomonitor, Delsys Inc, Boston, MA). EMG data were collected at a sampling frequency of 2000 Hz using EMGworks software (Delsys Inc, Boston, MA). Retroreflective markers (Fig. 1) were placed on subjects' bony landmarks. Marker co-ordinates were captured using six optical cameras at a sampling frequency of 160 Hz (Vicon Bonita 10, Oxford Metrics Ltd, Oxford, UK). Ground

Marker co-ordinates and force plate data were filtered using a fourth-order, zero-phase-shift Butterworth filter at a low-pass cutoff frequency of 15 Hz (Kristianslund et al., 2012). EMG data were filtered at a high-pass cutoff frequency of 20 Hz and a low-pass cutoff frequency of 450 Hz. Filtered EMG data were rectified and filtered at a low-pass cutoff frequency of 10 Hz to obtain the linear envelope EMG (Dai et al., 2012). EMG data during jump-landing were normalized as a percentage of the maximum 1-second average EMG during MVIC. Marker coordinates, force, and EMG data were time-synchronized to 160 Hz using a linear interpolation method. The hip joint center was determined according to Bell et al. (1990). The knee joint center was determined as the midpoint between the medial and lateral femoral epicondyles (Kadaba et al., 1989). The ankle joint center was determined as the midpoint between the medial and lateral malleoli (Kadaba et al., 1989). The pelvis reference frame was defined by the right and left anterior superior iliac spines and midpoint between the right and left posterior superior iliac spines (Wu et al., 2002).

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B. Dai et al. / Journal of Biomechanics 47 (2014) 3674–3680

Fig. 2. Jump–landing–jump task. The thigh reference frame was defined by the hip joint center, knee joint center, and lateral femoral epicondyle (Grood and Suntay, 1983). The shank reference frame was defined by the knee joint center, ankle joint center, and tibia tuberosity (Dai et al., 2012). The joint centers and markers used to define segment reference frames were calibrated during the static trial, and recreated during the jump-landing trials using a singular decomposition method (Soderkvist and Wedin, 1993). Joint angles were calculated as the Cardan angles in a flexion–extension, abduction–adduction, and internal–external rotation order (Grood and Suntay, 1983). An inverse dynamic approach was used to calculate knee and hip joint resultant moments (Kingma et al., 1996). Segment anthropometric data were based on the method described by de Leva (1996). The length–force relationship of the band was fit with a first order polynomial function (force¼ A  lengthþ B). The force vectors applied by the band were defined as external forces in inverse dynamics. The mass and moment of inertia of the band were ignored. Joint resultant moments were transferred to distal segment reference frames and expressed as internal loading. Joint resultant moments were normalized to body weight (BW) and body height (BH). Peak ACL loading may occur within 100 ms before landing (Taylor et al., 2011) or during early landing (Cerulli et al., 2003). ACL injuries usually occur within 100 ms after landing (Koga et al., 2010; Krosshaug et al., 2007). Therefore, the pre-

landing and early-landing phases were examined. The pre-landing phase was defined as 100 ms before initial ground contact (Dai et al., 2012), and the earlylanding phase was defined as 100 ms after initial ground contact (Kristianslund and Krosshaug, 2013). The average internal hip abduction moment and gluteus medius EMG during the pre-landing and early-landing phases were calculated. The knee flexion, knee abduction, knee internal rotation, hip flexion, and hip adduction angles at initial ground contact as well as the maximum internal knee varus moments and maximum internal knee external rotation moments during the earlylanding phase were calculated to assess changes in knee and hip biomechanics (Kristianslund and Krosshaug, 2013). The maximum knee flexion angle during the entire stance phase was extracted to assess the overall knee joint motion. The stance time and jump height were calculated to evaluate jumping performance. The calculations were performed using customized subroutines developed in MATLAB 2009a (MathWorks Inc. Natick, MA).

2.4. Statistical analysis Procedures to assess for statistical outliers included identifying data with Z-values greater than 3 or less than  3. Repeated measure ANOVAs with the band

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size with d4 0.8 considered “large”, 0.8 4d 40.5 considered “medium”, and d o0.5 considered “small” (Cohen, 1988). The standard deviations used to calculate Cohen's d were the pooled standard deviation from four different gender and band conditions (Howell, 2013). Statistical tests were performed using IBM SPSS Statistics 21 (IBM Corporation, Armonk, New York).

3. Results

Fig. 3. Resistance band.

One subject's EMG data were identified as outliers and eliminated from the analysis. The kinematic and kinetic data for this subject were still included. The means 7standard deviations of the coefficients A and B for the length–force equations were 322.87 25.1 N/m and  19.7 74.8 N, respectively. The R2 of polynomial fit were 0.95 70.01.The average forces applied by the band were 43.37 18.0 N during pre-landing and 41.0717.2 N during early-landing. No significant interaction effect was observed for any variable. ANOVAs showed significant main effects for the band condition for the hip abduction moment (Fig. 5, Table 1) and gluteus medius EMG (Fig. 6). Applying the band significantly increased hip abduction moments during pre-landing (F(1,26) ¼ 130.9, p o0.001, d¼ 2.8) and early-landing (F(1,26) ¼ 65.6, p o0.001, d ¼1.5), and gluteus medius EMG during pre-landing (F(1,25) ¼ 41.9, p o0.001, d¼ 1.0) and early-landing (F(1,25) ¼11.1, p ¼0.003, d ¼0.55). ANOVAs showed significant main effects for the band condition for the initial hip flexion angle, initial hip adduction angle, maximum knee flexion angle, and jump height (Table 2). Applying the band significantly decreased the initial hip flexion angle (F(1,26) ¼5.4, p ¼0.028, d ¼0.25), initial hip abduction angle (F(1,26) ¼38.1, po 0.001, d ¼0.91), maximum knee flexion angle (F(1,26) ¼4.4, p ¼0.046, d ¼0.17), and jump height (F(1,26) ¼10.3, p¼ 0.004, d ¼0.16). No significant main effects for the band condition were observed for the other variables.

4. Discussion The results supported our primary hypothesis. Subjects increased gluteus medius activation to generate greater internal hip abduction moments to counterbalance the external hip adduction moment produced by the band. Distefano et al. (2009) showed that gluteus medius activation during the side-lying hip abduction exercise (81% MVIC) was the greatest among 12 exercises. These findings indicate that gluteus medius activation may be best achieved with exercises that isolate the muscle action. In the current study, gluteus medius activation was less as compared to isolated muscle exercises

Fig. 4. Calibration for the resistance band. condition as a within-subject factor and gender as a between-subject factor were used. Because gender may affect gluteus medius activation and jump-landing mechanics (Hart et al., 2007; Zazulak et al., 2005), it was included as a variable to examine the potential interaction between the band condition and gender. The primary hypothesis of the study would be supported if the two ANOVAs for average hip abduction moments and the two ANOVAs for average gluteus medius EMG showed significant increases for the with-band condition. To control the study-wise type-I error rate, the type I error rate was set at 0.05/4 ¼ 0.0125 for the ANOVAs for the hip abduction moment and gluteus medius EMG. Type I error rate was still set at 0.05 for ANOVAs for the other variables. Cohen's d (d) was used to evaluate effect

Fig. 5. Hip adduction ( þ )/abduction (  ) moments (BH  BW) during pre-landing (100 ms before initial contact) and early-landing (100 ms after initial contact). Lines represent the ensemble means. Error bars represent 1.96* standard errors of the means.

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Table 1 Means (standard deviations) and p-values of ANOVAs regarding hip moment and gluteus medius (GM) EMG. Dependent variables

Males and females

Males

Without band

With band

Without band

With band

Without band

With band

Gender Band

 0.030 (0.013)  0.021 (0.013) 0.59 (0.24) 0.68 (0.44)

0.005 (0.012)  0.005 (0.012) 0.33 (0.15) 0.37 (0.17)

 0.026 (0.014)  0.026 (0.010) 0.53 (0.26) 0.61 (0.51)

0.001 (0.010) 0.000 (0.013) 0.41 (0.19) 0.58 (0.36)

 0.034 (0.011)  0.017 (0.014) 0.64 (0.22) 0.75 (0.37)

0.11

o 0.001 0.54

0.11

o 0.001 0.38

0.22 0.18

o 0.001 0.68 0.003 0.54

Hip adduction ( þ )/abduction (  ) moment during pre0.003 landing (BW  BH) (0.011) Hip adduction ( þ )/abduction (  ) moment during early-  0.002 landing (BW  BH) (0.013) GM EMG during pre-landing (%MVIC) 0.37 (0.17) GM EMG during early-landing (%MVIC) 0.48 (0.30)

Fig. 6. Gluteus medius EMG (% MVIC) during pre-landing (100 ms before initial contact) and early-landing (100 ms after initial contact). Lines represent the ensemble means. Error bars represent 1.96* standard errors of the means.

(Distefano et al., 2009). The benefits of performing jump-landing with a resistance band are the increased hip abduction moments and gluteus medius activation during pre-landing and early-landing of jump-landing tasks. These increases are important because peak ACL strain occurs during pre-landing (Taylor et al., 2011) or early landing (Cerulli et al., 2003), and ACL injuries occur during early-landing (Koga et al., 2010; Krosshaug et al., 2007). Increased anticipatory hip abduction contraction may decrease the risk of ACL injuries during landing (Chaudhari and Andriacchi, 2006). Gooyers et al. (2012) evaluated the effects of resistance bands applied to the distal thigh on frontal plane knee mechanics during a vertical jump. The resistance band slightly decreased the normalized knee width, and increased the peak external knee abduction moment during certain phases of the jump. The authors reported that their findings were not consistent with the positive outcomes identified in a clinical case study, during which elastic tubing was used to exaggerate the perturbed movement patterns to provoke corrective neuromuscular responses (Cook et al., 1999). The current study indicates that the beneficial effects of a resistance band may not be directly improved kinematics, but the increased involvement of muscles. Long-term training may result in increased muscle capability to resist perturbations and maintain correct movement patterns. Investigators found that ACL injuries were associated with perturbations before landing (Boden et al., 2009; Krosshaug et al., 2007; Olsen et al., 2004). Jump-landing training with a resistance band may improve an individual's ability to maintain neutral frontal plane hip and knee alignments, which may decrease the risk of ACL injuries (Chaudhari and Andriacchi, 2006). The major significance of applying a resistance band is to directly train the hip abductors during jump-landing tasks, during which ACL injuries commonly occur. Therefore, it was important to identify the effects of the band on jumping performance and

Females

p-values Interaction

biomechanical risk factors for ACL injuries that have been previously observed, particularly the knee abduction angle. The band significantly decreased the initial hip flexion angle, initial hip abduction angle, maximum knee flexion angle, and jump height. Although subjects performed practice trials before testing, the novelty of landing with a band might have caused slightly upright landing postures and decreased jump heights. The band decreased the initial hip abduction angle, suggesting that the band might have moved the two limbs closer. Although the subjects were instructed to maintain the same movement patterns as landing without a band, they still gave up a certain degree of hip abduction in the with-band condition because of the medial forces applied by the band. Considering that the decreased knee flexion angle and hip abduction angle may be associated with increased risks of ACL injuries, these movements should not be encouraged during an exercise intervention. One strategy is to provide additional instructions and feedback regarding the hip abduction, hip flexion, and knee flexion angles during training to ensure that the band does not induce these negative changes. In addition, future studies may explore the possibility of incorporating other training methods (Padua and Distefano, 2009) with a resistance band to train the gluteus medius and improve lower extremity biomechanics at the same time. The use of bands in jump-landing training should not be encouraged and other strategies may be required if individuals demonstrate poor movement patterns. Researchers have examined the relationships between the hip abductor strength, knee abduction angle, and knee abduction moment during landing tasks (Homan et al., 2013; Jacobs and Mattacola, 2005; Jacobs et al., 2007; Wallace et al., 2008). Several investigators found an inverse relationship between the isometric or eccentric peak hip-abductor strength and peak knee abduction during landing in females (Jacobs and Mattacola, 2005; Jacobs et al., 2007; Wallace et al., 2008). On the other hand, Homan et al. (2013) did not find differences in the knee abduction during landing between high and low hip abduction strength groups, but they found that the low strength group demonstrated greater gluteus medius activation during landing. The researchers suggested that it was the hip strength as well as the muscle activation that affected the knee abduction motion. Investigators found that long-term plyometric and basic resistance programs induced increased gluteus medius activation as well as improved joint kinematics, such as increased knee and hip flexion angles, during pre-landing and early-landing in females (Lephart et al., 2005). A hip-focus plyometric and balance perturbation training increased the isometric strength of the hip abductors and peak knee and hip flexion, and decreased the peak knee abduction angle during landing in females (Stearns and Powers, 2014). Future studies are needed to compare the effectiveness between resistance band training and other traditional training methods to determine whether resistance band training has advantages in application. We only quantified the immediate changes. Studies are needed to study the effects of long-term jump-landing training with a resistance

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Table 2 Means (standard deviations) and p values of ANOVAs regarding other kinematic and kinetic variables. Dependent variables

Initial knee flexion ( þ) angle (deg) Initial knee adduction ( þ )/abduction (  ) angle (deg) Initial knee internal ( þ )/external rotation (  ) Angle (deg) Initial hip flexion (  ) angle (deg) Initial hip adduction ( þ)/abduction (  ) angle (deg) Maximum knee adduction ( þ ) moment during early-landing (BH  BW) Maximum knee external rotation (  ) moment during early-landing (BH  BW) Maximum knee flexion ( þ) angle during stance phase (deg) Stance time (ms) Jump height (m)

Males and females

Males

Without band

With band

Without band

With band

Without band

With band

17.6 (6.1)  1.5 (3.7)

17.0 (5.9)  1.5 (3.6)

17.0 (7.8) 0.1 (3.2)

15.5 (7.1)  0.2 (2.8)

18.2 (4.2)  2.8 (3.7)

18.3 (4.5)  2.7 (3.9)

0.37 0.045

0.30 0.83

0.26 0.51

 10.2 (7.9)

 9.2 (7.2)

 11.1 (8.6)

 11.0 (7.9)

 9.4 (7.5)

 7.7 (6.3)

0.38

0.20

0.27

 42.9 (9.6)  8.1 (3.6)

 40.5 (9.7)  5.3 (3.1)

 45.6 (11.2)  42.3 (10.1)  40.5 (7.7)  9.6 (2.7)  7 (2.1)  6.8 (3.8)

 38.9 (9.4)  3.8 (3.1)

0.23 0.009

0.037 (0.018)  0.006 (0.005) 95.1 (17.8)

0.030 (0.018)  0.005 (0.004) 92.2 (16.6)

0.032 (0.017)  0.006 (0.003) 97.1 (18.1)

0.030 (0.019)  0.006 (0.003) 92.5 (18.0)

0.040 (0.018)  0.006 (0.006) 93.4 (17.9)

0.030 (0.017)  0.004 (0.004) 92 (16.0)

0.46

0.08

0.28

0.77

0.17

0.20

0.75

0.046 0.27

572.9 (147.6) 0.40 (0.11)

587.4 (163.0) 0.39 (0.10)

578.8 (138.4) 0.5 (0.06)

587.8 (166.0) 0.49 (0.07)

567.8 (160.0) 0.31 (0.06)

586.9 (166.3) 0.31 (0.06)

0.92

0.26

band on lower extremity biomechanics as well as to compare the training effects with other training strategies. We only collected data for one side. Collecting bilateral data would demonstrate whether stance width changed between conditions. We examined gluteus medius activation as a factor related to knee abduction without including other factors such as ankle muscles and ankle motion (Bell et al., 2013; Padua et al., 2012a, 2012b). Only one resistance band and one band placement were evaluated. Different band lengths, stiffness, and placements could affect jump-landing mechanics differently. Hip strength was not assessed, so it is unclear whether individuals with different hip strength had different responses to the band. In conclusion, a resistance band applied to the lower shanks increased internal hip abduction moments and gluteus medius EMG during the pre-landing and early-landing phases of a jump–landing– jump task. Applying a resistance band provides a strategy to potentially train the strength and muscle activation for the hip abductors during jump-landing. Additional instructions and feedback regarding hip abduction, hip flexion, and knee flexion angles may be required to minimize negative changes to other kinematic variables.

Conflict of interest statement The authors have no financial or personal conflicts of interest to declare.

Acknowledgments The authors would like to thank Samantha Ellis for her help with data collection. We also thank Dr. Mark Byra for his help with editing the manuscript. The study was supported by a grant from the College of Health Science at University of Wyoming.

References Agel, J., Arendt, E.A., Bershadsky, B., 2005. Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: a 13-year review. Am. J. Sports Med. 33, 524–530. Bell, A.L., Pedersen, D.R., Brand, R.A., 1990. A comparison of the accuracy of several hip center location prediction methods. J. Biomech. 23, 617–621. Bell, D.R., Oates, D.C., Clark, M.A., Padua, D.A., 2013. Two- and 3-dimensional knee valgus are reduced after an exercise intervention in young adults with demonstrable valgus during squatting. J. Athl. Train. 48, 442–449.

Females

p-values Gender

o 0.001

Band

Interaction

0.028 0.43 o 0.001 0.68

0.68

0.004 0.91

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