Investigate the relationship between hip abduction muscle strength, hip abduction muscle activation, and the EPKAM
The study included 163 athletes (107 female) in soccer or team handball, median age 16 (IQR = 15-17). Hip abduction strength (% body weight) was measured with a handheld dynamometer secured with a belt 2cm distal to the knee joint line. Three trials were taken on each side and the highest used for the analysis. During the same test, the maximum voluntary isometric contraction measure for the EMG was captured and used to normalize the EMG signal. EMG was collected at 1000Hz and RMS filtered with a 101 frame moving window. The mean EMG signal was calculated at 25ms intervals from 50ms before initial contact.
Retro-reflective markers were placed on subjects and captured at 400Hz and a resolution of 0.3MP. Ground reaction forces (GRF) were captured with a force plate capturing at 2000Hz. The athletes performed 10 repetitions per leg of a change of direction task from a ready position, and the kinetics / kinematics were calculated.
KAM results were transformed to signed differences, and a Ward D2 clustering algorithm was used to categorize them into either the EPKAM waveform, or other waveforms. Kinematics, vertical GRF, emg, and hip strength were independent variables and EPKAM was the depdenent variable. A boosted regression model was calculated from 80% of subjects with an interaction depth of 3, shrinkage of 0.01, and a maximum of 7000 trees. The model was cross validated on the remaining 20% of subjects, and the hypothesis test was the spearman correlation between the predicted and observed EP KAM waveform. The effect size was the difference in spearman correlation between models with and without the emg and hip abduction strength variables.
A model that included the vertical GRF and body posture at initial contact could predict an EPKAM waveform with a spearmans correlation of 0.46 (p 0.0001). A model that additionally included the hip abduction strength and hip abductor EMG performed worse, and could only predict an EPKAM waveform with a spearmans correlation of 0.42 (p 0.0001).
Hip abductor muscle strength and activation during a change of direction task was not related to the chance of an EPKAM waveform.
If hip abductor muscle strength or activation affects the risk of ACL injury, it does not appear to be through a reduced frequency of EPKAM waveforms during a change of direction task. The magnitude of KAM could potentially be modified by these variables.
biomechanics
hip strength
