Muscle activation of the gluteus maximus and hamstrings during prone hip extension with knee flexion in three hip abduction positions

Abstract

The direction of fiber alignment within a muscle is known to influence the effectiveness of muscle contraction. However, most of the commonly used clinical gluteus maximus (GM) exercises do not consider the direction of fiber alignment within the muscle. Therefore, the purpose of this study was to investigate the influence of hip abduction position on the amplitude and onset time of the GM and hamstrings (HAM) during prone hip extension with knee flexion (PHEKF) exercise. Surface electromyography (EMG) signals were recorded from the GM and HAM during PHEKF exercise in three hip abduction positions: 0°, 15°, and 30°. Thirty healthy subjects (age: 22.8 ± 2.9 yrs, body mass: 66.9 ± 10.8 ㎏, height: 170.3 ± 4.1 ㎝) voluntarily participated in this study. EMG amplitude was transformed into the root-mean-square (RMS) and expressed as a percentage of the maximal voluntary isometric contraction (MVIC) for each muscle. The relative onset difference between activation of the GM and HAM was calculated by subtracting the HAM onset from the GM onset (in ms). Repeated measures analysis of variance (ANOVA) with Bonferroni post hoc tests was used to compare muscle activation between the three hip abduction positions. The level of statistical significance was set at p < 0.05. The results show that GM amplitude was greatest in the 30° hip abduction position, followed by 15° and then 0° hip abduction during PHEKF exercise (mean ± SD: 29 ± 11% MVIC, 23 ± 9% MVIC, and 20 ± 8% MVIC, respectively). On the other hand, the HAM amplitude was greatest at 0° hip abduction, followed by 15° and then 30° (mean ± SD: 17 ± 15% MVIC, 15 ± 15% MVIC, and 14 ± 14% MVIC, respectively). Additionally, the relative onset difference between the GM and HAM was positive at 0° hip abduction, meaning that GM firing was delayed relative to that of the HAM (mean ± SD, 0.17 ± 0.17 ms). In contrast to the 15 and 30° hip abduction positions, the relative onset difference was negative, which means the GM onset occurred earlier than the HAM (mean ± SD: -0.02 ± 0.11 ms and -0.21 ± 0.20 ms, respectively). These findings indicate that the 30° hip abduction position maximizes GM amplitude and minimizes HAM amplitude. The EMG onset of the GM was significantly earlier relative to the HAM at 15° and 30° hip abduction. Therefore, performing PHEKF exercise in the 30° hip abduction position may be recommended as an effective way to selectively activate the GM and to reduce the delay in GM firing in asymptomatic individuals. This finding provides preliminary evidence that amplitude and onset time during PHEKF can be modified by the extent of hip abduction.