Measurement of muscle length-related electromyography activity of the hip flexor muscles to determine individual muscle contributions to the hip flexion torque

This study aimed to investigate muscle length-related electromyography (EMG) of the iliopsoas (IL) and other hip flexor muscles to determine individual muscle contributions to the hip flexion torque. Ten healthy sedentary young men participated in the EMG experiment. A subgroup of six subjects underwent a magnetic resonance imaging (MRI) measurement to confirm the region of the skin over the IL. Surface EMG signals were sampled from the IL, rectus femoris (RF), sartorius (SA), and tensor fasciae latae (TFL) using an active electrode. The subjects performed maximum voluntary isometric hip flexion with the right hip joint set at -10°, 0°, 30°, and 60°. The root mean square (RMS) value for the TFL at 30° (0.81 ± 0.19, p <0.005) and 60° (0.66 ± 0.17, p <0.001) and the SA at 60° (0.62 ± 0.24, p <0.005) were significantly decreased compared with those at 0°. However, the RMS value for the IL and RF did not change significantly. The RMS value and muscle length changes were significantly correlated in the IL (r =0.39, p <0.05), SA (r =0.51, p <0.001), and TFL (r =0.70, p <0.001), but not in the RF (r =0.22, p =0.180). We conclude that, in a hip joint flexed position, the contribution of the IL to hip flexion movement is relatively larger than that of the other hip flexor muscles.