Correspondence between the directional patterns of hip muscle activation and their mechanical action in man.

The hypothesis that the pattern of muscle activation during a static voluntary effort exerted in different directions is oriented in the direction of the muscle's mechanical action was evaluated. The electromyographical (EMG) activation patterns of five hip muscles (gluteus medius, rectus femoris, tensor fasciae latae, gracilis and semitendinosus) and one knee muscle (vastus lateralis) were characterized in 11 normal subjects during static efforts at the hip joint. Subjects were asked to generate torques (10 and 20 Nm) in 24 directions covering 360 degrees at increments of 15 degrees in the transverse plane of the femur whereas torques at the knee were to be kept at zero. Using vector summation of the rectified EMGs, a mean angular value of muscle activation was calculated for each muscle across subjects. It was observed that the mean angular values of muscles acting at the hip were significantly oriented, whereas the activity of the vastus lateralis was not significantly oriented. In addition, the angular values of activation of the rectus femoris, tensor fasciae latae and semitendinosus muscles were not significantly different from the direction of mechanical action of these respective muscles as determined using a biomechanical model of the hip. However, the angular values of the gluteus medius and gracilis activations were found to be significantly different from their anatomical line of action. The angular values of activation of two muscles (rectus femoris and gluteus medius) were also compared to the mechanical pulling direction of these muscles as determined following electrical stimulation of the recorded muscle regions. The analysis revealed that the angular value of the gluteus medius and rectus femoris during voluntary static efforts at the hip was not significantly different from the direction of mechanical action of these muscles as determined by the electrical stimulation. In general, these results support the hypothesis that the degree of a muscle's activation in one direction is determined as a function of its mechanical action.