Relationship between muscle length, muscle activity, and torque of the hamstring muscles

The relationship between muscle length, integrated electromyographic activity, and torque of the biceps femoris muscle was investigated while the line of action of the muscle at the knee was held constant. Muscle length was changed by varying the hip joint angle. Sixteen subjects produced 1) maximal isometric contractions, 2) contractions with constant submaximal torque, and 3) contractions with constant submaximal muscle activity at four different hip positions (0, 45, 90, and 135 degrees of flexion). Simultaneous readings of hip angle, muscle torque, and raw and integrated electromyographic activity revealed that changes in muscle length influence production of integrated muscle activity and development of torque differently. During maximal isometric contraction, an increase in integrated electromyographic activity and a decrease in torque occurred as the muscle was shortened; the opposite occurred when the muscle was at lengthened positions. A greater difference in this relationship was noted when the respective electromyographic activity and torque were held constant. Some clinical questions were raised.