A method for determining the moment arm ratio of biarticular muscles in vivo using electrical stimulation

The ratio of the biceps brachii moment arm at the shoulder to that at the elbow was determined for a range of arm positions in the horizontal plane (arm abducted 90 degrees). The biceps muscle was electrically stimulated and the resulting force at the hand measured. The joint torques about the shoulder and elbow were calculated and expressed as a ratio, representing the shoulder to elbow moment arm ratio. The moment arm ratio ranged from 0.21 to 1.71 with a mean value of 0.75 across subjects. The relation between joint angles and the moment arm ratio was modeled for individual subjects using a third order polynomial. The R² value for the model ranged from 0.70 to 0.90. The relation had a characteristic saddle shape which shifted in the direction normal to the joint angle plane for different subjects.