The Basics 04: Joint Torques

The human angular motion machine uses joint torques to create joint rotations. A joint torque is create by a muscle pulling on body segments on each side of a joint. For example, when the brachialis muscle contracts, it pulls equally on the distal, anterior surface of the humerus (it's origin) and on the coronoid process of the ulna (it's insertion).  If you connect a line between a muscle's origin and it's insertion, you will have drawn the muscle force's line of pull.  Since, the brachialis muscle's line of pull is located anterior to (i.e., in front of ) the elbow joint, a contraction of this muscle will create a torque that results in elbow flexion. The magnitude (i.e., size) of a joint torque is directly proportional to the magnitude of the muscle force (i.e., if muscle force increases, the joint torque increases) and to the perpendicular distance (i.e., the moment arm) from the joint to the line of pull of the muscle (i.e., if the moment arm increases, the joint torque increases).  For the vertical jumping example, three joint torques must be created:

• a hip extension torque 
• a knee extension torque
• a ankle plantar flexion torque