Joint Torque

Joint torque is the rotational effect of forces acting about a joint. In biomechanics, the closely related term joint moment is often used interchangeably with joint torque. The magnitude of a torque depends on both the force applied and its moment arm, the perpendicular distance between the force’s line of action and the joint’s axis of rotation. Torque is commonly expressed in newton-metres (N·m) .¹

Muscles generate joint torque by pulling through tendons on bones. For a given muscle force, a larger moment arm generally produces greater torque about the joint. Because muscle moment arms change with joint geometry and often with joint angle, the torque-producing capacity of a muscle can also vary as the joint moves .^1,2

Torques about a joint can arise from internal and external forces. Internal joint torques can be produced by muscles and passive tissues acting across the joint. External torques can result from gravity, ground-reaction forces, externally held loads, or other forces acting on the body. Joint motion depends on the net rotational effect of these forces together with the inertial properties of the body segments involved .¹

A net joint moment calculated using inverse dynamics should not be interpreted as the torque produced by a single muscle. It represents the net effect of internal moments acting about the joint, including contributions from multiple muscles and passive tissues, while opposing muscles may be active simultaneously. Additional modeling or measurements are required to estimate how the net moment is distributed among individual muscles .³