Moment Arm
Short definition
Perpendicular distance from the muscle line of action to the joint axis, determining the torque.
Full definition
The moment arm is the distance from the joint rotation axis to the muscle line of action. Torque = force × moment arm. Larger moment arms produce greater torque for the same muscle force, and vary with joint angle and model geometry.
Examples
Triceps moment arm at the elbow is largest near 90°.