A moment arm is the perpendicular distance from an axis of rotation to the line of action of a force. Multiplying a force by its moment arm gives the torque that force produces about that axis. The longer the moment arm, the greater the rotational effect of the same force.
Moment arms are a property of geometry, not of the force itself. As body position changes, the moment arms of both muscles and external loads change, which is one reason the difficulty of a movement varies through its range.
Internal and External Moment Arms
- Internal moment arm: The distance from the joint axis to a muscle or tendon's line of pull. It depends on anatomy and joint angle.
- External moment arm: The distance from the joint axis to the line of action of an external force, such as gravity acting on a barbell or the ground reaction force.
A muscle usually has a much shorter moment arm than the external load it opposes, so it must produce a force many times larger than the load. This arrangement trades force for speed and range of motion at the distal end of a segment.
Observing It in Video
External moment arms are often the easiest part of joint mechanics to visualize from video. For a load whose weight acts vertically, such as a barbell, a vertical line drawn through the load and a horizontal distance measured to the estimated hip or knee center shows the moment arm in a side view. Comparing frames can show how leaning forward, sitting back, or changing bar position shifts that distance.
Limits and Pitfalls
- The moment arm must be measured perpendicular to the line of action, not along the limb
- The line of action of a ground reaction force is not always vertical and cannot be seen without force data
- Joint centers estimated from surface landmarks introduce error
- Measurements are valid only in the plane being viewed and are affected by perspective error
- Internal muscle moment arms cannot be observed from external video
Moment arms provide a practical way to discuss why technique changes can redistribute demand between joints, without requiring force measurements.