Define the moment arm.

Define the moment arm.

• A. Magnitudes of the force
• B. Distance from muscle insertion to origin
• C. Perpendicular distance from the force's line of action to the axis of rotation
• D. Angle between force and lever

Answer: (C)

The moment arm is the perpendicular distance from the joint’s axis of rotation to the line along which the force is applied. This perpendicular distance is what converts a force into rotational effect about the joint. In other words, torque about the joint equals the force times this moment arm. Think of bending the elbow with the biceps. The elbow joint is the axis, and the biceps pull along a certain line. The shorter that perpendicular distance is, the less torque the same muscle force produces; a longer perpendicular distance increases the torque for that same force. If the force acts directly through the joint axis, the moment arm is zero and it produces no rotation. This isn’t about how hard the muscle pulls (the force’s magnitude), nor about where the muscle attaches on the bone (the insertion-to-origin distance). It’s specifically the shortest, perpendicular distance from the joint’s center to the force’s line of action. The moment arm can change with joint angle as the geometry changes, which is why torque can vary even if the muscle force stays the same.

The moment arm is the perpendicular distance from the joint’s axis of rotation to the line along which the force is applied. This perpendicular distance is what converts a force into rotational effect about the joint. In other words, torque about the joint equals the force times this moment arm.

Think of bending the elbow with the biceps. The elbow joint is the axis, and the biceps pull along a certain line. The shorter that perpendicular distance is, the less torque the same muscle force produces; a longer perpendicular distance increases the torque for that same force. If the force acts directly through the joint axis, the moment arm is zero and it produces no rotation.

This isn’t about how hard the muscle pulls (the force’s magnitude), nor about where the muscle attaches on the bone (the insertion-to-origin distance). It’s specifically the shortest, perpendicular distance from the joint’s center to the force’s line of action. The moment arm can change with joint angle as the geometry changes, which is why torque can vary even if the muscle force stays the same.