Biomechanics Conceps: Moment Arm · CSCS Accelerator Community

Biomechanics Conceps: Moment Arm

Moment Arm

In the context of strength and conditioning, a moment arm explains why an exercise feels harder or easier at certain joint angles, because it determines how much torque the muscle must produce to move or control a load. It is tightly related to concepts of lever classes and, in part, helps to determine the sticking point of an exercise.

Definition: The moment arm is the perpendicular distance between a force’s line of action and the axis of rotation at a joint.

In resistance training:

It determines how much torque (rotational force) is produced at a joint.
A longer moment arm = more torque required from the muscle.
A shorter moment arm = less torque required.

Example: During a biceps curl, the moment arm of the external load is greatest when the forearm is around 90° of elbow flexion, which is why the lift feels hardest there.

Lock this concept in with your own training. When an exercise feels harder at certain joint angles or through a specific ROM, it’s likely because the moment arm is larger and the muscle has to produce more force.

Question: Have you ever tried or prescribed targeted isometric or partial ROM work at the sticking point of a lift? For example, pause squats, pause at the knee deadlifts or cleans, spoto press, etc.?

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