Torque Is Why Your Triceps Pull 17 Times What You Hold

Opener

This is a physics lecture. The muscle is the triceps and the exercise is a lying triceps extension, but the subject is torque: what it is, how to calculate it, and why a modest bar asks a muscle for hundreds of pounds of force. Once you can do this calculation for one joint, you can do it for any joint, and a lot of training advice starts to make more sense.

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1. Torque is force times lever arm

A force applied to something that can rotate around a pivot produces torque, the turning effect of that force. For a force acting at right angles to the lever, the formula is

torque = force × lever arm

The lever arm is the perpendicular distance from the pivot to the force's line of action. If a force points straight through the pivot, its lever arm is zero, and so is its torque, however large the force is. If it acts at right angles to the lever, it produces as much torque as it can.

For a limb held still, or moving at a steady speed, the torques around the joint have to cancel. The torque that tries to bend the joint has to equal the torque that tries to straighten it. This is the rotational version of Newton's first law, and it is the only rule we need below.