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A force exerted by an active or passively stretched muscle. Its contribution to rotation about an anatomical joint depends on the muscle moment arm, rather than its magnitude alone.

A lever is a body free to rotate about a fulcrum. Its usefulness comes from a balance of torques, not from producing energy. For a stationary ideal lever, let $d_E$ and $d_L$ be the perpendicular lever arms of the effort $E$ and load $L$. ThenThe mechanical advantage uses perpendicular distances to the lines of action, which need not equal distances measured along a bent or obliquely loaded arm. In an ideal slow motion, the effort moves through the larger distance when its force is the smaller: work is conserved.

For the three familiar straight arrangements, the fulcrum lies between effort and load in a first-class lever, the load lies between fulcrum and effort in a second-class lever, and the effort lies between fulcrum and load in a third-class lever. The second class has ideal mechanical advantage greater than one; the third has it less than one. The first can have either, depending on the lever arms.

The forearm bends by elbow flexion. The biceps brachii and brachialis pull through short tendons close to the elbow, while a load in the hand acts farther away. This is predominantly a third-class arrangement: a large muscle force buys a comparatively large and rapid hand displacement. The biceps brachii also contributes to forearm supination. Elbow extension is produced chiefly by the triceps brachii, whose tendon acts on the olecranon behind the elbow. The extensor insertion and forearm load lie on opposite sides of the effective pivot, giving the corresponding first-class analogy. Actual anatomical joint motion has changing muscle moment arms, and the joint reaction force also enters the full force balance; the body is not a set of frictionless pins. ^[ref]