...While letting gravity keep your forearm fully pronated, slowly and repeatedly flex your elbow. Palpation of your upper arm during this movement should quickly verify that your biceps muscle is not active. If it were, your forearm would supinate. The most active muscle is your deeper brachialis—a muscle that cannot pronate or supinate. Next, while continuing to palpate your upper arm as you flex and extend your elbow, quickly and forcefully supinate your forearm. The immediate increase in tension in your biceps while supinating reflects the strong activation of this muscle. The nervous system recruits the biceps muscle because its combined actions exactly match the task at hand. The brachialis likely remains relentlessly active during both scenarios.
...As stated, the biceps crosses the shoulder, elbow, and forearm joints and therefore is often referred to as being multi-articular. Many movements of the upper extremity can influence the length at which the biceps is activated. Consider the natural motion of pulling, which combines elbow flexion with shoulder extension. Such a motion occurs when one attempts to start a lawnmower with a pull cord. By crossing the shoulder and elbow, the biceps, in effect, contracts (and shortens) across the elbow as it simultaneously lengthens across the shoulder. By contracting at one end and lengthening at the other, the muscle actually shortens a small net distance. This offers a physiologic advantage based on the muscle’s length-tension relationship.
A muscle is considered more actively efficient when a given effort level produces a greater amount of force. This occurs when (1) a muscle contracts, and the muscle fibers shorten a relatively small amount per instant in time; and (2) a muscle remains at a nearly optimal length (to create contractile force) throughout an active movement.