Final answer:
The biceps brachii concentrically contracts on the downswing when hammering a nail, which is an isotonic concentric contraction resulting in the shortening of the muscle and movement of the forearm toward the body.
Step-by-step explanation:
When hammering a nail, the biceps brachii concentrically contracts on the downswing. During a concentric contraction, the muscle shortens, causing the elbow joint angle to decrease and the forearm to move toward the body. This type of contraction is isotonic, meaning there is movement and the length of the muscle changes while the tension remains relatively constant.
As the biceps brachii contract, the large mass in the center of the muscle, called the belly, shortens and pulls on the tendons connected to the forearm bones, thus producing the motion required for hammering a nail. If the triceps brachii were to contract after this action, it would extend the elbow joint and move the arm in the opposite direction, preparing it for another downswing.