Final answer:
The contraction of the biceps brachii that decreases the angle between the forearm and humerus is a concentric contraction. This shortening of the muscle moves a load, such as lifting a hand weight. The triceps would extend the forearm in the opposite action.
Step-by-step explanation:
When the biceps brachii muscle contracts and the angle between the forearm and the humerus decreases, this is known as a concentric contraction. Concentric contractions involve the muscle shortening to move a load, and in the case of the biceps brachii, it occurs when a hand weight is brought upward with increasing muscle tension. As the muscle fibers shorten and cross-bridges form, the myosin heads pull the actin, leading to a decrease in the angle of the elbow joint as the forearm is brought closer to the body.
If the triceps were to contract after this action, the forearm would extend, increasing the angle between the forearm and the humerus as the triceps muscle lengthens while producing force.