Final answer:
During the lowering phase of a biceps curl, the elbow flexors perform an eccentric contraction where the muscle elongates while still under tension. This action controls the descent of the weight. If the triceps were to contract afterward, it would extend the arm, indicating a concentric contraction for the triceps.
Step-by-step explanation:
During the lowering phase of a biceps curl, the elbow flexors are performing an eccentric contraction. This type of muscle action occurs when a muscle lengthens while generating force. It's different from a concentric contraction, where the muscle shortens, as it would during the lifting phase of a biceps curl.
In an eccentric contraction, the muscle elongates under tension, essentially acting like a brake to help control the descent of the weight and preventing it from dropping quickly. This is important for safely managing motion and is utilised in everyday activities like walking down stairs or lowering objects. During this phase, as the biceps are lengthening, the angle of the elbow joint increases as the forearm moves away from the body.
If the triceps were to contract after this phase, it would further help in extending the arm, as the triceps are primarily responsible for straightening the elbow joint. This action is an example of a concentric contraction for the triceps.