Final answer:
The biceps contract as the forearm moves up, and the triceps contract as the forearm moves down. During the upward movement, the biceps engage in an isotonic contraction while the triceps relax. When lowering the forearm, the triceps contract to extend the elbow.
Step-by-step explanation:
When considering the actions of the biceps and triceps as the forearm moves, it is important to understand their roles. The biceps act to flex the elbow, bringing the forearm upwards, while the triceps act to extend the elbow, straightening the arm and moving the forearm downwards. Therefore, the correct response to the question of what the biceps and triceps are doing as the forearm moves up and down is:
a) Biceps contract as the forearm moves up, triceps contract as it moves down.
b) Biceps relax as the forearm moves down, triceps contract as it moves up.
This means that the biceps muscle is the primary mover when lifting the forearm, for example when performing a bicep curl, which is an example of an isotonic contraction because the muscle changes length during the contraction, resulting in movement. Conversely, when the forearm is lowered back down, it is the triceps muscle that contracts, working against gravity to control and extend the elbow joint. Hence, part a) of the student's question reflects the isotonic contraction of the biceps when the forearm moves up. As for part c), if the triceps were to contract, it would help to straighten the arm by moving the forearm downward.