Final answer:
The arrows in the question are pointing to the biceps and triceps muscles, which function as opposing muscles to control the movement of the forearm at the elbow.
Step-by-step explanation:
Muscles are arranged in pairs known as antagonistic pairs, where one muscle contracts and the other relaxes to facilitate movement. One common example of such a pairing is the biceps and triceps muscles in the human arm. The biceps muscle functions as a flexor, meaning it contracts to bend the arm at the elbow. In contrast, the triceps muscle operates as an extensor, contracting to extend or straighten the arm. This dynamic enables coordinated movement and is essential in performing everyday activities that require the bending and stretching of the elbow.
Given that muscles can only contract and not extend, opposing muscle pairs are crucial in human anatomy. They allow limbs to move in opposite directions by a muscle contracting to move a limb one way and another muscle contracting to move it back. The biceps and triceps exemplify this, working in opposition to control the movement of the forearm. When the forearm needs to be extended, the triceps contracts while the biceps relaxes, and conversely, the biceps contracts while the triceps relaxes to flex the forearm.