Final answer:
Muscles can only contract; they do not actively extend. The unilateral action of a muscle is always contraction, such as the biceps brachii flexing the forearm and the triceps brachii extending it. The action of a muscle is influenced by the location of its insertion and origin.
Step-by-step explanation:
When discussing the unilateral action of a highlighted muscle, it's important to note that muscles can only contract. They do not actively extend but rather relax back into their non-contracted position. Therefore, the unilateral action of a muscle is contraction.
Regarding the biceps and triceps in the arm, the biceps brachii is responsible for flexing the forearm (decreasing the angle between bones), which is an example of a contraction. In contrast, the triceps brachii is responsible for extending the forearm (increasing the angle between bones) when it contracts. This extension is due to the triceps brachii opposing the biceps muscle, not because the muscle itself is extending.
The location of a muscle's insertion and origin can determine its action. For example, since the biceps are attached to the bone via tendons close to the elbow joint, when they contract, the forearm is flexed. Conversely, the triceps brachii is the muscle that extends the forearm when it contracts.