Is the action of a particular muscle always the same?

No, the action of a particular muscle is not always the same; it can vary depending on factors such as the position of the body, the presence of other muscles, and the specific movements being performed. For example, the biceps brachii primarily flexes the elbow, but it can also assist in shoulder flexion and supination of the forearm. Additionally, muscles can work synergistically or antagonistically with others, altering their function in different contexts.