Final answer:
Skeletal muscles include multiple muscles which can act together or individually for different movements, exemplified by the deltoid and quadriceps. They contract through organized motor unit recruitment that allows continuous activity without fatigue. The arrangement and interaction of muscle fibers determine the range of motion and power of the muscles.
Step-by-step explanation:
The term skeletal muscles refers to muscles attached to the skeleton that may be called into play during various actions. For instance, the deltoid muscle is a multipennate muscle, meaning it can contract as a whole to achieve arm abduction or can contract in parts, using specific fascicles, to change the direction of pull and create movements like arm abduction and flexion. The muscle fiber arrangement within these muscles, such as being parallel, pennate, or fusiform, contributes to their function and range of motion. Muscles like the quadriceps for knee extension and hamstrings for knee flexion demonstrate how multiple muscles can collaborate or oppose each other to achieve complex movements.
Agonists, or prime movers, are primarily responsible for a specific movement, whereas synergists assist, and antagonists provide opposition to prevent overextension and control the movement. Controlled muscle contraction is maintained through the recruitment of motor units, which prevents complete fatigue by allowing some muscle fibers to rest while others are active. This is crucial for the longevity of muscle contractions and overall efficiency. Ultimately, anatomical features such as the length-tension relationship in the sarcomere determine the optimal power output during muscle contraction.