Final answer:
Muscle fibers contract when sarcomeres shorten, with motor neurons stimulating individual fibers forming a motor unit. The recruitment of more fibers results in a stronger contraction, while varying active units prevents fatigue. Physical conditioning can lead to muscle hypertrophy or atrophy.
Step-by-step explanation:
Each muscle fiber can contract and this capability is crucial for producing movement in the body. These fibers contract when sarcomeres, which are the smallest contractile units within the myofibrils of muscle cells, shorten. This process happens due to the interaction of two key proteins, actin and myosin, that slide past each other. Muscle fibers are activated by motor neurons, and each fiber is controlled by only one neuron in a setup known as a motor unit. The density and recruitment of these motor units determine the force a muscle can generate.
When more muscle fibers are recruited in unison, the muscular force is greater. However, to prevent muscle fatigue, not all motor units are active at once. Instead, a process known as recruitment occurs allowing some fibers to rest while others work. This ability to vary the number of active motor units is crucial for allowing muscles to sustain contractions over time and avoid fatigue. Moreover, the physical condition of a muscle, including its strength and endurance, is influenced by factors like hormones, stress, and the level of physical activity, which can lead to changes such as muscle hypertrophy or atrophy.