Final answer:
The statement is true; muscle contraction strength depends on the size and number of motor units activated, with larger units providing greater force and smaller units controlling finer movements. This recruitment process allows muscles to respond to various demands without immediate fatigue.
Step-by-step explanation:
True, the strength of a muscle contraction is determined by the size and number of motor units being stimulated. This process is called recruitment.
Motor units are comprised of a motor neuron and the muscle fibers it innervates. Small motor units with more excitable, lower-threshold motor neurons are activated first, leading to a weaker muscle contraction. As greater strength is required, larger motor units with bigger, higher-threshold motor neurons are recruited, leading to a stronger contraction. The addition of more motor units results in increased muscle tension and contraction strength. Progressive recruitment allows for fine-tuned muscle control, such as picking up a feather versus lifting a heavy weight using the same muscle. Muscle responsiveness thus relies on both the motor unit size and number involved in the contraction.
Moreover, the muscle fiber's ability to generate tension is also influenced by the length-tension relationship, as the optimal overlap between actin and myosin filaments within a sarcomere determines the maximal force production. However, a balance is maintained to avoid fatigue, where not all units are active simultaneously, allowing muscles to contract longer.