Final answer:
The statement is true; not all motor units in a skeletal muscle contract at once due to the body's efficient recruitment process, allowing various degrees of strength in muscle contractions and preventing fatigue.
Step-by-step explanation:
The statement that not all motor units of one entire skeletal muscle contract at once is true. Within skeletal muscles, there are varying motor units that the nervous system controls for muscle contractions of different strengths. Initially, smaller motor units with lower-threshold motor neurons fire to their corresponding muscle fibers, generating a small contractile force. As the need for force increases, larger motor units with higher-threshold neurons are recruited to activate larger muscle fibers, in a process known as recruitment. This leads to a graded increase in muscle contraction strength. However, not all motor units are active simultaneously, as some may rest while others work to prevent fatigue, which facilitates longer muscle contractions.
During maximum force contractions, all motor units may be activated, but such activation is short-lived due to the intense energy requirements. The nervous system's recruitment mechanism efficiently manages the utilization of skeletal muscle. Additionally, the amount of force a muscle can generate is influenced by the length-tension relationship within a sarcomere, as this dictates the number of cross-bridges that can form between actin and myosin filaments during muscle contraction.