Final answer:
The number of myofibers receiving an action potential from the controlling neuron determines force production in a muscle. Increasing the frequency of action potentials and the length of sarcomeres also affect force production.
Step-by-step explanation:
The primary variable determining force production in a muscle is the number of myofibers receiving an action potential from the controlling neuron. When a muscle is used to perform a light task, only a few neurons in the muscle are signaled, and only a few myofibers respond. However, when the muscle is used to perform a heavy task, all neurons in the muscle are signaled, and every myofiber participates, resulting in close to the maximum force the muscle can produce.
Increasing the frequency of action potentials (the number of signals per second) can also increase force production to some extent. Additionally, the length-tension relationship influences force production. The length of the sarcomeres, the functional units of a muscle fiber, directly affects the force generated when the sarcomeres shorten. When the zone of overlap between thin and thick filaments in the sarcomeres is greatest, the muscle produces optimal force.