Final answer:
During a voluntary muscle contraction, the change in muscle force depends on the number of myofibers that receive an action potential from the neuron controlling the muscle. For light objects, only a few myofibers respond, but for heavy objects, all myofibers participate. Increasing the frequency of action potentials can also increase force production.
Step-by-step explanation:
The change in muscle force during a voluntary contraction involves the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber. When picking up a light object, the motor cortex of the brain only signals a few neurons of the muscle, and only a few myofibers respond. However, when picking up a heavy object, the motor cortex signals all of the neurons in the muscle, and every myofiber participates, leading to close to the maximum force the muscle can produce. Increasing the frequency of action potentials can further increase the force generated.