Question

muscles increase the power with which they contract by increasing the ______ with which their motor neurons are firing.

frequency

force

Explanation:

In muscle physiology, the force (power) of muscle contraction can be increased by increasing the frequency of motor neuron firing. Motor neurons send action potentials to muscle fibers, and a higher firing frequency leads to summation of muscle contractions, increasing the overall force. "Force" is what the muscle produces, not what the motor neurons' firing modulates in this context. So the correct term is frequency.

Answer:

frequency (the option with "frequency" should be selected)