Whole muscles can respond in a graded fashion to stimuli by varying Select one: a. the force of contraction of individual muscle fibers. b. the number of motor units recruited. c. the amplitude of the action potential. d. the frequency of stimulus. e. thresholds.

b. the number of motor units recruited

Explanation:

Recruitment of motor units is defined as the activation of several additional motor units which results into high contractile force in a muscle. A motor unit consists of the stimulating muscle fibres and one motor neuron.

Activation of motor neuron, stimulates and contracts all the muscle fibres inherited by the motor neuron. When only one motor neuron is activated, it results into a weak but distributed contraction of the muscle. Activating more motor neurons will result in activation of more muscle fibres, and thus a stronger muscle contraction.

Hence, the correct option is b.