Recruitment and Modulation. The plot (top) emphasizes the relatively higher recruitment thresholRecruitment and Modulation. The plot (top) emphasizes the relatively higher recruitment threshold of larger motor units (e.g. fast versus slow-twitch) and their firing rate modulation until the common drive (U) saturates the firing rate of all motor units. The schematic (bottom) provides a mechanistic explanation for this phenomenon, namely the size principle. The same synaptic current produces a smaller excitatory post synaptic potential (EPSP) in the larger motor neuron because its input impedance is lower. Therefore, larger input currents are necessary for overcoming the threshold for action potential generation and motor unit recruitment. Bottom figure source: Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 4th ed. New York (NY): McGraw-Hill; c2000. Figures 34-11; p.687. d of larger motor units (e.g. fast versus slow-twitch) and their firing rate modulation until the common drive (U) saturates the firing rate of all motor units. The schematic (bottom) provides a mechanistic explanation for this phenomenon, namely the size principle. The same synaptic current produces a smaller excitatory post synaptic potential (EPSP) in the larger motor neuron because its input impedance is lower. Therefore, larger input currents are necessary for overcoming the threshold for action potential generation and motor unit recruitment. Bottom figure source: Kandel ER, Schwartz JH, Jessel TM. Principles of Neural Science. 4th ed. New York (NY): McGraw-Hill; c2000. Figures 34-11; p.687.