The order in which motor units are recruited is highly correlated with several indices of motor unit size, including:• the size of the motor neuron cell bodies• the diameter and conduction velocity of the axons• the amount of force that the muscle fibers can exert.• Because the recruitment threshold of a motor unit depends on the membrane resistance of the motor neuron, which is inversely related to its surface area, a given synaptic current will produce larger changes in the membrane potential of small-diameter motor neurons