Motor unit recruitment is the activation of additional motor units to accomplish an increase in contractile strength in a muscle. A motor unit consists of one motor neuron and all of the muscle fibers it stimulates. All muscles consist of a number of motor units and the fibers belonging to a motor unit are dispersed and intermingle amongst fibers of other units. The muscle fibers belonging to one motor unit can be spread throughout part, or most of the entire muscle, depending on the number of fibers and size of the muscle. When a motor neuron is activated, all of the muscle fibers innervated by the motor neuron are stimulated and contract. The activation of one motor neuron will result in a weak but distributed muscle contraction. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction. Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. The higher the recruitment the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons, and thus slow to fast twitch) as contraction increases. This is known as Henneman's size principle. Neuronal mechanism of recruitment Henneman proposed that the mechanism underlying the size principle was that the smaller motor neurons had a smaller surface area and therefore a higher membrane resistance. He predicted that the current generated by an excitatory postsynaptic potential (EPSPs) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and therefore larger EPSPs in smaller motoneurons. Burke later demonstrated that there was a graded decrease of both EPSP and inhibitory postsynaptic potential (IPSP) amplitudes from small to large motoneurons. This seemed to confirm Henneman's idea, but Burke disagreed, pointing out that larger neurons with a larger surface area had space for more synapses. Burke eventually showed (in a very small sample of neurons) that smaller motoneurons have a greater number of synaptic inputs from a single input source. The topic is probably still regarded as controversial. In their 1982 paper, Burke and colleagues propose that the small cell size and high surface-to-volume ratio of S motor units allows for greater metabolic activity, optimized for the "highest