ABSTRACT
A single stimulus to a muscle fiber will result in a brief contraction followed by an immediate relaxation and is called a muscle twitch. Additionally, a series of low-frequency stimuli will result in a series of twitches. The recruitment of additional motor units acts to increase the force that a muscle can exert. The order of motor unit recruitment is influenced by size; that is, the unit with the smallest motor neuron is recruited first and that with the largest last. Recruitment is also influenced by membrane sensitivity and the number of synapses on a specific motor neuron. The force generated by muscle is also dependent upon the velocity of fiber shortening or lengthening. This is known as the force-velocity relationship. Muscle contractions occurring as the muscle elongates are termed eccentric contractions. The force generated by muscle during this type of contraction is greater than Po and varies with velocity.
