<P> Force--velocity relationship relates the speed at which a muscle changes its length (usually regulated by external forces, such as load or other muscles) to the amount of force that it generates . Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity . The reverse holds true for when the muscle is stretched--force increases above isometric maximum, until finally reaching an absolute maximum . This has strong implications for the rate at which muscles can perform mechanical work (power). Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). Instead, the optimal shortening velocity for power generation is approximately one - third of maximum shortening velocity . </P> <P> These two fundamental properties of muscle have numerous biomechanical consequences, including limiting running speed, strength, and jumping distance and height . </P> <P> Smooth muscles can be divided into two subgroups: single - unit (unitary) and multi-unit . Single - unit smooth muscle cells can be found in the gut and blood vessels . Because these cells are linked together by gap junctions, they are able to contract as a syncytium . Single - unit smooth muscle cells contract myogenically, which can be modulated by the autonomic nervous system . </P> <P> Unlike single - unit smooth muscle cells, multi-unit smooth muscle cells are found in the muscle of the eye and in the base of hair follicles . Multi-unit smooth muscle cells contract by being separately stimulated by nerves of the autonomic nervous system . As such, they allow for fine control and gradual responses, much like motor unit recruitment in skeletal muscle . </P>

Spontaneous contraction of random groups of muscles is called