beyond a given length, there is an entirely passive tension, which opposes lengthening. Combined, there is a strong resistance to lengthening an active muscle far beyond the peak of active tension. Force-velocity relationships 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 intrinsic property of active muscle tissue plays a role in the active damping of joints that are actuated by simultaneously-active opposing muscles. In such cases, the force-velocity profile enhances the force produced by the lengthening muscle at the expense of the shortening muscle. This favoring of whichever muscle returns the joint to equilibrium effectively increases the damping of the joint. Moreover, the strength of the damping increases with muscle force. The motor system can thus actively control joint damping via the simultaneous contraction (co-contraction) of opposing muscle groups. Smooth muscle Smooth muscles can be divided into two subgroups: single-unit and multiunit. 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 functional syncytium. Single-unit smooth muscle cells contract myogenically, which can be modulated by the autonomic nervous system. Unlike single-unit smooth muscle cells, multiunit smooth muscle cells are found in the muscle of the eye and in the base of hair follicles. Multiunit 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. Mechanisms of smooth muscle contraction The contractile activity of smooth muscle cells can be tonic (sustained) or phasic (transient) and is influenced by multiple inputs such as spontaneous electrical activity, neural and hormonal inputs, local changes in chemical composition, and stretch. This is in contrast to the contractile activity of skeletal muscle cells, which relies on a single neural input. Some types of smooth muscle cells are able to generate their own action potentials spontaneously, which usually occur following a pacemaker potential or a slow