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.