Muscle activation results in muscle actions that can be
classified into three basic types: static, dynamic concentric,
and dynamic eccentric. A static (also known as isometric)
muscle action is characterized by the generation of force in
the absence of movement of the joint or limb. Under these
conditions, the resistance is higher than the force generated
(i.e., pushing against a wall). On the other hand, a dynamic
action involves force generation and joint movement.
Dynamic (previously known as isotonic) actions can be
divided into concentric or eccentric actions. Concentric
muscle actions result in the shortening of a muscle because
its origin and insertion come closer together (i.e., flexing
the elbow). Eccentric actions result in the lengthening of
the muscle because its origin and insertion move farther
apart (i.e., lowering a load from a bent elbow position). A
type of dynamic action produced in the laboratory using a
special device is known as isokinetic because the velocity
of the movement is constant. This type of muscle action is
artificial and does not occur in normal human movement
because during the performance of daily activities there is a
combination of acceleration and deceleration.