<P> In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length . This occurs when a muscle's force of contraction matches the total load on the muscle . </P> <P> In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts . This occurs when the force generated by the muscle exceeds the load opposing its contraction . </P> <P> During a concentric contraction, a muscle is stimulated to contract according to the sliding filament theory . This occurs throughout the length of the muscle, generating a force at the origin and insertion, causing the muscle to shorten and changing the angle of the joint . In relation to the elbow, a concentric contraction of the biceps would cause the arm to bend at the elbow as the hand moved from the leg to the shoulder (a biceps curl). A concentric contraction of the triceps would change the angle of the joint in the opposite direction, straightening the arm and moving the hand towards the leg . </P> <P> In eccentric contraction, the tension generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract . Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load . This can occur involuntarily (e.g., when attempting to move a weight too heavy for the muscle to lift) or voluntarily (e.g., when the muscle is' smoothing out' a movement). Over the short - term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone . However, exercise - induced muscle damage is also greater during lengthening contractions . </P>

Contraction of a muscle requires all the motor units to be undergoing tetanic contraction