<P> Unlike skeletal muscle, the contractions of smooth and cardiac muscles are myogenic (meaning that they are initiated by the smooth or heart muscle cells themselves instead of being stimulated by an outside event such as nerve stimulation), although they can be modulated by stimuli from the autonomic nervous system . The mechanisms of contraction in these muscle tissues are similar to those in skeletal muscle tissues . </P> <P> Muscle contractions can be described based on two variables: force and length . Force itself can be differentiated as either tension or load . Muscle tension is the force exerted by the muscle on an object whereas a load is the force exerted by an object on the muscle . When muscle tension changes without any corresponding changes in muscle length, the muscle contraction is described as isometric . If the muscle length changes while muscle tension remains the same, then the muscle contraction is isotonic . In an isotonic contraction, the muscle length can either shorten to produce a concentric contraction or lengthen to produce an eccentric contraction . Furthermore, if the muscle length shortens, the contraction is concentric . But if the muscle length lengthens, then the contraction is eccentric . In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time - varying manner . Therefore, neither length nor tension is likely to remain constant when the muscle is active during locomotor activity . </P> <P> An isometric contraction of a muscle generates tension without changing length . An example can be found when the muscles of the hand and forearm grip an object; the joints of the hand do not move, but muscles generate sufficient force to prevent the object from being dropped . </P> <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>

What form of exercise does the muscle contract and does not lengthen nor shorten