Isotonic contraction is a form of muscular exertion principally characterized by a change in both muscle length and joint angle. Also known as dynamic contraction, isotonic exercises typically involve the rhythmic, repetitive motion of large muscle groups. This is the type of muscular exertion that is most often used during strength training and cardiovascular exercise, resulting in net gains in muscular size, strength, and endurance.
Other forms of muscular contraction include isometric contraction and auxotonic contraction. Isometric contraction, in which there is no change in muscle length and no visible movement of the joints, occurs when muscular force is exerted against an immovable object. Isometric training is sometimes used by athletes to overcome specific weaknesses in the dynamic range of motion of a particular muscle group, or to prevent muscle atrophy when a limb is immobilized. Auxotonic contraction, in which the resistance increases as the force is applied, is most commonly seen in cardiac muscle.
During isotonic contraction there is a distinct physiological response that is not seen during isometric contraction. As the working muscles consume oxygen, the heart rate increases and blood is shunted toward areas of demand. Along with a boost in heart rate, the heart's stroke volume — the amount of blood pumped with each heartbeat — also becomes elevated. As the isotonic contraction continues, there is a progressive rise in systolic blood pressure combined with a stable, or slightly decreased, diastolic blood pressure.
In this way, isotonic contraction imposes an increased volume load on the heart muscle. The heart adapts to the increased load by building up strength and endurance. This adaptation is known as the cardiac training effect, and occurs most often in response to the demands of dynamic exercise. Isometric exercise, by contrast, typically results in a rise in both systolic and diastolic blood pressure, accompanied by a moderate increase in cardiac output without significant increase in blood flow to working muscles.
Isotonic contraction can be further subdivided into eccentric and concentric contraction. Concentric contraction occurs when muscular force is greater than the force of resistance, and the muscle shortens. The shortening of the muscle results in a net decrease in the angle of the working joint. In resistance training, this is generally the phase of motion that moves against gravity — for example, the portion of a bicep curl when the elbow is flexed and the barbell is moved upward.
Eccentric contraction occurs when the force of resistance exceeds the force exerted by the muscle. In this case, there is typically an overall lengthening of the muscle, and an increase in the angle of the joint. A muscle's weight bearing limit is up to 40% greater during eccentric contraction than concentric contraction. Both forms of isotonic contraction are effective for building muscular strength, but there are other adaptations that are particular to eccentric exercise.
Extreme athletes, such as bodybuilders and ultra-marathoners, tend to engage in more eccentric exercises than the general population. As a result, these athletes appear to have a greater than average amount of connective tissue around the muscles. This is thought to be an adaptation to protect the muscles from the high levels of force associated with this form of exercise. Conversely, exercise programs that reduce or eliminate the eccentric phase of contraction have been associated with stress injuries and limited gains in muscular strength.