Isotonic contraction involves rhythmic movement of large muscle groups and results in gains in muscle size, strength, and endurance. Isometric contraction involves no change in muscle length and is often used to overcome specific weaknesses. Isotonic contraction imposes a greater volume load on the cardiac muscle, resulting in the cardiac training effect. Isotonic contraction can be subdivided into eccentric and concentric contraction, both effective for building muscle strength, but eccentric exercise is associated with higher levels of connective tissue and reduced risk of stress injuries.
Isotonic contraction is a form of muscular effort that is characterized primarily by a change in both muscle length and joint angle. Also known as dynamic contraction, isotonic exercises generally involve the repetitive, rhythmic movement of large muscle groups. This is the type of muscular effort most often used during strength training and cardiovascular exercise, resulting in net gains in muscle size, strength, and endurance.
Other forms of muscle 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 muscle force is exerted against an immovable object. Athletes sometimes use isometric training 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 resistance increases as 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 working muscles consume oxygen, heart rate increases and blood is diverted to areas of demand. Along with an increase in heart rate, the heart’s stroke volume, the amount of blood pumped with each heartbeat, also rises. As isotonic contraction continues, there is a progressive increase in systolic blood pressure combined with a stable or slightly decreased diastolic blood pressure.
In this way, isotonic contraction imposes a greater volume load on the cardiac muscle. The heart adapts to the increased load by building strength and stamina. 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, generally results in an increase in systolic and diastolic blood pressure, accompanied by a moderate increase in cardiac output without a significant increase in blood flow to working muscles.
Isotonic contraction can be subdivided into eccentric and concentric contraction. Concentric contraction occurs when the muscle force is greater than the resistance force, and the muscle shortens. Shortening of the muscle results in a net decrease in the angle of the working joint. In resistance training, this is generally the movement phase that moves against gravity; for example, the portion of a bicep bends when the elbow is flexed and the bar is moved up.
Eccentric contraction occurs when the resistance force exceeds the force exerted by the muscle. In this case, there is usually a general lengthening of the muscle and an increase in the angle of the joint. The weight load limit of a muscle is up to 40% greater during eccentric contraction than concentric contraction. Both forms of isotonic contraction are effective for building muscle strength, but there are other adaptations that are particular to eccentric exercise.
Extreme athletes, such as bodybuilders and ultra-marathon runners, tend to perform more eccentric exercises than the general population. As a result, these athletes appear to have a higher than average amount of connective tissue around their muscles. It is believed to be an adaptation to protect the muscles from the high levels of force associated with this form of exercise. In contrast, exercise programs that reduce or eliminate the eccentric phase of contraction have been associated with stress injuries and limited gains in muscle strength.
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