An isotonic exercise is one in which the tension applied to muscle remains unchanged while the length of opposing muscles changes. Lifting weights is a good example of an isotonic exercise. Contractions that occur during isotonic exercises are classified as either concentric or eccentric. In a concentric contraction the muscular force overcomes the resistance whereas in an eccentric contraction the resistance is too great to be overcome by muscular force.

This leads to a lengthening of the muscle being subjected to the load. An example of an eccentric contraction would be trying to do bicep curls with a weight that is too heavy to get off the ground. Almost all bodybuilding exercises done with weights execute isotonic movements, primarily of the concentric kind.