According to Springfield Technical Community College, isotonic contraction occurs when a muscle length changes and causes movement of a body part. As Wikipedia explains, the name isotonic implies that the tension in the muscle during the movement remains unchanged.
There are two types of isotonic contractions, according to Wikipedia: concentric and eccentric. During a concentric contraction, the external force applied is lesser than the force generated by the muscle, resulting in shortening. During an eccentric contraction, the external force is greater than the muscle force, causing the muscle to lengthen. Isotonic contractions are present in all lifting exercises and almost all physical activities, including eccentric tension-stimulated muscle strengthening.