Reference.com states that an isotonic exercise is any exercise in which the length of a person's muscles change. They are the opposite of isometric exercise, in which muscle length does not change.
Reference.com states that common examples of isotonic exercise include things such as lifting objects, running, and walking. There are two different kinds of isotonic contractions: concentric and eccentric. Concentric contractions occur when the force applied to a muscle is less than the amount of force the muscle is creating. Eccentric contractions occur when the force applied to a muscle is greater than what the muscle can produce. Eccentric contractions can be more useful than concentric contractions in muscle strengthening, but injury or other harm can sometimes result from them.
According to Reference.com, isometric exercise requires that the participant not change their body position during the entirety of the exercise. This is often done by placing the participant against an immovable force. Examples of this kind of exercise include holding weights in place, pressing the palms of the hands against each other, and pushing against objects that are not easily moved.
Both isotonic and isometric exercises are useful in different ways, according to Reference.com. Isotonic exercises are helpful in increasing muscle twitch force, whereas isometric exercises are better at increasing overall muscle power. Many people incorporate isometric exercise into their normal isotonic workout so that they can get some of the benefits of both exercise types.