Muscles are comprised of many individual fibers, in much the same way a rope is. When these fibers are stimulated, the muscle contracts. If there is a force acting in opposition, such as a weight, the fibers stretch, increasing tension. Different types of activities require your muscles to work in different ways, requiring certain types of muscle contraction.
The word "isotonic" is derived from two Greek words: "iso," meaning "same," and "tonikos," meaning tension. As such, an isotonic contraction is one in which the muscle maintains the same tension as it shortens. Examples of activities that involve isotonic contractions include walking, running or lifting a light object. Isotonic contractions come in two varieties: concentric and eccentric. In a concentric contraction, the muscle shortens when its tension is greater than the force opposing it, such as your biceps does when performing an arm curl. In an eccentric contraction, the force is greater than the muscle tension, causing the muscle to elongate; this happens when going down stairs or sitting down in a chair, as the effects of gravity add to the opposing force.
"Isometric" means "same length," and in contractions of this variety, the muscle does not shorten and its tension never exceeds the opposing force. Examples of isometric exercises include holding a weight in place above the ground or pushing against a stationary object. While the entire muscle does not change length during an isometric contraction, the individual muscle fibers will shorten. As such, isometric exercises can help to strengthen a muscle.
A third type of contraction is "isokinetic," or "same motion." In these contractions, the muscle shortens at a consistent rate throughout the motion. Exercises involving isokinetic contractions are often used in rehabilitation settings, and generally require the use of special equipment capable of providing consistent resistance and velocity throughout the activity.
When undertaking a strength training or general fitness regimen, most of the exercises you perform will be isotonic in nature, targeting the body's major muscle groups. In addition to weight training, these include exercises such as walking, cycling and performing calisthenics. This also includes the operation of many of the fitness devices designed for use in the home, such as ellipticals and steppers. After a workout, it is important to allow the targeted muscle group a day or two to recover before performing the same set of exercises again.