Professionals in the field consider exercise which involves taking a muscle, or muscle group, through a relatively full range of motion through a path of little resistance to be of the isotonic type. Activities such as running and swimming fit into this category.
Isometric exercise requires using the muscle to move a short distance against very high resistance.
Generally, isometrics are used for development of the larger muscles, while isotonics benefit the cardiovascular system by increasing the heart's capacity to pump blood more efficiently. Isotonic exercises like running or swimming also have the benefit of maximizing the amount and effectiveness of small blood vessels which feed the muscles. All of this information seems to tell us that endurance athletes greatly benefit from isotonic training. Two months ago, my son the triathlete suggested that I start doing weight lifting workouts for both the upper and lower body. In a deja-vu kind of role reversal, my initial resistance finally gave way to what I really knew was a good idea.