Isotonic Contraction

The most commonly thought of muscular contraction is call an isotonic contraction. In this type of contraction the myofibrils are actually sliding across one another causing shortening of the muscle, which causes movement. Example curling a barbell up to your chest.

Isometric Contraction

Any time a muscle contracts but does no movement occurs is called an isometric contraction. When an isometric contraction occurs the myosin myofibrils are basically trying to slide together but can't because the resistance is too great. A good example of this type of contraction is doing the "plank" exercise.