The three types of muscle contraction are Isometric, Isotonic, and Isokinetic. Isometric is defined as that type of contraction where muscle tension and muscle length remain constant. This type of exercise provides muscle strength gains but only at the joint angle held during the exercise. Isotonic contraction is defined as that where the muscle tension remains constant and muscle length varies. Isokinetic contraction is defined as varying tension and length.


In each exercise there are four main functions of the associated muscles, Agonists (prime movers), Antagonists, Stabilizers and Assistors. The Agonists is generally the muscle we are exercising. The Antagonist is the opposing muscle and acts in contrast to the agonist. The Stabilizer muscles are those that hold a joint in place so that the exercise may be performed. The Assistors help the Agonist muscle doing the work. The stabilizer muscles are not necessarily moving during exercise, but provide stationary support.


For example, when doing biceps curls, the biceps are the agonists, the triceps are the antagonists and various muscles including the deltoids are the stabilizer muscles. However, when doing a triceps push down, now the triceps are the agonists and the biceps are the antagonists. Again the deltoid muscles are the stabilizer muscles. The agonist/antagonist relationship changes depending on which muscle is expected to do the work. However, every muscle group has an opposing muscle group. The following table lists muscles and their opposing counterparts:



In reference to Agonist and Antagonist, this above list could easily be reversed when exercising the muscles in the right hand column. Muscle balance is that relationship between the Agonist and Antagonist. It is important to have muscle balance to prevent injury. If the Agonist is much stronger than the Antagonist is, the Agonist can overpower and injure the Antagonist.