The muscular system permits the body to enjoy an incredibly wide range of movements. It is often a common practice that when the word muscle is heard that people tend to think of all contractile tissues like smooth muscles, cardiac and skeletal muscles. However, one must take note that when specifically talking about the muscular system, the skeletal muscles take centre stage.
Muscles work hand in handCredit: http://stevegallik.org/sites/histologyolm.stevegallik.org/htmlpages/HOLM_Chapter07_Page04.html
Depending on how the muscles are arranged, they can either work together for an action to be possible or work in opposition to permit a variety of movements. Working in opposition is as important as working in unison, just imagine raising your fork towards your mouth when eating, if no muscle group is working in opposition then your hand will stay in that position forever.
Even with the marvellous ability of the muscles, it can only pull and it can never push. The general rule for a working muscle is that what a muscle or group of muscles another muscle or group of muscles does the opposite in order to undo the action.
Functional Classification of MusclesCredit: http://www.t-nation.com/article/performance_training/maximal_force&cr=
There are four known functional classification of muscles, they are the prime movers, the antagonists, the synergists and the fixators. Bearing the word “prime” in its name somehow gives the action of this muscle. Prime movers are the source of the majority of the force needed for a specific action to be done. The other name for prime movers is the agonist of a movement. An example of prime mover muscle is the biceps brachii muscle, which is a prime mover of the elbow when performing flexion movement.
Conversely, the muscle or muscle group that reverses or opposes the movement of the agonist are called antagonists. Muscle do show respect to one another in the sense that whenever a prime mover is in action, antagonists stretch and relax; when the antagonists are in action, it is the time for prime movers to relax. Antagonists also serve another purpose, that is to provide some resistance to the prime movers, preventing them from overshooting their movement.
Most of the movements done either by the agonist or antagonist involved the use of synergists. Synergists help prime movers in two ways, namely to amplify the contractile force of the prime mover and to limit the unnecessary movements while the prime mover contracts.
Undesirable or unnecessary actions happen when a muscle crosses two or more joints. As the muscle contracts and crosses the joints, the contractile strength of the muscle resonates with the joint and causes movement of the spanned joint. In order to prevent this, some muscles act to stabilize the joint. With this ability, the prime movers are able to direct all of its force in performing its desired action in a specific direction.
A more precise name is given to synergists that act to immobilize or stabilize a bone. Fixators, as they are specifically called, immobilize a muscle at its origin. Furthermore, fixators help in attaining only the desired movement of the muscle and are also instrumental for humans to attain an upright posture.