Broadly, when a muscle contracts, it shortens by one-third (30%) of its belly-length, and brings about a movement. The range of movement depends on the length of fleshy fibers, and the power or force of movement on the number of fibers.
However, the actual behavior of muscle contraction is more complex. During contraction the length of the muscle may decrease (isotonic contraction), may remain unchanged (isometric contraction) or may increase, according to the functional demands of the body. In each circumstance the tension generated at the ends may either increase, persist, or decrease, depending upon the number and state of its active motor units and the external conditions like loading.
Types of Skeletal muscles on the basis of action:
Each movement at a joint is brought about by a coordinated activity of different groups of muscles. These muscle groups are classified and named according to their function.
Prime movers (agonists) bring about the desired movement. When a prime mover helps opposite action by active, controlled lengthening against gravity, it is known as action of paradox. For example, putting a glass back on the table is assisted by gravity but controlled by a gradual active lengthening of biceps (paradoxical or eccentric action).
Antagonists (opponents) oppose the prime movers. They help the prime movers by active controlled relaxation, so that the desired movement is smooth and precise. Thus, the antagonists cooperate rather than oppose the prime movers. This is due to reciprocal innervation of the opposite groups of muscles, regulated by the spinal cord through stretch reflex.
Fixators are the groups of muscles which stabilize the proximal joints of a limb, so that the desired movement at the distal joint may occur on a fixed base. Muscles acting on shoulder joint fix it for better movement of fingers.
When the prime movers cross more than one joint, the undesired actions at the proximal joints are prevented by certain muscles known as synergists. For example, during making a tight fist by long digital flexors the wrist is kept fixed in extension by the synergists (extensors of wrist). Thus, the synergists are special fixators and partial antagonists to the prime movers. Two or more muscles causing one movement are synergist.