Different aspects of contraction of muscle

Aspects of Muscle Contraction 
When muscle contracts, one bone remains fairly station­ary, and the other one moves. The origin of a muscle is on the stationary bone, and the insertion of a muscle is on the bone that moves.
Frequently, a body part is moved by a group of mus­cles working together. Even so, one muscle does most of the work and is called the prime mover. The assisting muscles are called the synergists. When muscles con­tract, they shorten; therefore, muscles can only pull; they cannot push.
Muscles have antagonists, and antagonistic pairs work opposite one another to bring about movement in opposite directions. For example, the biceps brachii and the triceps brachii are antagonists; one flexes the forearm, and the other extends the forearm. The discus­sion that follows will point out other antagonistic pairs.
When muscles cooperate to achieve movement. some act as prime movers. others as synergists. and still others as antagonists.
Isotonic Versus Isometric Contraction 
Ordinarily, when muscles contract, they shorten and a movement occurs. This is called isotonic contraction. For example, when a barbell weight is moved upward, the biceps contracts isotonically. On occasion, muscles con­tract but do not shorten or produce movement. This is called an isometric contraction. When a stationary bar is pulled on, the biceps contracts isometrically, and no movement occurs.
Muscle Tone 

Whole skeletal muscles have tone, a condition in which some fibers are always contracted. Muscle tone is particu­larly important in maintaining posture. If the muscles of the neck, trunk, and legs suddenly relax, the body collapses.
Maintenance of the right amount of tone requires the use of special sense receptors called muscle spindles. A muscle spindle consists of a bundle of modified muscle fibers, with sensory nerve fibers wrapped around a short, specialized region. A spindle contracts along with muscle fibers, but thereafter, it sends stimuli to the central nervous system, which then regulates muscle contraction so that tone is maintained.