Answer
The layman generally thinks of muscle contraction as a process in which muscles become shorter. But not all muscle contraction types result in reduction of muscle length. There are several types of muscle contractions, for example, isometric and isotonic.
Isometric contraction:
In this type of muscle contraction the muscle develops tension, but there no changes of muscle length.
If one grabs a weight, or a dumb-bell, and tries to lift it, at first, the muscle fibers (cells) of the elbow flexors will will contract--but there will be no movement. Then the elastic tissue associated with the muscle will stretch to absorb the tension , but still there will be no shortening.
Isometric contractions of postural muscles keep us in desired positions for long periods. The maintenance of joint stability is also facilitated by isometric contraction of antagonistic muscles.
Work Step by Step
Isotonic contraction:
In isotonic contraction the contracting muscle builds internal tension until it overcomes the resistance of the load or weight. Then the muscle shortens and moves the load but does not increase its tension
There are two forms of isotonic contraction--concentric, and eccentric. In concentric isotonic contraction, the muscle shortens as it maintains tension; for example, the elbow flexors ( biceps brachii or brachialis) contract as they flex the elbow to lift a weight. After the tension of the weight is overcome, the lifting involves shortening of the muscles, but the muscle tension remains the same
In eccentric isotonic contraction, the muscle lengthens as it maintains tension; for example, the biceps brachii ( or brachialis) lengthens during elbow extension, but if one is holding a weight, the muscle it keeps the tension on to keep the weight from falling.
