One of the main characteristics of muscles is that they have a rich network of blood vessels, supplying nutrients and oxygen and removing waste products. Muscle contraction, producing tension or pull, is an active process, requiring energy. The shortening of the length of muscles is brought about by the protein of sarcomeres (actin, myosin) interweaving, like the 'teeth' of two combs sliding into each other. This tension/shortening pulls bones, the surfaces on which the tendons of muscles are attached, and causes movements.
Some motor units within any particular muscle are always active, even when the muscle is not contracting. The contractions of these muscle fibres do not produce enough tension to cause movement but they do tense and firm up muscles. This resting tension in a skeletal muscle is called muscle tone. Lack of muscle tone can make a muscle look limp and flaccid, whereas even with the slightest tone the muscles become alert. It is muscle tone that makes the biceps look as they are, even when they are not contracting. Muscle shapes are produced by muscle tone even in the resting position when most muscle fibres relax. Resting muscle tone stabilises bones and joints, while lack of it can allow a joint to collapse. For example, stroke victims with loss of muscle power in one arm often have the shoulder joint dislocated with the weight of the arm. The deltoid (around the shoulder joint) muscles are so flaccid that they cannot keep the head of the humerus bone fixed into the socket of the shoulder joint.
Muscle tone also helps to act as a shock absorber that cushions the impact of a sudden bump or shock. A strong muscle tone is essential for preparedness for sports and exercises where sudden movements happen. Exercises also build up muscle tone.