Inactive muscle loses strength, endurance, and mass very quickly. Perhaps you have seen an arm or a leg shriveled up with atrophied muscle (often called disuse atrophy) after it has been confined to a cast for several weeks. Loss of muscular strength due to immobility progresses at a rate of about 12% each week. After 3 to 5 weeks of immobility due to bed rest almost one-half of the muscular strength is lost. Correct positioning and reduction of abnormal stress on immobilized muscles and joints are important because these structures may stretch or shorten, resulting in abnormal fixation of a joint, altering biomechanics. For example, an ankle may develop contractures when a tight, heavy blanket or improper positioning puts excessive and inappropriate pressure on the foot (Fig. 25-1). Generally flexor muscles are stronger than the opposing extensor muscles (which atrophy more than the flexor muscles), and this imbalance may allow an inactive joint to take an abnormal position if flexibility is not maintained by range-of-motion exercises. With inactivity, tendons and ligaments shorten and lose elasticity. Prolonged immobility causes fibrous tissue to replace muscle cells, leading to muscle wasting and weakening, decreased flexibility, further possibly irreversible deformity (contracture), and loss of function.

Bone deteriorates with inactivity. Bone is a “living” tissue in which new bone is constantly forming (osteoblastic activity) and other bone is being resorbed (osteoclastic activity). Bone demineralization occurs because the lack of weight bearing and muscle action reduces osteoblastic activity or bone formation; however, osteoclastic activity continues. This process leads to loss of bone mass and osteoporosis with the potential for spontaneous fractures if undue stress is placed on the bones (see Chapters 24 and 9).

The breakdown of muscle and bone tissue initially results in elevated serum levels of nitrogen wastes such as creatinine and in elevated serum calcium. Hypercalcemia may cause renal calculi or kidney stones if fluid intake is inadequate and the urine becomes too concentrated (see Chapter 18). Also a high serum calcium level can further impede muscle activity because it decreases muscle tone and leads to flaccidity or loss of muscle tone. Passive range-of-motion exercises and weight bearing if tolerated on a regular basis are helpful in preventing these complications.

Tendons and ligaments that connect the muscles to bone and maintain joint structure also require movement to maintain their structure and functionality. After 4 to 6 days of immobility these forms of connective tissue begin to shorten and the density of the tissue increases limiting flexibility and range of motion.

The skin breaks down easily when its circulation is impaired and cell regeneration is reduced. Blood supply is often reduced in places where the skin is stretched over bony projections and there is little fatty or muscular tissue to cushion the weight of the body. Areas that are particularly vulnerable to poor blood perfusion include the ischial tuberosities, sacrum, the greater trochanter of the hip, the heels, and the elbows. Pressure at these points causes ischemia and necrosis of tissue (Fig. 25-2), leading to decubitus ulcers (pressure sores or bedsores). Other factors that promote skin breakdown and development of decubiti include:

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