Deconditioning

Muscle strength of itself matters little; what is important is how changes in muscle strength affects the ability to perform daily activities. As the strength of a muscle decreases with age, activities relying on that muscle require a greater proportion of the maximum strength of the muscle. Eventually, a threshold is reached where the maximum strength available to an individual for a particular action is the minimum strength required for that action. Any further decline in muscle strength will make the activity impossible. If that activity is essential to an independent existence, a small decline in muscle function, such as following a brief period of inactivity due to acute illness, may be sufficient to cause dependence. For example, the quadriceps (thigh muscle) is the most important muscle used in rising unaided from a toilet or from a low chair. The threshold for quadriceps contraction needed to perform these activities is reached at about the age of eighty in women, and a few years later in men.

Appropriate patterns of muscle contractions in the leg (and trunk) are used to adjust and maintain balance (following a stumble, for instance). Deconditioning can adversely affect balance in a number of ways. Disuse atrophy will reduce the functional reserve of muscles needed to maintain balance. For example, loss of strength in the muscles that flex the ankle joint (dorsiflexors) has been associated with falls in nursing-home residents. Prolonged bed rest may cause the brain to adapt to the recumbent position and hence lead to imbalance when the patient eventually tries to walk.