A variety of clinical and environmental conditions can reduce the use of skeletal muscle and give rise to a state of deconditioning. The most common circumstances in which this happens are illness or surgical operations which lead to the individual being confined to bed for prolonged periods of time, or after fractures of the spine/lower limb where whole-body or limb immobilisation is necessary to promote healing. A less common example of this deconditioning is the microgravity associated with spaceflight, where a variety of different strategies to prevent loss of muscle mass and function (countermeasures) have been developed.

Deconditioning of skeletal muscle is accompanied by a reduction in physical activity energy expenditure. When this occurs during recovery from illness/surgery, or after a fracture, then there is often no alteration in appetite such that positive energy balance can occur. This would lead to an increase in body fat content such that body weight may not decrease even though muscle mass is reduced. The consequence of this is a need to not only improve muscle mass and function after recovery from the illness/fracture, but also to decrease body fat content.

The situation in spaceflight appears to be somewhat different, with a reduction in appetite and loss of weight being reported to accompany the decrease in muscle mass. There has been substantial interest in the possibility that nutritional supplements may be an effective countermeasure to prevent the loss of muscle mass and function during microgravity. However, in a recent study in women using prolonged head down tilt (bed-rest) as a model of microgravity, protein supplementation did not prevent the loss of muscle mass during the bed-rest period. By contrast, a different study of prolonged bed-rest in men which assessed the combined effects of resistance training and essential amino acid supplementation showed a beneficial effect on muscle mass and strength when the amino acids were consumed immediately before the exercise bouts.

Less is known about the influence of diet composition or nutritional supplements on muscle mass and function seen with illness, surgery or fracture, or during recovery from the deconditioning which occurs in these situations. Recent work has indicated that older individuals may be more dependent of the quantity and timing of protein intake for optimal improvement in muscle function during training, but little has been done in the more clinical situations.