Muscle atrophy in immobilization and senescence in humans.

PURPOSE OF REVIEW: Recent reports exploring the mechanisms thought to be responsible for the determination of muscle mass during health, ageing and immobilization in humans have presented findings that have wide ranging implications. This brief review highlights some of the more important findings. RECENT
FINDINGS: Contrary to expectations, recent findings suggest an apparent dissociation between muscle signalling pathways and their associated events in humans, particularly in relation to muscle protein synthesis. Although debate concerning the relative importance of muscle protein synthesis and degradation to muscle mass loss during immobilization continues, and the mechanisms responsible for this loss and its restoration during rehabilitation remain unclear, new evidence has emerged showing that anabolic resistance to protein nutrition develops during immobilization. This latter observation is in agreement with earlier evidence pointing to anabolic resistance of muscle to protein nutrition existing in the elderly, which is of clinical importance. Recent observations also suggest that a sex difference exists in the rate of muscle protein synthesis under postabsorptive conditions in the elderly and may explain why women lose muscle mass at a slower rate with age than men.
SUMMARY: These recent findings highlight our current lack of understanding of the mechanisms that regulate muscle mass in humans.