Muscle Disuse as a Pivotal Problem in Sarcopenia-related Muscle Loss and Dysfunction.

An age-associated loss of muscle mass and strength--sarcopenia--begins at around the fifth decade of life, with mass being lost at ~0.5-1.2% per year and strength at ~3% per year. Sarcopenia can contribute to a variety of negative health outcomes, including an increased risk for falls and fractures, the development of metabolic diseases like type 2 diabetes mellitus, and increase the chance of requiring assisted living. Linear sarcopenic declines in muscle mass and strength are, however, punctuated by transient periods of muscle disuse that can accelerate losses of muscle and strength, which could result in increased risk for the aforementioned conditions. Muscle disuse is recognizable with bed rest or immobilization (for example, due to surgery or acute illness requiring hospitalization); however, recent work has shown that even a relative reduction in ambulation (reduced daily steps) results in significant reductions in muscle mass, strength and possibly an increase in disease risk. Although reduced ambulation is a seemingly "benign" form of disuse, compared to bed rest and immobilization, reports have documented that 2-3 weeks of reduced daily steps may induce: negative changes in body composition, reductions in muscle strength and quality, anabolic resistance, and decrements in glycemic control in older adults. Importantly, periods of reduced ambulation likely occur fairly frequently and appear more difficult to fully recover from, in older adults. Here we explore the consequences of muscle disuse due to reduced ambulatory activity in older adults, with frequent comparisons to established models of disuse: bed rest and immobilization.