What changes in muscle contribute to a loss of function with aging?

A progressive loss of muscle mass and function generally begins around the fourth to fifth decade of life, and the rate of decline increases with advancing age.^[1] The rate of the age-related decline in muscle strength (about 1%–3% per year) and power (about 3%–4% per year) exceeds that of muscle loss (about 0.5%–1.5% per year).^[1] What mechanisms explain this comparatively accelerated loss of muscle strength and power (i.e., muscle function)?

Total muscle mass and muscle strength are strongly correlated,^[2][3] so one might expect that the rate of decline for muscle mass and muscle strength would be more similar than they are. The primary reason for this discrepancy seems to be the type of muscle atrophy occurring.

Muscle fibers come in different types based on their speed of contraction and metabolism, and they are generally categorized as slow-twitch (type I) and fast-twitch (type II). Type I muscle fibers contract relatively slowly and are more resistant to fatigue, whereas type II muscle fibers contract with more velocity and produce significantly more power but are quick to fatigue.

Older adults predominantly experience a reduction in the size of type II muscle fibers, while type I muscle fiber size (and perhaps even their potential for hypertrophy) is preserved.^[1] Furthermore, there’s evidence to suggest that type II muscle fibers in older adults are less sensitive to calcium,^[4][5][6] which is detrimental for force production, as calcium triggers muscle contraction.

Other factors thought to contribute to the decline in muscle function with aging are a progressive increase in myosteatosis (i.e., fat infiltration within skeletal muscle),^[7] satellite cell loss and dysfunction,^[8][9] and mitochondrial dysfunction.^[10]