The decline in skeletal muscle mass with aging is mainly attributed to a reduction in type II muscle fiber size

BackgroundThe loss of skeletal muscle mass with aging has been attributed to a decline in muscle fiber number and muscle fiber size.ObjectiveTo define to what extent differences in leg muscle cross-sectional area (CSA) between young and elderly men are attributed to differences in muscle fiber size.MethodsQuadriceps muscle CSA and type I and type II muscle fiber size were measured in healthy young (n = 25; 23 ± 1y) and older (n = 26; 71 ± 1y) men. Subsequently, the older subjects performed 6 months of resistance type exercise training, after which measurements were repeated. Differences in quadriceps muscle CSA were compared with differences in type I and type II muscle fiber size.ResultsQuadriceps CSA was substantially smaller in older versus young men (68 ± 2 vs 80 ± 2 cm2, respectively; P < 0.001). Type II muscle fiber size was substantially smaller in the elderly vs the young (29%; P < 0.001), with a tendency of smaller type I muscle fibers (P = 0.052). Differences in type II muscle fiber size fully explained differences in quadriceps CSA between groups. Prolonged resistance type exercise training in the elderly increased type II muscle fiber size by 24 ± 8% (P < 0.01), explaining 100 ± 3% of the increase in quadriceps muscle CSA (from 68 ± 2 to 74 ± 2 cm2).ConclusionReduced muscle mass with aging is mainly attributed to smaller type II muscle fiber size and, as such, is unlikely accompanied by substantial muscle fiber loss. In line, the increase in muscle mass following prolonged resistance type exercise training can be attributed entirely to specific type II muscle fiber hypertrophy.

► Age-related differences in muscle size can be attributed to smaller muscle fibers. ► Training-induced increase in muscle size is attributed to muscle fiber hypertrophy. ► Large in muscle fiber numbers changes in aging or after training are unlikely. ► Interventions for sarcopenia should target type II muscle fiber hypertrophy.