The decline in skeletal muscle mass with aging is mainly attributed to a reduction in type II muscle fiber size.
Level 3 - non-randomized controlled study
Cross-sectional cohort comparison combined with an uncontrolled prospective pre-post exercise intervention.
PubMed 23425621 · doi:10.1016/j.exger.2013.02.012
What was done
Quadriceps muscle cross-sectional area (CSA) and type I and type II muscle fiber sizes were measured in 25 healthy young men (23 ± 1 years) and 26 healthy older men (71 ± 1 years). Following baseline comparisons, the older cohort completed 6 months of resistance-type exercise training, after which quadriceps CSA and fiber size measurements were repeated to evaluate changes in muscle mass relative to fiber hypertrophy.
What was found
Quadriceps CSA was significantly smaller in older versus young men (68 ± 2 vs 80 ± 2 cm², P < 0.001). Type II muscle fiber size was 29% smaller in older men (P < 0.001), with a trend toward smaller type I fibers (P = 0.052); differences in type II fiber size fully explained the difference in quadriceps CSA between groups. Following 6 months of resistance training in the older men, type II muscle fiber size increased by 24 ± 8% (P < 0.01), accounting for 100 ± 3% of the increase in quadriceps muscle CSA (which increased from 68 ± 2 to 74 ± 2 cm²).
Why it matters
This study demonstrates that muscle loss in healthy aging is driven predominantly by type II muscle fiber atrophy rather than permanent fiber loss. It shows that prolonged resistance training can substantially reverse this type II fiber atrophy and restore muscle mass.
Limits
The study was conducted only in healthy men, limiting generalizability to women or frail populations. The training phase used an uncontrolled pre-post design with no non-exercising older control group. Muscle fiber number was not directly counted, only inferred from cross-sectional area calculations.
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- supports Type 2 fast-twitch muscle fibers atrophy and lose strength first during aging, before type 1 slow-twitch fibers.