Effects of age on human skeletal muscle: a systematic review and meta-analysis of myosin heavy chain isoform protein expression, fiber size, and distribution.
Level 3 - non-randomized controlled study
Systematic review and meta-analysis of observational cross-sectional studies
PubMed 39374077 · doi:10.1152/ajpcell.00347.2024
What was done
A systematic review and meta-analysis of 27 studies compared skeletal muscle myosin heavy chain (MyHC) characteristics between young (18-49 years) and older (≥60 years) adults. Outcomes evaluated included relative MyHC protein expression, fiber cross-sectional size, and fiber type distribution, with subgroup analyses evaluating sex, measurement technique, physical activity, and muscle type.
What was found
The abstract reports no numerical values, effect sizes, or confidence intervals. Directionally, aging was associated with higher relative protein expression of slow-contracting MyHC I and lower expression of fast-contracting MyHC II and IIA. Fiber size was preserved with age for MyHC I but smaller for MyHC II and IIA. Fiber distributions did not differ with age. Female-specific analyses showed atrophy of MyHC II and IIA fibers with age but no change in MyHC protein expression.
Why it matters
This synthesis demonstrates that age-related muscle loss and power decline stem primarily from selective atrophy of fast-twitch fibers rather than a complete redistribution of fiber types.
Limits
The abstract provides only qualitative directions without effect sizes, variance, or heterogeneity metrics. Primary data rely on cross-sectional age comparisons rather than longitudinal tracking. The evidence base is heavily skewed toward male participants and vastus lateralis biopsies, with limited separate assessment of MyHC IIA versus IIX isoforms and inconsistent objective measurement of physical activity.
Cited by
- supports Humans experience a progressive loss of type II muscle fibers starting after the fourth decade of life.