The impact of ageing, physical activity, and pre-frailty on skeletal muscle phenotype, mitochondrial content, and intramyocellular lipids in men.
Level 4 - case-series / case-control
Cross-sectional comparative observational study in humans
PubMed 27897402 · doi:10.1002/jcsm.12139
What was done
Vastus lateralis muscle biopsies were collected from men across six groups defined by age, physical activity, and frailty: recreationally active young adults (20–30 yo; YA), active and sedentary middle-aged men (50–65 yo; MA-ACT and MA-SED), active and sedentary older men (65+ yo; 65+ACT and 65+SED), and pre-frail older men (65+PF). Biopsies were analyzed on cross-sections for muscle phenotype (myosin heavy chain isoforms immunolabelling), fibre type-specific mitochondrial content (succinate dehydrogenase stain intensity), and fibre type-specific lipid content (Oil Red O stain intensity).
What was found
No quantitative values, sample sizes, or p-values were reported in the abstract. Compared to YA, 65+SED and 65+PF showed significantly smaller overall and type IIa fibre sizes. 65+SED had a lower proportion of type IIa fibres, while MA-SED and 65+SED had higher hybrid type IIa/IIx proportions. Mitochondrial content was lower across all fibre types in sedentary and pre-frail men, but preserved in active older men compared to YA. Intramyocellular lipid in type I fibres was elevated only in 65+SED versus YA. Mitochondrial content (not lipid content) positively correlated with muscle function, functional capacity, and insulin sensitivity across all subjects.
Why it matters
The findings demonstrate that several skeletal muscle phenotypes traditionally attributed to biological aging—including mitochondrial decline and type IIa atrophy—are largely driven by physical inactivity rather than aging alone.
Limits
The study is cross-sectional, precluding causal inference. The abstract provides no participant numbers, exact effect sizes, or variability metrics. The sample was restricted exclusively to men, and physical activity was evaluated observationally.
Cited by
- supports Type 2 fast-twitch muscle fibers atrophy and lose strength first during aging, before type 1 slow-twitch fibers.