A claim you hear on longevity podcasts

Do you lose type 2 muscle fibers as you age?

Leaning for. Of 10 studies that test it, 6 agree, 3 partly agree and 1 disagrees.

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The claim checked: You lose type 2 muscle fibers as you age Hear it at 51:26

What do the studies say?

Leaning for

Type 2 fibers are the fast ones used for power. Six of ten studies agree they shrink or drop with age. In 221 adults, type IIa fibers had shrunk by 61. In another study, type II fibers were 26% smaller in older adults. One study disagrees: 12 older adults followed for about ten years, from a mean age of 71.1 to 80.0, showed no shrinkage, though only 9 had biopsies. Three partly agree: a pooled analysis found fast fibers shrink but the mix of fiber types stays the same, and sprinters aged 48-85 who kept training saw no significant change. Most of this is about fibers getting smaller. Fewer studies counted the fibers.

Computed from 10 studies that test this point, each judged by two readers, in full. 10 more studies are waiting for a full read and do not count yet.

What was said on the podcast?

“We know that as we age, obviously, we're losing type 2 muscle fibers. we're we're we're getting weaker. We're we're losing the muscle mass itself.”
Jeff Cavaliere, a guest on The Drive · 414 ‒ Resistance training as we age: exercise selection, training intensity, and much more · 51:26 on YouTube
Episode check Lift fewer, harder sets as you age, the episode says. Of four studies, none backs it and two favor more volume. 5 claims checked against 34 studies · 25 chapters not checked yet See the whole episode →
The point checked Leaning for

People lose type 2 muscle fibers as they age

Agree 6

other primary study · 2025 · landmark

“In conclusion, based on a comprehensive histological analysis of aged muscle, our results show that ageing is associated with a type I myofibre‐dominant muscle, marked deficits in type II myofibre size and shape, a lesser abundance of stem cells and capillaries specifically surrounding type II myofibres.”

Ageing leads to selective type II myofibre deterioration and denervation independent of reinnervative capacity in human skeletal muscle. Exp Physiol
Who: Healthy, lean, recreationally active non-smoking men, young (22 ± 3 years) and older (69 ± 3 years); vastus lateralis biopsies
PMID 39466960

other primary study · 2018

“At the histological/myocellular level, type IIx atrophy was noted by 37 y and type IIa atrophy by 61 y (P < 0.05).”

Human neuromuscular aging: Sex differences revealed at the myocellular level. Exp Gerontol
Who: 221 untrained adults (119 female, 102 male) in four age cohorts with means of 25, 37, 61 and 72 years, Birmingham, Alabama
PMID 29481967

other primary study · 2018

“There was no significant difference between young and old in type I FCSA ( p = .487), but type II fibers had 26% lower FCSA in old ( p < .001; Table 2 ).”

The Contributions of Fiber Atrophy, Fiber Loss, In Situ Specific Force, and Voluntary Activation to Weakness in Sarcopenia. J Gerontol A Biol Sci Med Sci
Who: 31 young adults (mean age about 22-23) and 40 older adults (mean age about 71-72, sarcopenic by ALM/h2); 23 older adults followed longitudinally; biopsies from 21 young and 30 older
PMID 29529132

other primary study · 2023

“YC had higher type II fiber distribution than OC ( P = 0.007) and borderline higher than OE ( P = 0.075).”

The impact of life-long strength versus endurance training on muscle fiber morphology and phenotype composition in older men. J Appl Physiol (1985)
Who: 42 men: young recreationally active (25 yr, n=11), old strength athletes (73 yr, n=10), old endurance athletes (72 yr, n=8), old recreationally active (75 yr, n=13)
PMID 37881849

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other primary study · 2018

“The proportion of MHC I was significantly higher in older adults than in young adults ( p =0.005), whereas the proportion of MHC IIa was significantly lower in older adults than in young adults ( p =0.008).”

Age- and sex-related differences in myosin heavy chain isoforms and muscle strength, function, and quality: a cross sectional study. J Exerc Nutrition Biochem
Who: 53 healthy Korean adults: 28 young (20-40 years; 17 men, 11 women) and 25 older (>65 years; 15 men, 10 women), vastus lateralis biopsy
PMID 30149426

other primary study · 2016

“In both biceps and quadriceps, we found a significant reduction in the number of type II fibers with age ( p < 0.005), while this association was not significant for type I fibers ( Table 2 ).”

An Age-Related Morphometric Profile of Skeletal Muscle in Healthy Untrained Women. J Clin Med
Who: Muscle specimens from cadavers of 23 healthy untrained women: 11 under 41 years and 12 over 41 years
PMID 27827979

Partly agree 3

other primary study · 2017

“Taken together, our results suggest that ageing in sedentary men is associated with (i) complex changes in muscle phenotype (fibre type and size) preferentially affecting type IIa fibres; (ii) a decline in mitochondrial content affecting all fibre types; and (iii) an increase in lipid content in type I fibres.”

The impact of ageing, physical activity, and pre-frailty on skeletal muscle phenotype, mitochondrial content, and intramyocellular lipids in men. J Cachexia Sarcopenia Muscle
Who: Men aged 20 to 94: young adult, active and sedentary middle-aged, and active, sedentary and pre-frail aged 65 and older; vastus lateralis biopsies
PMID 27897402

pooled analysis · 2024

“In addition, our findings indicate that the relative MyHC protein expression shift to the slower isoform in older males is due to the age-related atrophy of fast-contracting MyHC II or IIA fibers and not due to changes in fiber type distribution.”

Effects of age on human skeletal muscle: a systematic review and meta-analysis of myosin heavy chain isoform protein expression, fiber size, and distribution. Am J Physiol Cell Physiol
Who: Healthy young adults (18-49 years) versus older adults (mean age at least 60 years), mostly male, mostly vastus lateralis biopsies
PMID 39374077

other primary study · 2025

“On a fibre type specific level, there were also no significant changes in FCSA of Type I, IIa and IIx fibres (Table 2 ).”

A 10-Year Longitudinal Study of Muscle Morphology and Performance in Masters Sprinters. J Cachexia Sarcopenia Muscle
Who: 24 healthy male Finnish masters sprinters aged 48-85 at baseline with long-term sprint training and championship success
PMID 40289323

Disagree 1

other primary study · 2008

“Of importance, both type I and IIA fibers showed no atrophy after 10 yr using diameter or cross-sectional area as indexes of fiber size.”

Muscle fiber size and function in elderly humans: a longitudinal study. J Appl Physiol (1985)
Who: 12 older adults (5 men) followed from mean age 71.1 to 80.0 years; 9 had muscle biopsies
PMID 18556434

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