REF-310 · Costill DL, Fink WJ & Pollock ML, 1976 · Medicine and Science in Sports, · Paper · PubMed

Muscle Fiber Composition and Enzyme Activities of Elite Distance Runners

Gastrocnemius biopsies in 14 elite distance runners, 18 middle-distance runners and 19 untrained men: elite runners had 79% slow-twitch fibers, ST fibers 22% larger than FT, and SDH activity 3.4× (elite) / 2.8× (middle-distance) that of untrained men; glycogenolytic enzymes did not differ

Blimited evidenceMeasured on elite to world-class athletesWritten before October 2026, being re-checked

Participants

n=51: 14 elite distance runners, 18 middle-distance runners (all highly trained, but significantly slower than the elite group at distances over 3 miles) and 19 untrained men

Protocol

Gastrocnemius muscle biopsies
Fiber composition and mean cross-sectional areas from histochemically stained sections; succinate dehydrogenase (SDH), lactate dehydrogenase (LDH) and phosphorylase activities from muscle homogenates
Maximal oxygen uptake on a treadmill in all subjects

Key Results

Elite runners' gastrocnemius: 79% slow-twitch (ST) fibers ("as previously demonstrated by others")
In the elite runners, the cross-sectional area of ST fibers was on average 22% larger than that of FT fibers (P < 0.05)
SDH activity of whole-muscle homogenates was 3.4-fold (elite) and 2.8-fold (middle-distance) higher than in untrained men
LDH and phosphorylase activities were similar in runners and untrained men — training for distance running has little influence on the enzymes of glycogenolysis
The abstract gives no ST percentage for the middle-distance or untrained groups

Mechanism

Cross-sectional data — the study cannot separate selection from adaptation. The pattern is consistent with a high ST proportion, larger ST fibers and high oxidative (SDH) capacity being the muscle signature of elite distance running, while glycolytic enzyme capacity is not what distinguishes runners from untrained men.

Practical Application

In 14 elite distance runners the gastrocnemius was 79% slow-twitch, the ST fibers were 22% larger than the FT fibers, and oxidative enzyme (SDH) activity was 3.4× that of untrained men (2.8× in 18 highly trained middle-distance runners); glycogenolytic enzymes (LDH, phosphorylase) were no higher than in untrained men
What separates distance runners from the untrained at the muscle level is oxidative capacity and slow-fiber dominance, not glycolytic machinery — the enzyme side of this is what endurance training can clearly move (the 3.4× / 2.8× SDH gap)
Inference, not tested: how much of the 79% ST is inherited versus trained cannot be read from a cross-sectional sample; for longitudinal change in this population see Trappe 1995 REF-311, for the review of fiber-type shifting see Wilson 2012 REF-308
Not from this paper; the base's own practice: an athlete who recovers slowly from hard sessions is not thereby "low type I" — fiber type is not inferred from feel; see Lievens 2020 REF-307 for the recovery link

Limitations

Cross-sectional — cannot determine whether high ST proportion is cause or effect of training
1976 — older methodology
Only gastrocnemius sampled; men only
No DOI line in this entry (PMID only)
How this entry was checked and changed
Why this level

regraded 2026-09-27 from B+: cross-sectional, n=51 (<100)

Content correction 2026-09-27
"Good runners ~60% Type I; untrained ~50%" and "82.9% of total cross-sectional area was ST" -> not in the abstract; removed.
Added the abstract's actual numbers: ST fibers 22% larger than FT in elite runners; SDH 3.4× / 2.8× untrained; LDH and phosphorylase unchanged.
Participants: first n= now the total (n=51), groups listed after it; "18 good runners" -> 18 highly trained middle-distance runners.
Level B+ -> B (cross-sectional, n=51); Sport "running, strength" -> running.