REF-308 · Wilson JM, Loenneke JP, Jo E, Wilson GJ, Zourdos MC & Kim JS, 2012 · Journal of Strength and Conditioning Research, · Paper · PubMed
The Effects of Endurance, Strength, and Power Training on Muscle Fiber Type Shifting
Narrative review: IIA↔IIX interconversion is well established; whether type I and type II interconvert is disputed; high-velocity/ballistic work is discussed as a route toward more type II, long-duration high-volume endurance work as a route toward type I
Key Results
Type I fibers specialise in long-duration contractile activity and are abundant in elite endurance athletes; type IIA and IIX fibers serve short-duration anaerobic activity and are proportionally higher in elite strength and power athletes
Interconversion between type IIA and IIX is well recognised in the literature
Whether type I and type II fibers interconvert is CONFLICTING in the literature — the review analyses the evidence rather than settling it
Variables discussed as possibly increasing type II and decreasing type I: high-velocity isokinetic contractions, ballistic movements such as bench-press throws and sprints
A shift from type II toward type I is discussed as possible under longer-duration, higher-volume endurance-type events
Detraining "overshoot" of MHC IIX above pre-training levels is a finding of Andersen & Aagaard 2000 REF-309, not of this review
Mechanism
Not from this paper (textbook physiology, unlabelled in the abstract): sustained low-frequency motor-unit activation → calcineurin/NFAT signalling → slow myosin heavy-chain expression; the review's own contribution is to sort the training modes by the direction of shift they are associated with.
Practical Application
Narrative review (Wilson et al. 2012): the well-established shift is within type II (IIX ↔ IIA); a shift from type II to type I with long-duration, high-volume endurance training is discussed as possible but the evidence is conflicting — do not tell an athlete that endurance training will "convert" fast fibers to slow ones as an established fact
The training modes the review associates with more type II are high-velocity and ballistic (isokinetic high-speed work, bench-press throws, sprints); with more type I, long-duration high-volume endurance work
Not from this paper; the base's own practice: the anecdote of former sprinters becoming "slow-fiber" endurance athletes is a coaching observation, consistent with the review's direction but not evidence; the longitudinal human data live in Trappe 1995 REF-311
For the reverse direction after a training stop (IIX overshoot) see Andersen & Aagaard 2000 REF-309
Limitations
Narrative review — no systematic search, no pooled data
Most cited evidence is cross-sectional; true longitudinal data are limited
Degree of type II → type I conversion is explicitly described as conflicting
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B+: narrative review without systematic method
Content correction 2026-09-27
"Elite endurance athletes: 70-90% Type I" -> number not in the abstract; removed (Costill 1976 REF-310 carries 79% for elite runners).
"True Type II → Type I conversion possible but requires years" -> the abstract calls the type I/II interconversion evidence conflicting and mentions no time frame; "years" removed.
Detraining IIX overshoot attributed to REF-309, not to this review; Bakken anecdote relabelled as house observation.
Level B+ -> C+ (narrative review); Tier "mixed→Tier 4-5" -> mixed; Sport "sprint" label dropped.