Myosin Heavy Chain IIX Overshoot in Human Skeletal Muscle
In sedentary men, 3 months of heavy resistance training cut vastus lateralis MHC IIX from 9.3% to 2.0% (IIA 42.4% → 49.6%); after 3 months of detraining MHC IIX overshot to 17.2%, above the pre-training level
Participants
Adult sedentary men (the abstract does not state the sample size)
Protocol
3 months of heavy-load resistance training, then 3 months of detraining
Vastus lateralis biopsies before training, after training and after detraining
MHC isoform distribution by SDS-PAGE; fiber type composition by ATPase histochemistry and immunocytochemistry; fiber size; maximal isometric quadriceps strength
Key Results
After resistance training: MHC IIX fell from 9.3 ± 2.1% to 2.0 ± 0.8% (P < 0.01) with a reciprocal rise in MHC IIA from 42.4 ± 3.9% to 49.6 ± 4.0% (P < 0.05)
After detraining: MHC IIX reached 17.2 ± 3.2% (P < 0.01) — higher than both before and after training (about 1.8× the pre-training value)
Fiber-type composition changed in parallel with the MHC isoforms
Type II fibers hypertrophied significantly with training
Maximal isometric quadriceps strength rose with training and returned to pre-training level after detraining
Mechanism
Heavy-load resistance training lowers MHC IIX and raises MHC IIA; removing the training stimulus reverses this and, for a period, overshoots — IIX ends up well above where it started. Inference, not tested: the overshoot reflects a transient rebound in IIX gene expression once the chronic activity signal is withdrawn.
Practical Application
In sedentary men, 3 months of heavy resistance training took MHC IIX from 9.3% to 2.0% and IIA from 42.4% to 49.6%; 3 months of detraining then pushed IIX to 17.2% — above baseline. Training suppresses the fastest isoform; stopping brings it back with an overshoot
Strength gained over the 3 training months was lost again after 3 detrained months, even as IIX overshot — a higher IIX share did not preserve maximal isometric strength in this group
Inference, not tested: a short, deliberate reduction of training (a taper) might transiently raise the fastest-isoform share; this study used 3 full months of complete detraining in sedentary men and measured no sprint or power performance, so the "taper → sprint boost" idea is an extrapolation, not a finding
Not from this paper; the base's own practice: taper design for racing rests on the tapering literature, not on this biopsy study
Limitations
Resistance training in sedentary men — transfer to endurance-trained athletes is untested
Sample size not stated in the abstract
Only 3 months of training and 3 months of detraining; no performance outcome beyond isometric strength
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A-: uncontrolled pre-post, sample size not given in the abstract
Content correction 2026-09-27
Participants line "n=adult sedentary men" -> the abstract gives no sample size; stated as such.
Added abstract facts missing from the entry: type II hypertrophy with training; isometric strength rose with training and returned to baseline after detraining.
Practical bullets on taper / "enhanced sprint capacity for races" relabelled as inference and house practice (no performance outcome in the abstract).
Level A- -> B (uncontrolled pre-post); Tier "mixed→Tier 2-3 (review...)" -> Tier 0 (sedentary); Sport "cycling, strength, sprint" -> strength.