Potent myofiber hypertrophy during resistance training in humans is associated with satellite cell-mediated myonuclear addition: a cluster analysis.
Level 3 - non-randomized controlled study
Prospective non-randomized single-arm cohort with post-hoc response clustering
PubMed 18436694 · doi:10.1152/japplphysiol.01215.2007
What was done
Sixty-six humans underwent 16 weeks of knee extensor resistance training. Vastus lateralis biopsies were obtained at baseline and week 16. Using K-means cluster analysis based on mean myofiber hypertrophy, participants were classified as extreme responders (Xtr, n = 17; +58% hypertrophy), modest responders (Mod, n = 32; +28%), or non-responders (Non, n = 17; 0%). Satellite cells (identified by NCAM surface expression) and myonuclei counts were quantified immunohistochemically to determine whether satellite cell activation and myonuclear addition differentiate hypertrophic responses.
What was found
Baseline myofiber size did not differ among clusters, but baseline satellite cell counts were significantly higher in Xtr than in Mod and Non (P < 0.01). With training, satellite cell numbers increased significantly only in Xtr (+117%, P < 0.001). Myonuclear addition occurred in Mod (+9%, P < 0.05) and was greatest in Xtr (+26%, P < 0.001). After training, Xtr had 23% more myonuclei per fiber than Non (P < 0.05) and tended to have more than Mod (+19%, P = 0.056). Both Mod and Xtr expanded their myonuclear domain to meet or exceed 2,000 µm² per nucleus.
Why it matters
The study provides human mechanistic evidence that baseline satellite cell availability and subsequent myonuclear addition may set the upper limit on skeletal muscle hypertrophy in response to resistance training.
Limits
Responder groups were categorized post hoc via cluster analysis rather than a priori. The abstract does not report participant demographics (such as age, sex, or prior training background), and measurements were limited to a single muscle (vastus lateralis) using an isolated knee extension exercise protocol. The association between myonuclear addition and extreme hypertrophy remains correlational.
Cited by
- supports High responders to resistance training achieve greater muscle growth due to superior satellite cell activation, proliferation, and myonuclear addition compared to non-responders.