Muscular and systemic correlates of resistance training-induced muscle hypertrophy.
Level 3 - non-randomized controlled study
Prospective single-cohort follow-up study without a control group
PubMed 24130904 · doi:10.1371/journal.pone.0078636
What was done
Twenty-three adult males completed 4 sessions per week of resistance training for 16 weeks. Muscle biopsies were obtained before and after the training period, and acutely at 1 and 5 hours after the first training session, to assess skeletal muscle androgen receptor protein content and p70S6K phosphorylation. Serum free testosterone, growth hormone, insulin-like growth factor 1, and interleukin 6 concentrations were measured immediately, 15, 30, and 60 minutes following the first and last training sessions.
What was found
Mean muscle fiber area increased by 20% (range: -7% to 80%, P < 0.001). Skeletal muscle androgen receptor protein content was unchanged overall with training (fold change = 1.17 ± 0.61, P = 0.19), but changes in androgen receptor content correlated with changes in fiber area (r = 0.60, P = 0.023). Phosphorylation of p70S6K was elevated 5 hours post-exercise and correlated with gains in mean fiber area (r = 0.54, P = 0.007). The post-exercise interleukin 6 response also correlated with hypertrophy (r = 0.48, P = 0.019). There was no relationship between pre- or post-training exercise-induced changes in free testosterone, growth hormone, or insulin-like growth factor 1 and muscle fiber hypertrophy.
Why it matters
These findings suggest that resistance training-induced muscle hypertrophy tracks with local intramuscular signaling and receptor availability rather than acute post-exercise systemic hormonal spikes.
Limits
The study was limited to a small sample of 23 young males with no female participants and no non-exercising control group. The correlational design cannot establish direct causality between the measured markers and muscle hypertrophy.
Cited by
- supports Acute, transient post-exercise elevations in systemic testosterone and growth hormone do not drive long-term muscle hypertrophy.