Resistance exercise-induced S6K1 kinase activity is not inhibited in human skeletal muscle despite prior activation of AMPK by high-intensity interval cycling.
Level 2 - randomized trial
Randomized crossover trial in humans
PubMed 25605643 · doi:10.1152/ajpendo.00486.2014
What was done
Eight trained male subjects performed either resistance exercise alone (R) or high-intensity interval cycling followed by resistance exercise (ER) in a randomized crossover design. Vastus lateralis muscle biopsies were obtained before and after endurance exercise and repeatedly during recovery following resistance exercise to evaluate glycogen content, kinase activity, protein phosphorylation, and gene expression. Mixed muscle fractional synthetic rate was measured at rest and during 3 hours of recovery using a stable isotope technique.
What was found
In the ER trial, AMPK activity was elevated immediately post-endurance and post-resistance exercise (~90%, P < 0.05) but was unchanged in R. Thr(389) phosphorylation of S6K1 increased severalfold immediately post-exercise (P < 0.05) in both trials and rose further throughout recovery. After 90 and 180 min recovery, S6K1 activity was elevated (~55% and ~110%, P < 0.05) and eukaryotic elongation factor 2 phosphorylation was reduced (~55%, P < 0.05) with no difference between trials. ER induced a significant increase in MuRF1 gene and protein expression (P < 0.05), which did not occur in R. Numerical values for fractional synthetic rate were not reported in the abstract.
Why it matters
These findings challenge the rodent-based model that endurance-induced AMPK activation blunts mTORC1 signaling during subsequent resistance exercise in human skeletal muscle. The interference effect of concurrent training may instead involve enhanced pathways of muscle protein breakdown.
Limits
The sample size is very small (n = 8) and restricted entirely to trained young males. The study assessed only acute molecular markers over 3 hours of recovery, not chronic phenotypic adaptations such as hypertrophy or strength gains.
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- contradicts Performing cardiovascular exercise prior to strength training impairs the muscle protein synthesis signaling response compared to doing cardio after strength training.