The order of concurrent endurance and resistance exercise modifies mTOR signaling and protein synthesis in rat skeletal muscle.
Level 5 - mechanism / opinion, no new human data
Non-human animal mechanistic study
PubMed 24691029 · doi:10.1152/ajpendo.00647.2013
What was done
Male Sprague-Dawley rats were divided into five groups: endurance exercise alone (treadmill running at 25 m/min for 60 min), resistance exercise alone (percutaneous electrical stimulation producing maximal isometric contractions, 3 x 10 s, 5 sets), endurance before resistance exercise, endurance after resistance exercise, or a non-exercise control. The researchers measured muscle protein synthesis and phosphorylation markers of mTORC1 (p70S6K, mTOR, Raptor, Akt) and AMPK signaling at 3 and 6 hours after resistance exercise.
What was found
The abstract reports directional changes without numeric values. Phosphorylation of p70S6K at 3 hours post-exercise significantly increased in both combined exercise groups, but the increase was smaller when endurance exercise occurred after resistance exercise. Muscle protein synthesis was greatly increased at 6 hours only in the endurance-before-resistance group. Increases in AMPK and Raptor phosphorylation occurred exclusively in the endurance-after-resistance group. Akt and mTOR phosphorylation increased in both sequence groups without between-group differences.
Why it matters
This study provides mechanistic evidence that the order of concurrent exercise affects muscle anabolic signaling, showing that a subsequent bout of endurance exercise can downregulate prior resistance-induced mTORC1 activation and protein synthesis.
Limits
The study was conducted in a rodent model using electrically stimulated contractions, which may not translate directly to human training. The abstract provides no exact sample sizes (n), quantitative values, effect sizes, or p-values, and it measures acute molecular signaling rather than chronic muscle hypertrophy.
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