High Exercise Intensity Increases Myocellular Signaling In Resistance Exercise Modes Equalized For Time Under Tension

PURPOSE: How intensity and time under tension during resistance exercise (RE) link to anabolic signaling and muscle growth in human skeletal muscle is incompletely understood. We hypothesized high intensity was more important than time under tension for the anabolic signal transduction in response to resistance exercise. METHODS: To test our hypothesis, 22 male subjects (23±2 years, 185±7cm, 8±5kg) performed 3 distinct RE modes in the fed state: 1. (HIT n=8) High intensity single set RE; 20 reps with 100% maximum force (MF); 2. (STD n=7) Standard RE, 3 sets - 10 reps, 75% of MF; 3. (ECC n=7) 3 sets - 8 reps, 100% eccentric MF. Vastus lateralis skeletal muscle biopsies were collected at baseline and 15, 30, 60, 240 min and 24 h upon RE. Anabolic signaling was assessed as the phosphorylation of growth-related signaling proteins via western blotting and separately in type I and II myofibers by immunohistochemistry. RESULTS: Concentric and eccentric force generation in HIT dropped significantly already after the 6th repetition during exercise (P < 0.01) however not in STD and ECC. Phosphorylation of mechanosensitive markers pFAKTyr397 and pJNKThr183/Tyr185 were significantly higher in ECC than STD upon RE (P < 0.01 - P < 0.05). AKT, mTOR, 4EBP1, p70s6k and S6 showed at several time points post RE significantly higher phosphorylation levels in ECC than STD and HIT (P <0.01 - P <0.05). HIT offered generally the weakest signaling response of all disposed RE modes. Skeletal muscle signaling as observed in western blotting was also reflected by fiber type specific responses in the phosphorylation of JNK, mTOR and S6. Specifically, sarcoplasmic levels of pJNKThr183/Tyr185, mTORSer2448 and S6Ser235/236 were 30 min post RE significantly higher in type II myofibers in ECC compared to STD and HIT (P < 0.01). CONCLUSIONS: We conclude that ECC > STD > HIT with respect to the resultant anabolic response to resistance exercise. This suggests that “high intensity tension over time” is more important than time under tension alone for subsequent anabolic signaling. We further assume high intense resistance exercise to recruit collectively more myofibers and by that to emphasize anabolic signaling in recruited fiber populations.