Strength and muscle mass development after a resistance-training period at terrestrial and normobaric intermittent...

This study investigated the effect of a resistance training (R_T) period at terrestrial (HH) and normobaric hypoxia (NH) on both muscle hypertrophy and maximal strength development with respect to the same training in normoxia (N). Thirty-three strength-trained males were assigned to N (FiO₂ = 20.9%), HH (2,320 m asl) or NH (FiO₂ = 15.9%). The participants completed an 8-week R_T program (3 sessions/week) of a full body routine. Muscle thickness of the lower limb and 1RM in back squat were assessed before and after the training program. Blood markers of stress, inflammation (IL-6) and muscle growth (% active mTOR, myostatin and miRNA-206) were measured before and after the first and last session of the program. Findings revealed all groups improved 1RM, though this was most enhanced by R_T in NH (p = 0.026). According to the moderate to large excess of the exercise-induced stress response (lactate and Ca²⁺) in HH and N, results only displayed increases in muscle thickness in these two conditions over NH (ES > 1.22). Compared with the rest of the environmental conditions, small to large increments in % active mTOR were only found in HH, and IL-6, myostatin and miR-206 in NH throughout the training period. In conclusion, the results do not support the expected additional benefit of R_T under hypoxia compared to N on muscle growth, although it seems to favour gains in strength. The greater muscle growth achieved in HH over NH confirms the impact of the type of hypoxia on the outcomes.