Neuromuscular Plasticity Induced by Resistance Training Under Different Hypoxic Environments

Background: This study compared the effects of 8 weeks of hypertrophy-oriented resistance training (R_T) under normoxia (N), moderate hypobaric hypoxia (HH), and moderate normobaric hypoxia (NH) on functional, neuromuscular and structural adaptations of the quadriceps. Methods: Twenty-nine resistance-trained males (22.5 ± 3.4 years) were assigned to N (n = 10), HH at 2320 m (n = 8), or equivalent NH (FiO₂ = 15.9%; ~2320 m; n = 11). Participants completed 22 supervised R_T sessions across 8 weeks. Primary outcomes were corticospinal excitability (CSE), assessed via transcranial magnetic stimulation (TMS), and neuromuscular activation, assessed with femoral nerve stimulation and surface electromyography. Secondary measures included back squat one-repetition maximum (1RM_SQ) and vastus lateralis (VL) muscle thickness (MTh) via ultrasound. Results: All groups improved maximal squat performance (p < 0.001; NH Δ29.10 kg, d = -1.72; HH Δ22.61 kg, d = -1.34; N Δ16.82 kg, d = -0.99), VL MTh (p < 0.001; HH Δ0.25 cm, d = -0.66; N Δ0.24 cm, d = -0.63; NH Δ0.10 cm, d = -0.25), and VL maximal compound muscle action potential (Mmax) (p = 0.035; HH Δ1.13 mV, d = -0.65; NH Δ1.07 mV, d = -0.61; N Δ0.61 mV, d = -0.35), regardless of environmental condition. Conversely, the maximum motor-evoked potential obtained from the recruitment curve (RC_MEPmax) decreased following RT (p < 0.001; NH Δ-0.16, d = 1.39; HH Δ-0.08, d = 0.73; N Δ-0.04, d = 0.36). Other CSE markers remained unchanged. Conclusions: Eight weeks of RT under moderate hypoxia induced increases in Mmax and reductions in RC_MEPmax, suggesting training-related neuromuscular adjustments enhancing motor efficiency under hypoxic stress. These adaptations were accompanied by meaningful strength gains, although the mechanisms underlying their relationship remain to be fully elucidated.