Neuromuscular Fatigue Induced by Cycling at a Fixed Level of Perceived Effort: Effects of Different Purported Hypoxic Methods

We compared neuromuscular fatigue induced by cycling at a fixed perceived effort in normoxic condition (NOR) and three purported hypoxia modalities: systemic hypoxia (SyH, FiO₂ = 0.13), blood flow restriction (BFR, 50% arterial occlusion pressure) and airflow restriction mask (ARM, calibrated to ~3500 m). Seventeen healthy young participants cycled for 20 min at a self-selected power output corresponding to a hard effort (15/20, Borg scale) on an innovative cycle ergometer allowing immediate neuromuscular evaluation. Isometric maximum voluntary contraction of the knee extensors (IMVC), central (voluntary activation, VA) and peripheral fatigue were measured before and every 5 min during cycling. Power output, peripheral oxygen saturation (SpO₂), quadriceps oxygenation (near-infrared spectroscopy, TSI) and pain were assessed throughout cycling. Power output was lower in BFR and SyH compared to NOR and ARM and was lower in BFR compared to SyH. SpO₂ was reduced only in SyH (mean 77% ± 4%). In all conditions, IMVC decreased from minute 5 and subsequently plateaued (~-10% to -20%), except in BFR, wherein it further declined to -40% ± 14% at minute 20 in the presence of lowered VA and exacerbated muscle pain compared to other conditions. Muscle TSI was further decreased in SyH compared to other conditions. Our results confirm the inability of ARM to induce hypoxia. Compared to other conditions, BFR showed a greater reduction in IMVC and VA, in the presence of a higher quadriceps pain and no greater muscle deoxygenation. These results underline the psychophysiological impact of quadriceps pain on both maximal and submaximal motor output.