The effect of hypoxia (10% FiO2) on neuromuscular activity during repeated cycle-sprints in young athletes

Abstract

Introduction: A decrease in oxygen concentration produced by hypoxia training in exercising muscles, or metabolic stress, is proposed to stimulate various physiological processes associated with muscle hypertrophy. One possible mechanism is an increase in motor unit recruitment (volume). Another is a change in the muscle fibre type recruitment ratio, away from type-l motor units (oxidative) contribution to force production toward type-2 motor units (glycolytic) to complete an exercise task. However there is conflicting evidence regarding whether such training produces similar hypertrophic and or strength/force production increases as traditional resistance training. but with lower injury risk. Aim: This study investigated neuromuscular recruitment patterns using electromyography (EMG), of the vastus lateralis (VL) during rest and after fatiguing exercise during either continuous normoxia or hypoxia (Fraction inspired oxygen = 10% - hypoxicator, G02Altitude). Methods: The effects on VL muscle strength (leg extensions) in a cross-over study were examined in eighteen athletes (13 males, 5 females) before and after completing 10 x of 6-second cycle-sprints. At 2 timepoints post-exercise, immediately after, and again 5 minutes later, EMG, heart rate variability, maximal voluntary contraction (MVC), near-infrared-spectroscopy, peak. power output and arterial oxygen saturation were compared to 2 pre-exercise sets ("Baseline."' and "Pre"). Results: Final post-exercise MVC was significantly lower (7.5%) than baseline across time but not by condition. Comparative frequency analysis of the percentage change from pre to post-exercise in EMG area, at low 1-29 Hz (type-1 fibre activity) and high 75-100 Hz (type-2 fibre activity), revealed a significant reduction in type-1 fibre area/activity relative to type-2, by between 20-30% during hypoxia with a 10% difference in type-1 fibre activity between conditions. Significantly higher non-linear analysed sympathetic nervous system (SNS) tone "Immediate" post-exercise occurred in hypoxia, with positive correlations between SNS index at low (1-29 Hz) and high band (75-100 Hz) EMG areas (R2 = 0.47, P< 0.002 and R2 0.56, P< 0.0003). Conclusion: Exercise in hypoxia appeared to cause a temporary increase in central command mediated SNS tone and greater recruitment of type-2 muscle fibers with an accompanying reduction in type-1. Acute hypoxia training may benefit type-2, fast twitch muscle fiber conditioning.