Active Voice: Respiratory Muscle Endurance Training in Normoxia & Hypoxia - Differential Effects on Whole-Body Endurance Performance?
By Keisho Katayama, Ph.D.

Keisho Katayama, Ph.D.
Respiratory muscle endurance training (RMET) has been extensively studied, but the efficacy of this intervention on whole-body endurance exercise performance remains controversial. Exertional dyspnea and respiratory muscle fatigue have detrimental influences on cardiovascular responses and blood flow redistribution (respiratory muscle-induced metaboreflex) during exercise. RMET may contribute to increased whole-body endurance performance by promoting favorable adaptations in these functions

Acute hypoxia, reduced O2 availability in inspired air/body tissues, exaggerates whole-body, exercise-induced respiratory muscle fatigue compared to normoxia. The higher levels of metabolites during the increased work of breathing in hypoxia may improve respiratory muscle buffer capacity, as has been shown in previous studies involving exercise with the lower limbs. Thus, RMET under hypoxic conditions may induce greater improvements in respiratory muscle endurance by attenuated respiratory muscle metaboreflex and, subsequently, enhance wholebody endurance performance.

In our study, as described in the July 2019 issue of Medicine & Science in Sports & Exercise® (MSSE), my colleagues and I evaluated respiratory muscle endurance and respiratory muscle metaboreflex in subjects during hyperpnea (subjects voluntarily augmented their rate/depth of breathing) at rest and also during whole-body, endurance exercise performance before and after six weeks of RMET in normoxia and hypoxia. The subjects were 21 collegiate male endurance runners who were assigned to one of three groups: a control group, a normoxic training group or a hypoxic training group. The RMET was performed using isocapnic (i.e., maintaining the level of carbon dioxide) hyperpnea under normoxic and hypoxic conditions (30 minutes per day). In the hypoxic group, arterial oxygen saturation during RMET was set to 90% in the first two weeks and 80% thereafter. Respiratory muscle endurance and cardiovascular response during hyperpnea was measured by the incremental respiratory endurance test. A constant demand exercise test was performed on a treadmill at 95% of each subjectís peak oxygen uptake.

Respiratory endurance time increased, and the magnitude of the increase in arterial blood pressure during incremental respiratory endurance test was lower in the hypoxic and normoxic groups after RMET. During the constant exercise test, the dyspnea decreased, and time to exhaustion increased in both training groups after six weeks of RMET. The magnitude of these changes did not differ between the normoxic and hypoxic groups.

Our findings suggest that respiratory muscle endurance is improved by RMET in male endurance-trained runners. In part, this may be accompanied by a blunting of the respiratory muscle-induced metaboreflex ó leading to improvements in whole-body exercise endurance performance. However, there are no additional benefits when respiratory muscle endurance training is performed under hypoxic conditions. Future studies are needed to clarify the effect of RMET on exercise endurance performance in female runners and differences among individuals.

About the author:
Keisho Katayama, Ph.D., is an exercise physiologist at the Research Center of Health, Physical Fitness and Sports and the Graduate School of Medicine at Nagoya University in Japan. His research group focuses on how human respiratory and cardiovascular systems interact, respond and adapt to dynamic exercise and physical training. Dr. Katayama is a member of ACSM.

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