Muscle deoxygenation during sustained and intermittent isometric exercise in hypoxia

Area of Science:

• Exercise Physiology
• Sports Science
• Muscle Physiology

Background:

• Locomotor muscle fatigue develops faster during intermittent isometric exercise in hypoxia compared to normoxia.
• However, studies on sustained isometric contractions show inconsistent effects of hypoxia on fatigue development, potentially due to sustained ischemia from high intramuscular pressure.

Purpose of the Study:

• To investigate the hypothesis that differences in muscle deoxygenation between normoxia and hypoxia are smaller during sustained isometric exercise than intermittent exercise.
• To explore how these deoxygenation differences may explain inconsistent findings on hypoxia's effect on muscle fatigue.

Main Methods:

• Subjects performed sustained and intermittent isometric knee-extension exercises at 60% maximal voluntary contraction to exhaustion under normoxic and hypoxic conditions.
• Muscle oxygenation (using near-infrared spectroscopy to measure deoxyhemoglobin/myoglobin and tissue oxygenation index) and surface electromyography (EMG) of the vastus lateralis were recorded.

Main Results:

• Intermittent exercise in hypoxia showed significantly greater increases in deoxyhemoglobin/myoglobin and reductions in tissue oxygenation index compared to normoxia.
• The rate of increase in integrated EMG was also accelerated in hypoxia during intermittent exercise.
• Conversely, no significant differences in near-infrared spectroscopy variables or EMG were observed during sustained isometric exercise between normoxia and hypoxia.

Conclusions:

• Muscle deoxygenation is significantly exaggerated during intermittent isometric exercise in hypoxia compared to normoxia.
• During sustained isometric exercise, the degree of muscle deoxygenation remains similar between normoxic and hypoxic conditions.
• These differing deoxygenation patterns likely influence muscle fatigability by altering the rate of metabolite accumulation.