The effects of hypoxia on muscle deoxygenation and recruitment in the flexor digitorum superficialis during submaximal intermittent handgrip exercise

Abstract

Background: Decreased oxygenation of muscle may be accentuated during exercise at high altitude. Monitoring the oxygen saturation of muscle (SmO₂) during hand grip exercise using near infrared spectroscopy during acute exposure to hypoxia could provide a model for a test of muscle performance without the competing cardiovascular stresses that occur during a cycle ergometer or treadmill test. The purpose of this study was to examine and compare acute exposure to normobaric hypoxia versus normoxia on deoxygenation and recruitment of the flexor digitorum superficialis (FDS) during submaximal intermittent handgrip exercise (HGE) in healthy adults.

Methods: Twenty subjects (11 M/9 F) performed HGE at 50% of maximum voluntary contraction, with a duty cycle of 2 s:1 s until task failure on two occasions one week apart, randomly assigned to normobaric hypoxia (FiO₂ = 12%) or normoxia (FiO₂ = 21%). Near-infrared spectroscopy monitored SmO₂, oxygenated (O₂Hb), deoxygenated (HHb), and total hemoglobin (tHb) over the FDS. Surface electromyography derived root mean square and mean power frequency of the FDS.

Results: Hypoxic compared to normoxic HGE induced a lower FDS SmO₂ (63.8 ± 2.2 vs. 69.0 ± 1.5, p = 0.001) and both protocols decreased FDS SmO₂ from baseline to task failure. FDS mean power frequency was lower during hypoxic compared to normoxic HGE (64.0 ± 1.4 vs. 68.2 ± 2.0 Hz, p = 0.04) and both decreased mean power frequency from the first contractions to task failure (p = 0.000). Under both hypoxia and normoxia, HHb, tHb and root mean square increased from baseline to task failure whereas O₂Hb decreased and then increased during HGE. Arterial oxygen saturation via pulse oximetry (SpO₂) was lower during hypoxia compared to normoxia conditions (p = 0.000) and heart rate and diastolic blood pressure only demonstrated small increases. Task durations and the tension-time index of HGE did not differ between normoxic and hypoxic trials.

Conclusion: Hypoxic compared to normoxic HGE decreased SmO₂ and induced lower mean power frequency in the FDS, during repetitive hand grip exercise however did not result in differences in task durations or tension-time indices. The fiber type composition of FDS, and high duty cycle and intensity may have contributed greater dependence on anaerobiosis.

Keywords: Exercise; Hypoxia; Near-infrared spectroscopy; Skeletal muscle.

Fig. 1

Experimental protocol. ECG electrocardiogram, EMG electromyography, FDS flexor digitorum superficialis, MVC maximal voluntary contraction. NIRS near-infrared spectroscopy, TA tibialis anterior

Fig. 2

Muscle saturation of oxygen (SmO₂) and changes in oxy- (ΔO₂Hb), deoxy- (ΔHHb) and total hemoglobin (ΔtHb) during each quintile of hand grip exercise (HGE) from baseline until task failure (100%) in the flexor digitorum superficialis under hypoxic and normoxic conditions. Values are presented as mean ± SEM. Open markers indicate significant differences from baseline, asterisks denote significant differences from task failure, † indicates significant difference between hypoxic and normoxic conditions (p < 0.05)

Fig. 3

Root mean square and mean power frequency at the start of the trial and task failure during hand grip exercise (HGE) in the flexor digitorum superficialis. Trial start and task failure represent 1 s segments of sEMG signal from one of the first three contractions and one of the final three contractions during the HGE, respectively. Values are presented as mean ± SEM. Open markers indicate significant differences from trial start, dashed line indicates significant differences between gases (p < 0.05)

Fig. 4

Correlation between muscle saturation of oxygen (SmO₂) and arterial saturation of oxygen measured by pulse oximetry (SpO₂) (r = − 0.01, not significant). Points represent SmO₂ and SpO₂ values collected over the 6 time-points of the hand grip exercise trial for each subject during hypoxia