AltitudeOmics: exercise-induced supraspinal fatigue is attenuated in healthy humans after acclimatization to high altitude

Abstract

Aims: We asked whether acclimatization to chronic hypoxia (CH) attenuates the level of supraspinal fatigue that is observed after locomotor exercise in acute hypoxia (AH).

Methods: Seven recreationally active participants performed identical bouts of constant-load cycling (131 ± 39 W, 10.1 ± 1.4 min) on three occasions: (i) in normoxia (N, PI O2 , 147.1 mmHg); (ii) in AH (FI O2 , 0.105; PI O2 , 73.8 mmHg); and (iii) after 14 days in CH (5260 m; PI O2 , 75.7 mmHg). Throughout trials, prefrontal-cortex tissue oxygenation and middle cerebral artery blood velocity (MCAV) were assessed using near-infrared-spectroscopy and transcranial Doppler sonography. Pre- and post-exercise twitch responses to femoral nerve stimulation and transcranial magnetic stimulation were obtained to assess neuromuscular and corticospinal function.

Results: In AH, prefrontal oxygenation declined at rest (Δ7 ± 5%) and end-exercise (Δ26 ± 13%) (P < 0.01); the degree of deoxygenation in AH was greater than N and CH (P < 0.05). The cerebral O2 delivery index (MCAV × Ca O2 ) was 19 ± 14% lower during the final minute of exercise in AH compared to N (P = 0.013) and 20 ± 12% lower compared to CH (P = 0.040). Maximum voluntary and potentiated twitch force were decreased below baseline after exercise in AH and CH, but not N. Cortical voluntary activation decreased below baseline after exercise in AH (Δ11%, P = 0.014), but not CH (Δ6%, P = 0.174) or N (Δ4%, P = 0.298). A twofold greater increase in motor-evoked potential amplitude was evident after exercise in CH compared to AH and N.

Conclusion: These data indicate that exacerbated supraspinal fatigue after exercise in AH is attenuated after 14 days of acclimatization to altitude. The reduced development of supraspinal fatigue in CH may have been attributable to increased corticospinal excitability, consequent to an increased cerebral O2 delivery.

Keywords: adaptation; altitude; exercise; transcranial magnetic stimulation.

Figure 1

Mean area of motor evoked potentials (MEP) recorded from the vastus lateralis (VL) in response to stimulation over the motor cortex during varying contraction intensities pre- (○) and post-exercise (●) (mean for all conditions). The TMS responses were compared to the area of the maximal M-wave (M_max) evoked by peripheral stimulation of the femoral nerve. Data are means ± SE for 7 participants.

Figure 2

Cerebral oxygenation at resting baseline, during the final 30 s of a 3 min warm up (28 W) and during the final 30 s of constant-load exercise (131 W) in normoxia (N; panel a), acute hypoxia (AH; panel b) and chronic hypoxia (CH; panel c). Data are means ± SE for 7 participants. † P < 0.05 vs. respective baseline; ‡ P < 0.05 vs. respective warm up; * P < 0.05 vs. AH; # P < 0.05 vs. CH. Resting baseline in AH denotes the value after 10 min wash in of the hypoxic gas. O₂Hb, oxygenated haemoglobin; HHb, deoxygenated haemoglobin and THb, total haemoglobin.

Figure 3

Arterial oxygen saturation (S_pO₂) (a), cerebral blood flow velocity (MCA_v) (b) and middle cerebral artery O₂ delivery index (MCA_v × C_aO₂) during constant-load exercise (131 W) in normoxia (N), acute hypoxia (AH), and chronic hypoxia (CH). Values are plotted for the duration of the shortest trial (8 min) and extrapolated to the group mean exercise time (10.1 min). Data are means ± SE for 7 participants. † P < 0.05 vs. rest; * P < 0.05 vs. N; # P < 0.05 vs. CH.

Figure 4

Representative MEPs evoked during knee extensor contractions at 50% MVC before exercise in each condition. Traces are shown from a representative participant in each condition; 8 stimuli were delivered from which an average value was obtained. Note the increase in MEP amplitude (corticospinal excitability) after acclimatisation.

Figure 5

Cortical voluntary activation measured before (open bars) and immediately after (<2.5 min; closed bars) constant-load exercise (131 W) in normoxia (N), acute hypoxia (AH), and chronic hypoxia (CH). * P < 0.05 pre- vs. post-exercise.