Hypoxia and Fatigue Impair Rapid Torque Development of Knee Extensors in Elite Alpine Skiers

Abstract

This study examined the effects of acute hypoxia on maximal and explosive torque and fatigability in knee extensors of skiers. Twenty-two elite male alpine skiers performed 35 maximal, repeated isokinetic knee extensions at 180°s^-1 (total exercise duration 61.25 s) in normoxia (NOR, FiO₂ 0.21) and normobaric hypoxia (HYP, FiO₂ 0.13) in a randomized, single-blind design. Peak torque and rate of torque development (RTD) from 0 to 100 ms and associated Vastus Lateralis peak EMG activity and rate of EMG rise (RER) were determined for each contraction. Relative changes in deoxyhemoglobin concentration of the VL muscle were monitored by near-infrared spectroscopy. Peak torque and peak EMG activity did not differ between conditions and decreased similarly with fatigue (p < 0.001), with peak torque decreasing continuously but EMG activity decreasing significantly after 30 contractions only. Compared to NOR, RTD, and RER values were lower in HYP during the first 12 and 9 contractions, respectively (both p < 0.05). Deoxyhemoglobin concentration during the last five contractions was higher in HYP than NOR (p = 0.050) but the delta between maximal and minimal deoxyhemoglobin for each contraction was similar in HYP and NOR suggesting a similar muscle O₂ utilization. Post-exercise heart rate (138 ± 24 bpm) and blood lactate concentration (5.8 ± 3.1 mmol.l^-1) did not differ between conditions. Arterial oxygen saturation was significantly lower (84 ± 4 vs. 98 ± 1%, p < 0.001) and ratings of perceived exertion higher (6 ± 1 vs. 5 ± 1, p < 0.001) in HYP than NOR. In summary, hypoxia limits RTD via a decrease in neural drive in elite alpine skiers undertaking maximal repeated isokinetic knee extensions, but the effect of hypoxic exposure is negated as fatigue develops. Isokinetic testing protocols for elite alpine skiers should incorporate RTD and RER measurements as they display a higher sensitivity than peak torque and EMG activity.

Keywords: isokinetic; maximal torque production; near-infrared spectroscopy; neural drive; repeated knee extensions; simulated altitude.

FIGURE 1

Typical traces of deoxyhemoglobin concentration [HHb] (A) and total hemoglobin (tHb) (B) during the fatiguing protocol in normoxia (black line) and hypoxia (gray line) for one subject. Triangles represent maximal and minimal values measured during the last five contractions (mean values are presented in Table 1).

FIGURE 2

Maximal torque (A) and rate of torque development (B) values measured during the 34 maximal contractions performed in normoxia (black dots) and hypoxia (open circle). Data are means ± SD for 22 participants. #p < 0.05 hypoxia versus normoxia; ^∗p < 0.001 effect of fatigue.

FIGURE 3

Peak EMG (A) and rate of EMG rise (B) measured during the 34 maximal contractions performed in normoxia (black dots) and hypoxia (open circle). Data are means ± SD for 22 participants. #p < 0.05 hypoxia versus normoxia; ^∗p < 0.001 effect of fatigue.