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. 2018 Nov;596(22):5365-5377.
doi: 10.1113/JP276599. Epub 2018 Oct 13.

UBC-Nepal expedition: peripheral fatigue recovers faster in Sherpa than lowlanders at high altitude

Luca Ruggiero  1 , Ryan L Hoiland  1 , Alexander B Hansen  1 , Philip N Ainslie  1 , Chris J McNeil  1
Affiliations

UBC-Nepal expedition: peripheral fatigue recovers faster in Sherpa than lowlanders at high altitude

Luca Ruggiero et al. J Physiol. 2018 Nov.

Abstract

Key points: The reduced oxygen tension of high altitude compromises performance in lowlanders. In this environment, Sherpa display superior performance, but little is known on this issue. Sherpa present unique genotypic and phenotypic characteristics at the muscular level, which may enhance resistance to peripheral fatigue at high altitude compared to lowlanders. We studied the impact of gradual ascent and exposure to high altitude (5050 m) on peripheral fatigue in age-matched lowlanders and Sherpa, using intermittent electrically-evoked contractions of the knee extensors. Peripheral fatigue (force loss) was lower in Sherpa during the first part of the protocol. Post-protocol, the rate of force development and contractile impulse recovered faster in Sherpa than in lowlanders. At any time, indices of muscle oxygenation were not different between groups. Muscle contractile properties in Sherpa, independent of muscle oxygenation, were less perturbed by non-volitional fatigue. Hence, elements within the contractile machinery contribute to the superior physical performance of Sherpa at high altitude.

Abstract: Altitude-related acclimatisation is characterised by marked muscular adaptations. Lowlanders and Sherpa differ in their muscular genotypic and phenotypic characteristics, which may influence peripheral fatigability at altitude. After gradual ascent to 5050 m, 12 lowlanders and 10 age-matched Sherpa (32 ± 10 vs. 31 ± 11 years, respectively) underwent three bouts (separated by 15 s rest) of 75 intermittent electrically-evoked contractions (12 pulses at 15 Hz, 1.6 s between train onsets) of the dominant leg quadriceps, at the intensity which initially evoked 30% of maximal voluntary force. Trains were also delivered at minutes 1, 2 and 3 after the protocol to measure recovery. Tissue oxygenation index (TOI) and total haemoglobin (tHb) were quantified by a near-infrared spectroscopy probe secured over rectus femoris. Superficial femoral artery blood flow was recorded using ultrasonography, and delivery of oxygen was estimated (eDO2 ). At the end of bout 1, peak force was greater in Sherpa than in lowlanders (91.5% vs. 84.5% baseline, respectively; P < 0.05). Peak rate of force development (pRFD), the first 200 ms of the contractile impulse (CI200 ), and half-relaxation time (HRT) recovered faster in Sherpa than in lowlanders (percentage of baseline at 1 min: pRFD: 89% vs. 74%; CI200 : 91% vs. 80%; HRT: 113% vs. 123%, respectively; P < 0.05). Vascular measures were pooled for lowlanders and Sherpa as they did not differ during fatigue or recovery (P < 0.05). Mid bout 3, TOI was decreased (90% baseline) whereas tHb was increased (109% baseline). After bout 3, eDO2 was markedly increased (1266% baseline). The skeletal muscle of Sherpa seemingly favours repeated force production at altitude for similar oxygen delivery compared to lowlanders.

Keywords: contractile properties; electrical stimulation; hypoxia; muscle oxygenation; quadricep.

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Figures

Figure 1
Figure 1. Contractile parameters of the 15 Hz tetani during the fatiguing protocol and recovery
Mean values (± SEM) in lowlanders (circles, continuous line) and Sherpa (inverted triangles, dashed line). All values are expressed as percentage of baseline (mean value of the first 5 tetani of the fatiguing protocol). Open symbols represent data points significantly different from baseline. A, peak force was significantly decreased at all time points in lowlanders during the fatiguing protocol, while in Sherpa a decrease was evident only from the end of the first bout (P < 0.05). In addition, the decline from baseline was greater in lowlanders than Sherpa at the end of the first bout (* P < 0.05). During recovery, post hoc testing on pooled data showed a decrease in peak force throughout (P < 0.05). B, peak rate of force development (pRFD) was significantly slower in both groups (post hoc testing on pooled data) during the fatiguing protocol (P < 0.05). After the fatiguing protocol, pRFD remained depressed in lowlanders (P < 0.05) but recovered by 1 min in Sherpa. As a result, pRFD was faster in Sherpa than in lowlanders throughout recovery (* P < 0.05). C, peak rate of force relaxation (pRFR) was significantly slower in both groups (post hoc testing on pooled data) during both the fatiguing protocol and recovery (P < 0.05). D, half‐relaxation time (HRT) was slowed in both groups (post hoc testing on pooled data) at all time points during the fatiguing protocol (P < 0.05). Throughout recovery, HRT remained increased in both groups (P < 0.05) and was slower in lowlanders than in Sherpa (* P < 0.05).
Figure 2
Figure 2. Contractile impulse of the 15 Hz tetani during the fatiguing protocol and recovery
Mean values (± SEM) in lowlanders (circles, continuous line) and Sherpa (inverted triangles, dashed line). All values are expressed as percentage of baseline (mean value of the first 5 tetani of the fatiguing protocol). Open symbols represent data points significantly lower than baseline. A, contractile impulse (CI), calculated as the area under the whole force–time curve of the tetani, was lower than baseline in both groups (post hoc testing on pooled data) during both the fatiguing protocol and recovery (P < 0.05). B, contractile impulse of the first 200 ms of the tetani (CI200) was decreased from baseline in both groups (post hoc testing on pooled data) during the fatiguing protocol (P < 0.05). During recovery, CI200 was still decreased in lowlanders at all time points (P < 0.05) but only at 3 min in Sherpa (P < 0.05). When compared between groups, the recovery of CI200 was greater in Sherpa than in lowlanders (* P < 0.05).
Figure 3
Figure 3. Muscle near‐infrared spectroscopy parameters across the fatiguing protocol and recovery
Mean values (±SEM) in lowlanders (circles, continuous line) and Sherpa (inverted triangles, dashed line). All variables were calculated over 15 s time windows at 9 points: just before the start and in the middle of the first, second and third bout of stimuli, and just before the trains delivered at R1, R2 and R3. All values are expressed as percentage of baseline, which is represented by variables calculated just before the beginning of the first bout. For all measures, data were pooled for the two groups because the two‐way repeated measures ANOVAs reported only main effects of time. Open symbols represent data points significantly different from baseline. A, muscle tissue oxygenation index (TOI) was significantly decreased at all time points during the fatiguing protocol except the start of the third bout (P < 0.05), and greater than baseline throughout recovery (P < 0.05). B, total haemoglobin (tHb) was increased at all time points during both fatigue and recovery (P < 0.05). C, oxygenated haemoglobin (O2Hb) showed a decrease during the first bout (P < 0.05), whereas at the start of the second and third bout as well as during recovery it was increased compared to baseline (P < 0.05). D, deoxygenated haemoglobin (HHb) showed a significant increase throughout the fatiguing protocol (P < 0.05), whereas it was decreased from baseline at any time point during recovery (P < 0.05).
Figure 4
Figure 4. Superficial femoral artery mean blood flow and estimated delivery of oxygen during the fatiguing protocol and across recovery
Mean values (± SEM) in lowlanders (circles, continuous line) and Sherpa (inverted triangles, dashed line). A and B, absolute (A) and normalised (B) values of superficial femoral artery mean blood flow during the protocol, determined as the average across 1 min before the start of the fatiguing protocol (baseline), the 15 s of rest between each bout, and each 1 min window during recovery. C and D, absolute (C) and normalised (D) values of superficial femoral artery estimated delivery of oxygen. Open symbols represent data points significantly greater than baseline. For all measures, data were pooled for the two groups because the two‐way repeated measures ANOVAs resulted in main effects of time only. All variables were significantly greater than baseline throughout the fatiguing and recovery periods (P < 0.05).
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