Centre for Heart, Lung and Vascular Health, School of Health and Exercise Sciences, University of British Columbia, Kelowna, BC, Canada.
J Physiol. 2018 Nov;596(22):5365-5377. doi: 10.1113/JP276599. Epub 2018 Oct 13.
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.
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 (eDO ). 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 (CI ), and half-relaxation time (HRT) recovered faster in Sherpa than in lowlanders (percentage of baseline at 1 min: pRFD: 89% vs. 74%; CI : 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, eDO was markedly increased (1266% baseline). The skeletal muscle of Sherpa seemingly favours repeated force production at altitude for similar oxygen delivery compared to lowlanders.
高海拔地区的低氧张力会影响低地居民的表现。在这种环境下，夏尔巴人表现出出色的表现，但对这个问题知之甚少。夏尔巴人在肌肉水平上具有独特的基因型和表型特征，这可能使他们在高海拔地区比低地居民更能抵抗外周疲劳。我们研究了在年龄匹配的低地居民和夏尔巴人中，逐渐上升到 5050 米的高度和暴露在高海拔环境下对周围疲劳的影响，使用间歇性电诱发的膝关节伸展肌收缩。在协议的第一部分，夏尔巴人的外周疲劳（力量损失）较低。在协议结束后，夏尔巴人的力发展率和收缩冲动恢复速度比低地居民快。在任何时候，两组之间的肌肉氧合指数都没有差异。在不受意志疲劳影响的情况下，夏尔巴人的肌肉收缩特性变化较小。因此，收缩机制中的元素有助于解释夏尔巴人在高海拔地区出色的身体表现。
与海拔相关的适应过程的特点是明显的肌肉适应性变化。低地居民和夏尔巴人在肌肉的基因型和表型特征上存在差异，这可能会影响到高海拔地区的外周疲劳。在逐渐上升到 5050 米后，12 名低地居民和 10 名年龄匹配的夏尔巴人（分别为 32±10 岁和 31±11 岁）进行了三轮 75 次间歇性电诱发的股四头肌主动收缩（12 次脉冲，15 Hz，每次脉冲之间间隔 1.6 秒），起始强度可诱发 30%的最大自主力量。在协议结束后 1 分钟、2 分钟和 3 分钟，还进行了三次训练，以测量恢复情况。通过固定在股直肌上的近红外光谱探头来量化组织氧合指数（TOI）和总血红蛋白（tHb）。使用超声记录股浅动脉血流，并估计氧输送（eDO）。在第 1 轮结束时，夏尔巴人的峰值力大于低地居民（分别为 91.5%和 84.5%的基线，P<0.05）。夏尔巴人的峰值力发展率（pRFD）、收缩冲动的前 200 毫秒（CI）和半松弛时间（HRT）恢复速度比低地居民快（第 1 分钟的基线百分比：pRFD：89%和 74%；CI：91%和 80%；HRT：113%和 123%，P<0.05）。由于低地居民和夏尔巴人的血管措施在疲劳或恢复期间没有差异，因此将它们合并在一起（P<0.05）。在中间的第 3 轮，TOI 降低（90%的基线），而 tHb 增加（109%的基线）。在第 3 轮之后，eDO 明显增加（1266%的基线）。夏尔巴人的骨骼肌似乎在类似的氧气输送条件下更有利于在高海拔地区进行重复力量产生。
