Supraspinal Fatigue and Neural-evoked Responses in Lowlanders and Sherpa at 5050 m

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Abstract

Purpose

At high-altitude, Lowlanders exhibit exacerbated fatigue and impaired performance. Conversely, Sherpa (native Highlanders) are known for their outstanding performance at altitude. Presently, there are no reports comparing neuromuscular fatigue and its etiology between Lowlanders and native Sherpa at altitude.

Methods

At 5050 m, nine age-matched Lowlanders and Sherpa (31±10 vs. 30±12 years, respectively) completed a 4-minute sustained isometric elbow flexion at 25% maximal voluntary (MVC) torque. Mid-minute, stimuli were applied to the motor cortex and brachial plexus to elicit a motor evoked potential (MEP) and maximal compound muscle action potential (Mmax), respectively. Supraspinal fatigue was assessed as the reduction in cortical voluntary activation (cVA) from pre- to post-fatigue. Cerebral haemoglobin concentrations and tissue oxygenation index (TOI) were measured over the prefrontal cortex by near-infrared spectroscopy.

Results

Pre-fatigue, MVC torque and cVA were significantly greater for Lowlanders than Sherpa (79.5±3.6 vs. 50.1±11.3N·m, and 95.4±2.7 vs. 88.2±6.6%, respectively). With fatigue, MVC torque and cVA declined similarly for both groups (~24-26% and ~5-7%, respectively). During the task, MEP area increased more and sooner for Lowlanders (1.5min) than Sherpa (3.5min). Mmax area was lower than baseline throughout fatigue for Lowlanders but unchanged for Sherpa. TOI increased earlier for Lowlanders (2min) than Sherpa (4min). Total haemoglobin increased only for Lowlanders (2min). Mmax was lower while TOI and total haemoglobin were higher for Lowlanders than Sherpa during the second half of the protocol.

Conclusion

Although neither MVC torque loss nor development of supraspinal fatigue was different between groups, neural evoked responses and cerebral oxygenation indices were less perturbed in Sherpa. This represents an advantage for maintenance of homeostasis, presumably due to bequeathed genotype and long-term altitude adaptations.

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