UBC‐Nepal expedition: acclimatization to high‐altitude increases spinal motoneurone excitability during fatigue in humans

Abstract

The fatigue-induced failure of the motor cortex to drive muscles maximally increases in acute hypoxia (AH) compared to normoxia (N) but improves with acclimatization (chronic hypoxia; CH). Despite their importance to muscle output, it is unknown how locomotor motoneurones in humans are affected by hypoxia and acclimatization. Eleven participants performed 16 min of submaximal [25% maximal torque (maximal voluntary contraction, MVC)] intermittent isometric elbow flexions in N, AH (environmental chamber) and CH (7-14 days at 5050 m) (P O = 140, 74 and 76 mmHg, respectively). For each minute of the fatigue protocol, motoneurone responsiveness was measured with cervicomedullary stimulation delivered 100 ms after transcranial magnetic stimulation (TMS) used to transiently silence voluntary drive. Every 2 min, cortical voluntary activation (cVA) was measured with TMS. After the task, MVC torque declined more in AH (∼20%) than N and CH (∼11% and 14%, respectively, P < 0.05), with no differences between N and CH. cVA was lower in AH than N and CH at baseline (∼92%, 95% and 95%, respectively) and at the end of the protocol (∼82%, 90% and 90%, P < 0.05). During the fatiguing task, motoneurone excitability in N and AH declined to ∼65% and 40% of the baseline value (P < 0.05). In CH, motoneurone excitability did not decline and, late in the protocol, was ∼40% higher compared to AH (P < 0.05). These novel data reveal that acclimatization to hypoxia leads to a heightened motoneurone responsiveness during fatiguing exercise. Positive spinal and supraspinal adaptations during extended periods at altitude might therefore play a vital role for the restoration of performance after acclimatization to hypoxia.

Differences in muscle performance during fatigue may explain the differences in motoneurone excitability between acute and chronic hypoxia

“…Consistent with greater peripheral fatigue, the rise in EMG over the 16 min fatigue protocol was greatest in acute hypoxia (+58%), compared to normoxia (+22%) and chronic hypoxia (+11%) (see Table 2 of Ruggiero et al . ). The recruitment of more motor units (or increased firing rates) during the fatiguing contractions in acute hypoxia than in chronic hypoxia means that motoneurone activity was not well matched between conditions.…”

“…() compared the fatigue‐related declines in CMEPs of two sizes (15% and 50% of maximal M‐wave; elicited during a TMS‐induced silent period as in Ruggiero et al . ()) during a sustained submaximal contraction of 20% maximal EMG. The smaller CMEP declined more than the larger.…”

“…However, we urge caution with regard to the interpretation by Ruggiero et al . () given the differences in fatigue and associated differences in motoneurone activity across the experimental conditions. An experimental approach that would reduce the differences in motoneurone activity between conditions would be to perform contractions to only an EMG target instead of a force target.…”

Reply from Luca Ruggiero, Alexandra F. Yacyshyn, Jane Nettleton and Chris J. McNeil

“…5 A of Ruggiero et al . ), M max size increases with fatigue in N and AH but decreases in CH. To compensate for this reduction in peripheral excitability, greater descending drive would be required in CH compared to N and AH to achieve a targeted level of iEMG.…”

“…5 B of Ruggiero et al . ). Just prior to this time point, the absolute increase in integrated EMG (iEMG) during a matched torque contraction is 13% for AH, 5% for N, and 4% for CH.…”