Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC86
Assessment of the efficiency of two high-altitude walking strategies in humans
S. D. Myers1,2, K. Ashdown1, A. Wright2
1. Sport and Exercise Sciences, University of Chichester, Chichester, West Sussex, United Kingdom. 2. Birmingham Medical Research Expeditionary Society, Birmingham, West Midlands, United Kingdom.
Surveys of risk factors for acute mountain sickness (AMS) in trekkers report height gained as important (1), which is usually interpreted as the effect of the fall in the ambient partial pressure of oxygen. However, it could equally be due to or compounded by the effect of greater exercise required in achieving a higher altitude. Exercise at high-altitude has been linked with an increased risk of AMS (2) and is performed intermittently when the intensity is high or when working capacity is reduced (e.g. at high-altitude (3)). However, to-date the efficiency of exercise/work at high altitude has not been documented. Work efficiency, calculated from the total oxygen cost (VO2) of work, is lower in intermittent compared with continuous exercise, achieving the same average power output (4). We hypothesised that similar, but exaggerated results will be found at altitude and that perceived exertion would be greater with intermittent exertion (5). Our study assessed the efficiency of two trekking strategies, a steady continuous walk (CW) and an intermittent high-speed walk with rest (IW), when completed in hypoxia. Nine high altitude naive volunteers (age: 21±1 years; height 170±12 cm, body mass: 68.3±15.0 kg; hypoxia-specific peak uphill walking speed ((PWS) 2.1±0.2 m/s-1) completed two uphill (10%) walking sessions on a powered treadmill, following a crossover design, in a normobaric hypoxic chamber (simulated altitude 3500 m; O2 13.5%; CO2 0.04%; N2 balance; ambient temperature 10°C; relative humidity 20%); 1) CW: 30 min walk at 50% PWS; 2) IW: a timed walk to complete CW distance at PWS, with participants resting as they required. Breath-by-breath VO2 was measured concurrently with heart rate (HR) and arterial oxygen saturation (SpO2). Ratings of perceived exertion (RPE) and breathlessness were taken each min and blood lactate measured 3-min post-walk. Data were analysed with paired t-tests and Wilcoxon signed-ranks tests as appropriate. The mean total work was 124±37 kJ for which there was no difference in the time to complete between conditions (CW vs. IW: 1800 vs. 1729±291 s). Total VO2 was greater for the IW condition (3431.1±1551.9 vs. 5024.9±16361 L, p<0.05) as were peak and mean HR (peak: 168±24 vs. 192±13; mean: 129±13 vs. 161±11 b/min-1, p<0.01), but no difference in mean or minimum recorded SpO2. Ratings of perceived exertion and breathlessness were higher for the IW condition (RPE: 10±1 vs. 17±1; breathlessness: 2±1 vs. 5±1, p<0.01), as was blood lactate (1.23±0.47 vs. 7.15±1.13 mmol/L-1, p<0.01). The decreased efficiency and greater RPE for the IW condition in hypoxia agree with those findings reported by others for normoxia (4, 5). Importantly our findings indicate that a steady continuous walk is the most efficient strategy to adopt when high-altitude trekking and may reduce the risk of AMS.
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