The Runner's Breathing at Altitude
A 19-year-old club runner travels from sea level to train at 2,500 m for three weeks. On day 1 her resting breathing rate rises from 14 to 20 breaths per minute and her resting heart rate rises from 60 to 78 beats per minute. Her race pace over 5 km is 90 seconds per km slower than at sea level. By day 14 her resting breathing rate has fallen to 17 and her heart rate to 68, and her race pace has improved but is still 40 seconds per km slower than at sea level. A teammate suggests she should breathe pure oxygen during training to speed adaptation, and another suggests she should simply train harder to force adaptation.
1. Explain the day-1 changes in breathing and heart rate, and why race pace fell.
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At altitude the partial pressure of oxygen in air is lower, so the same volume of inspired air delivers less oxygen to the alveoli. The resulting lower oxygen level in the blood is detected by chemoreceptors, which increase the rate and depth of breathing. This raises oxygen uptake at the lungs and also blows off more carbon dioxide, making the blood slightly more alkaline. The heart rate rises to deliver the available oxygen around the body faster. Despite these responses, oxygen delivery to working muscles per minute is lower than at sea level, so aerobic respiration is limited and more energy must come from anaerobic respiration. Anaerobic respiration produces less ATP per glucose and builds up lactate, so the runner must slow down. The pace drop is therefore a direct consequence of reduced oxygen availability, not lack of effort.
2. Evaluate the two teammates' suggestions, and explain what the day-14 changes show about adaptation.
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Breathing pure oxygen during training would raise oxygen supply and remove the very stimulus that drives adaptation, so it would blunt rather than speed the useful response; it also does not mimic race conditions at altitude. Training harder is risky: the runner is already limited by oxygen delivery, so extra intensity adds fatigue and injury risk without increasing the adaptive stimulus beyond what the low oxygen already provides. The day-14 fall in resting breathing and heart rate, with improved pace, indicates genuine adaptation: increased red blood cell production raises oxygen-carrying capacity, and the initial alkalosis is corrected by the kidneys, reducing the chemoreceptor drive. The residual 40 seconds per km shows adaptation is incomplete at 14 days. The judgement is that the low oxygen itself is the stimulus, and adequate recovery plus time, not extra oxygen or extra intensity, is the sensible strategy.