Altitude acclimatization

Conventional wisdom recommends that endurance athletes either arrive about two weeks before the day before their high-altitude event. The body follows a predictable pattern of stress and adaptation during the first days at altitude, and this pattern makes days two through four the worst period to train or race¹.

Altitude’s Effect on the Body

Day 0 to 1 - Ventilation and resting heart rate rise as the body compensates for reduced oxygen. Plasma volume drops, increasing dehydration risk. Sleep is often disrupted by unstable breathing patterns¹.

Days 2 to 4 - This is the physiological low point. Athletes may experience acute mountain sickness symptoms such as headache, fatigue and nausea. Submaximal and maximal performance are impaired because acclimatization has not yet occurred. This is the main reason why arriving 2 to 4 days before a race typically feels much worse than expected^{1 2}.

Days 5 to 14 - Ventilatory efficiency starts to improve and sleep stabilizes. Perceived exertion decreases at a given workload. Meta-analysis of altitude exposure shows that hemoglobin mass begins increasing meaningfully during this period, which improves oxygen transport and endurance performance³.

Which strategy is better - Arriving two weeks early provides enough time for short-term acclimation plus early hematological changes, giving the best overall outcome^{1 3}. The day before is the next best because athletes race before the full burden of the day-two-to-four slump. The worst option is arriving mid-week, when the body is heavily stressed and nothing has adapted yet.

Performance at Altitude

Lower oxygen pressure reduces sustainable power. Based on the chart below, each additional 1,000 ft above sea level corresponds to roughly a 1% decrease in FTP up to about 5,000 ft; above that, the decline accelerates.

Heart rate trends higher, power trends lower, and RPE (rate of pereived exertion) becomes the most reliable guideline. Long climbs should feel like all-day endurance, not sea-level tempo.

Estimated available aerobic power for acclimatized and non-acclimatized athletes, adapted from Bassett et al. and Peronnet and Thibault^{4 5}.

What are my adjusted training zones?

Use the calculator below to determine your training zones at different altitudes:

Practical Examples

Sea level to Leadville - Ideally, arrive 12 to 14 days early. Spend the first days riding easily, hydrating well and stabilizing sleep. Introduce short tempo efforts after adaptation begins around days 5 to 7. Taper in the final days. If arriving the day before: finish taper at sea level, travel hydrated, keep pre-race activity minimal and pace conservatively by RPE.

Boulder to Leadville - Living at about 5,400 ft provides partial acclimatization, but 10,200 ft is still a significant jump. Five to seven days sleeping above 9,000 ft usually allows breathing, sleep and pacing to normalize¹. If arriving the day before, expect smaller losses than a sea-level athlete but still plan for reduced power on climbs.

Footnotes and Sources