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Power Models  :  Effects of Altitude Model
FTP (Sea Level, Watts)
Aero Bars (Typical)
Aero Bars Optimised (Typical)
Est: Height-Weight (RoadBike)
Est: Height-Weight (TTBike)
Est: Height-Weight (TriBike)
Pro Rider (RoadBike)
Pro Rider (TTBike)
Weight (Rider +Bike)
Road Gradient (%)
Power Decay Model
Non-Acclimatised Rider (Bassett et al. 1999)
Acclimatised Rider (Bassett et al. 1999)
Environment & Physiology
Combined Effect of Altitude
Air Pressure mb
Mouse over boxed values for percentage of sea level value
(% Sea level)
During the summer months in particular more and more cyclists of all levels find themselves taking on aspirational challenges in the high mountains, consciously or not exposing themselves to the effects of altitude. Most people have an idea that altitude poses certain challenges to human physiology, it’s one of the well-known reasons why climbers die on Everest, but serious cycling fans also know that the thin air of high altitude velodromes such as in Mexico City (2300 metres), Bogota (2600 metres) and La Paz (3417 metres) may conversely allow a cyclist to ride faster, in spite of those physiological challenges.
Effects of Altitude
There are two main effects of riding at altitude, one hindering the cyclist, the other helping him.
As we go higher the air becomes less dense, meaning one breath of air contains less oxygen molecules. It’s a common misconception that the percentage oxygen content of air reduces with altitude; in fact it’s a fairly constant 21% of dry air volume throughout the lowest level of the earth’s atmosphere because lower density air contains less of all gasses. There is no significant effect on human physiology below altitudes of 1500 metres because below that level we have no problems maintaining close to 100% oxygen saturation in our haemoglobin (the oxygen carrying component of blood), higher than 1500 metres though is a different story. Beyond 1500 metres the symptoms of oxygen scarcity familiar to many will be increased rates of respiration and increased heart rates as the body works harder and harder to extract oxygen from the air entering the lungs and to maintain the quantities reaching working muscle. The inevitable effect is a reduction in VO2max or the rate at which oxygen is available for energy release, something which has a very real impact on power output at all levels of performance. Check our
HR-VO2-Power Relationship Model
to understand why.
As air becomes less dense it poses less resistance to the forward movement of a cyclist. We have talked about and modelled the significance of low air density “float days” in time trials within our
Power Sector (Time Trial) Model
. We should keep in mind however that the benefits of lower density air are realised much more by a fast moving cyclist who is using the majority of his power to overcome aerodynamic drag than a cyclist using most of his power to ride slowly up a steep mountain gradient.
The question of whether a cyclist can ride faster at altitude hinges on a trade-off. Is his increase in speed due to reduced air density greater than the speed he loses due to the oxygen limited reduction in power output, something which itself varies with acclimatisation? As a general rule the answer tends to be “yes”, he will ride faster if his power output is high and he is riding on the flat (e.g. in a velodrome environment) whereas if he is slow and/or climbing then “no”, altitude only leads to slowness. At Cycling Power Models though we don’t trust “general rules” so we present this comprehensive model to help you investigate
the effects of altitude on any cyclist, of any power output, on any gradient, and through a range of potentially applicable altitudes.
FTP (Sea Level). The riders Functional Threshold Power or some other measure of sustainable aerobic power you are interested in as a sea level value.
Rider + Bike Weight (Kilos). Input total weight in kilos (e.g. 80).
CdA. This is the metric of aerodynamic drag (
rag x frontal
rea) applicable to the rider and bike combined. The figure is expressed in metres squared. Typical cycling values are in the range .20 to .30
Road Gradient (%). Select an applicable road gradient in percent.
Power Decay Model. Select an empirical model of the relationship between athlete VO2max and altitude. The Models of Bassett et al. (1999) in
Comparing cycling world hour records, 1967-1996: modeling with empirical data
are based on a large sample of data from elite athletes and demonstrate high predictive power. Given the constancy of cycling efficiency at different altitudes and of this relationship at fractional VO2max utilisations the percentage reduction in a riders FTP is taken to be equal to percentage recuction in VO2max.
An acclimatised rider is defined as having spent more than 7 days at ride altitude.
This model allows the study of power and speed effects through a range of altitudes from 0 metres (sea level) through to 4,500 metres (
the highest paved road in Europe, the Pico de Veleta near Granda in southern Spain, rises to 3,380 metres
the highest paved road in the USA, Mount Evans in Colorado, rises to 4,465 metres
). At each altitude we show "standard atmosphere" air pressure relative to sea level as well as the key physiological impact of such lower air pressures - reduced blood oxygen saturations. A nice resource detailing the physical and physiological implications of high altitude stems from the aviation safety literatuure and can be found
Outputs - Physiological Loss
Watts. The level of the riders FTP (according to the selected power decay model) at the specific altitude.
This demonstrates just the power output impairment expected at altitude.
Physiological Loss – Kph. The speed the rider would achieve (given selected choices for CdA, weight and road gradient) at the altitude specific FTP but assuming that the air pressure acting in aerodynamic drag on the rider and bike remains at a standard sea level value (1013 mb).
This isolates the effect on the riders speed (a reduction) due solely to the physiological impairment of altitude.
Outputs - Aerodynamic Gain
The riders achieves a certain speed "S" at his altitude specific FTP (given selected choices for CdA, weight and road gradient) and at the altitude specific air density.
Watts. The difference in altitude specific FTP and the power output that would be required to achieve the same speed "S" given sea level air density acting in aerodynamic drag on the rider and bike.
This isolates the effect on the rider’s power requirement (a reduction) due solely to the aerodynamic benefit of thinner air at altitude.
Kph. The difference between speed "S" and the speed that would be achieved given the same power output if riding with sea level air pressure acting in aerodynamic drag on the rider and bike.
This isolates the effect on the riders speed (an increase) due solely to the aerodynamic benefit of thinner air at altitude.
Outputs - Combined Effect
Watts. Physiological Loss (Watts) + Aerodynamic Gain (Watts). The expected level of the riders power output due to the physiological impairment of altitude (a reduction) increased by the aerodynamic benefit of thinner air at altitude expressed as a wattage saving.
Kph. The difference in the riders speed given altitude specific FTP and the aerodynamic impact of altitude specific air density (speed "S" above) and the riders speed given values at sea level.
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