I always knew I was slow hiking uphill at elevations of 8000 feet and above, which I attributed to lack of oxygen, but until I plotted data from my hikes some years ago, I never realized my uphill hiking speed began to noticeably slow down at about 5000 feet.
Everyone is affected by elevation at some level; the only variable is at what elevation it occurs. According to one reference below, I appear to be similar to "most" people. Another reference I read long ago places me among the most sensitive ~25% of the population.
This effect apparently catches many hikers unaware who go to high elevations, such as at Mt. Whitney, and expect their hiking speed will be the same as at lower elevations. It won't be; I've read articles about how surprised nearly all Mt. Whitney hikers are at how hard it is to breathe at those elevations, and how slowly they hike (step, step, step; rest; repeat). I've observed this same effect with many hikers at the Grand Canyon, who struggle to get out of the Canyon at elevations of ~6000 feet and above.
The following plot gives my average hiking speed for the entire trip, including uphill and downhill sections, and all rest stops:
Also shown is the median speed in each bin of 1000 feet, beginning with the bin of 0-1000 feet plotted at its average elevation of 500 feet. At about 5000 feet, the average declines noticeably. The average is not shown above 9000 feet since I have only a single data point there.
Only a small number of my hikes are in the above plot; the data come mostly from hikes that were done from mid-1999 to the end of 2000 when I was immediately digitizing my hike records. I have at least ten times more data that I will eventually fully digitize.
The following plot has attempted to isolate just the uphill sections, and includes no rest stops other than to catch my breath. It so far has many fewer points, since it is more laborious to gather the data for just the uphill sections. The data are not necessarily from the same hikes shown in the previous plot. The data are separated by the average elevation gain per mile on the uphill sections.
The precipitous decline in my uphill speed above 8000 feet elevation is very clear. You wouldn't want to hike with me at Mt. Whitney!
The plot also shows, not surprisingly, that my hiking speed declines as the trail gets steeper. The two points with over 900 feet of elevation gain per mile actually have 1400 and a whopping 2500 feet per mile, and are steep talus scrambles up the slope of Mt. Baldy and Dawson Peak from the north Devils Backbone Route.
Also shown are two models for the slowdown caused by the smaller amount of oxygen at higher elevations.
Model 1 comes from scientists who study human exercise physiology. Frank B. Wyatt reports in Physiological Responses to Altitude :
For most, oxygen consumption begins to decline at approximately 1500 m with a subsequent rate of decline of 3% per 300 m (1000 ft)[1]. There have been reports of declines in athletes as low as 580 m [4].1. Brooks, G.A., T.D. Fahey, T.P. White, and K.M. Baldwin (2000). Exercise Physiology: Human Bioenergetics and Its Applications, Third Edition. Mayfield Publishing Company. Mountain View, CA.
This agrees quite well with my data up to 8000 feet, with the decline beginning at 5000 feet (1500 m). However, above 8000 feet, my hiking speed falls off more rapidly than predicted by the model. (I note that Wyatt doesn't report over what elevation range his 3% decline applies; it might well be an approximation only below 8000 feet, for example.)
Model 2 assumes that above 5000 feet, my uphill hiking speed is directly proportional to the amount of oxygen in each breath, which in turn is proportional to the pressure at a given altitude. The Wikipedia article on Atmospheric Pressure gives a first-order approximation to the decline of oxygen with elevation, 5% per 1000 feet. This decline is larger than reported by Wyatt, and allows this model to fit my data up to 9000 feet.
Clearly, I slow down much more dramatically above 9000 feet than would be expected based solely on the amount of oxygen in the air.
Could there be something about the trails themselves that cause this slowdown? Not in general; the following plot shows that my downhill speed is unaffected by elevation overall for trails with less than 700 feet of elevation gain per mile:
Most of the points in the above plot are for the same sections of trails as in the previous plot. However, I didn't record data for all such sections, so I added a few points from other hikes.
Steeper trails do show a slower downhill speed due to the need to be careful in going steeply downhill.
One possible additional affect that might cause slower speeds at the highest elevations might be due to those elevations preferentially being at the end of a steep hike uphill. In that case, the slower uphill speed might be due to switching over to fat burning. The Wikipedia article on Aerobic exercise states:
Initially during aerobic exercise, glycogen is broken down to produce glucose, but in its absence, fat starts to decompose instead. This latter is a slow process, and is accompanied by a decline in performance level.
There is no doubt that I've used up my glycogen at the highest elevations, since I intentionally never eat while going uphill at those elevations. I learned a long time ago that my body could either digest food at high elevations, or hike uphill, and that it couldn't do both. I try to consume high-calorie liquids, like cola drinks and Tang, while going uphill, but I'm sure that isn't enough to prevent the switch to fat as fuel.
In contrast, I freely eat solid food on my way uphill when I am below 5000 feet elevation, so I am much less likely to have switched to fat-burning on those trails.
I'll plot more data in the future to see if I can tease that effect out of my data.