Cycling and Endurance Sport Training Information :
Physiological Effects of Acute Altitude Exposure

By
Chris Harnish

   Possibly the most common misunderstanding about altitude is the belief that high altitude air is “thinner”, or contains less oxygen. However, the air on Mt. Everest contains the same percentage of oxygen, carbon dioxide and nitrogen as the air I’m breathing in Cape Cod, albeit without as much pollution. What is different is the barometric pressure (PB). Case in point, I’m pretty much at sea level here on the Cape and my barometric pressure is approximately 760 mmHg. Compare this to the summit of Mt. Everest (~29,000 ft) where barometric pressure is only 200 mmHg. Great, the air pressure is less, but what does that have to do with athletic performance at altitude? Everything! To understand why PB important we need to understand how our lungs and muscles deal with oxygen and carbon dioxide.

   When we breathe, oxygen (among other gases) enters the lungs and gets picked up by the blood through the interaction of tiny air sacks in the lungs (alveoli) to the capillaries, whereas carbon dioxide carried by the blood gets dumped off in the alveoli to be exhaled. The rate of diffusion is driven by gas concentrations (i.e., the amount of a gas in a particular space) and is affected by barometric pressure; when pressure decreases gas molecules expand and thus take up more space within a given area. So while the percentage of gases is the same, the actual concentration is lower at altitude. It is this last fact that is at the heart of your decreased performance at altitude, because as blood passes through the lungs it needs a certain amount of time to transfer gases. At sea level, transit time (of blood passing by those air sacs) is rarely so short that problems arise (assuming you’re an average healthy Joe), but when exercising at altitude transit time is a factor because gas concentrations are lower in the lungs (remember the gas molecules are bigger now). Figure 1 depicts this effect. The lower gas concentrations are why your breathing rate increases when you first arrive at altitude. Other changes occur as well, and are outlined in table 1 below. Unfortunately, these changes just aren’t enough at a mile or more above seas level.

Strategies for Racing at Altitude

   Riders planning on competing at altitude often consider early arrival to acclimate, but most misunderstand what is needed to do so. Current research supports to approaches:

   1. Arrive 10 - 30 days prior to competition (Best), but continue to train mainly at seas level.
   2. Arrive the day before competition (Practical), because research shows that 18 hours of acclimatization is as good as 47 hours of acclimatization.

   Obviously strategy 1 is the best, but most of us can’t hang out for 2 weeks before our race, leaving us with strategy 2. As indicated above, recent research showed that short acclimatization, while not as ideal as full acclimatization, offers a more practical approach for the masses. The reason being that the body’s initial response is to improve oxygen delivery to the muscles by increasing ventilation and plasma volume. The loss of body water can be compensated for by concerted hydration, but the dehydration occurring decrease muscle size making it easier to unload oxygen from the blood to the muscles. However, questions still remain as to why such a short period of time is as effective as a few days.

   Taking all this into consideration, some of you may still choose to arrive 2 weeks or more before competition, or perhaps you plan a foray to altitude to pick up a few extra red blood cells. While the long-term acclimatization is prudent, the effects of altitude training are still controversial. For example, research in the last few years indicates that there is little, if any change in the number of red blood cells. Nonetheless, if you plan to stay at altitude for an extended period of time some considerations must be made.

Live High, Train Low

    No doubt many of us have heard the title phrase, but my experience has shown that most cyclists still believe that training at high altitude is some how superior to sea level training. Unfortunately, research on this issue is clear; extended training periods at high altitude (1 mile or more above sea level) often leads to DECREASED performance. When you consider that your performance will always be lower at altitude than at sea level, then the reason why performance goes down is pretty clear. If it isn’t consider this example:

   Joe Ryder needs to train at 320 watts to improve his aerobic power (AP) while at sea level because its about 20 watts over his threshold (i.e., it creates and overload on his body) and training at this wattage feels very hard and elicits a heart rate (HR) of 180 beats. Now Joe spends 2 weeks in Utah (~6000 ft) and continues his AP training, but repeatedly finds that he can only hold 270-280 watts, despite his HR being over 180 and his effort very high. Upon return to sea level, Joe finds that his AP decreased to 300 watts and no change in threshold.

   This example, while a bit extreme, indicates the problems of training at altitude; namely, your effort feels sufficient, but your output is lower. To put it another way, you’re doing less training relative to the overall load on your body. If you reduce your load, you can actually detrain and lose fitness. This is exactly what happens with long-term training at altitude. This is also the premise behind the Live high, train low strategy; by living high, your body adapts to the reduced oxygen availability, but you train low to maintain your optimal intensity, thereby gaining the best of both worlds. Clearly, this strategy holds true for either race preparation, or training camps.

Parting Shots

   The goal of this article is to provide athletes with a better understanding of what happens to the body at altitude and how to compensate for those changes to optimize one’s training and/or performance with consideration to their budget. Altitude training can seem a panacea for poor results, but rarely proves to make a major impact. In contrast, competition at altitude can be intimidating, but adequate preparation can help mitigate its effects improving one’s performance. For those unaccustomed to altitude and unable to allow for full acclimatization (2-4 weeks), a few training rides at altitude can educate you on your individual response and helping prevent any race day confidence loss when you don’t feel like your usual self. Until next, keep the rubber side down…

Table 1. Effects of Acute Altitude Exposure

Change = Effect

Resting & Sub-max HR = Increases oxygen transport to tissues

Resting & Sub-max breathing = Helps offset some of the decrement in performance by:

1. Increases oxygen available in lungs for pick-up
2. Decreases carbon dioxide in the body
3. Increases waters loss (dehydration)

Resistance in blood vessels = Increases blood pressure

Catecholamine (e.g., adrenalin) = Increases lactate production and resistance in blood vessels secretion

Urination = Decreases plasma volume which increase hematocrit

Decreased plasma volume = Decreases stroke volume (blood pumped per heart beat). Decreased body weight and muscle size (may be beneficial)

VO2 Max =Decreases performance

Quality of sleep = Increases fatigue

* Adapted from Brooks, Fahey, White and Baldwin. Exercise Physiology (2000).