While writing this post, I got really excited. This info blew me away. Apparently though, not all of you will be as excited — my husband’s eyes glazed over with each fact I blurted out to him. Thus, I am issuing an official warning: If you do not care how altitude affects your training, don’t bother reading on.
Yesterday I ran the Slacker Half Marathon in Georgetown, Colorado. Despite its name, the course is somewhat challenging for a couple of reasons. It begins at 10,630 feet and rolls through hills on a course that ultimately declines to 8,400 feet. Luckily, my quads are still happy today – although many people warned me it was a “quad shredder.” What I did notice, however, is that during the race my heart rate was out of control. It was a good 10 to 15 beats higher than I am usually comfortable with – which got me to wondering…What is the effect of altitude on heart rate? I already live in the Springs – at about 6,500 feet (perhaps a little higher where my actual home is located), so I am accustomed to training at altitude. Going up another 4,000 feet is significant though.
While you may think there is less oxygen available at high altitude, it is actually that the oxygen has less partial pressure. The level of pressure in oxygen affects how the body transfers air from the lungs into the blood. When the pressure of the oxygen in the air is reduced, less oxygen is driven from the lungs into the blood – hence you feel like there is no air in the air. Your body responds by trying to pump more blood out and thus your heart rate speeds up, along with your respiratory rate.
Furthermore, you cannot reach your true VO2 max – your ability to take in oxygen and deliver it to working muscles – when exercising at altitude. According to Rice University researchers, you experience a 2 percent drop in VO2 max for every 300 m of elevation above 1,500 meters – even if you are acclimatized. So, during the Slacker – I was working with a VO2 max that was reduced by 8 percent of what I am used to training with in Colorado Springs, and 11 percent less than I would be training with at sea level. That does make me feel better about my time (which was about 4 minutes off my PR), and may explain why I felt a complete an utter inability to really push (I told myself it was because it was my birthday and I just didn’t want to).
On top of all this, the lower oxygen levels in your blood makes your anaerobic system work harder to produce energy when you are working hard. As a result, you have less tolerance for lactic acid buildup and fatigue earlier (which explains why the uphill during the last mile toasted me – usually I am stubborn, especially at mile 12.2, but I just could not force it.) You are also more likely to become dehydrated at altitude – further decreasing performance. I, for one, could not get enough to drink yesterday (and today) and made very few trips to the bathroom, despite four refills at lunch and dinner.
It takes 2 weeks to acclimate adequately to altitudes of about 7,500 feet and another week for every 2,000 feet above that. If you are coming from sea level to run the Slacker, it would take you about 3 ½ weeks to acclimate. True acclimation occurs after 4 to 6 weeks. Acclimitizing means your body increases the number of red blood cells (which help with oxygen delivery) and, despite the initial surge in heart rate, eventually decreases cardiac output resulting in a decrease in maximum heart rate. Your red blood cells also become more efficient at delivering oxygen to your tissues and your number of mitochondria increases. Mitochandria help you break down fat, carbs and protein into energy. They need oxygen to do this, so it makes sense that with less oxygen, you need more mitochondria working to create energy because each one is less efficient. Interestingly enough, no matter how long you live at altitude – you will never fully be able to deal with the lack of oxygen in your blood and can never gain the aerobic power or endurance that you have at sea level. You can get better exercising at altitude, however.
This next bit of info is quite disappointing. It appears we altitude-acclimatized folk may not be more efficient at sea level. Some of those adaptations our body makes (such as decreased maximal heart rate) actually hurt us when exercising at sea level. Studies have yet to show any consistent correlation between performance at sea level and training at altitude – but if you experience a benefit, heck keep it up.
Sleeping at altitude and training at sea level may offer some training benefit. Your body responds to high altitude by producing more red blood cells, but you can still push maximum heart rate and VO2 max when training at sea level. Studies, however, show this yields only about a 1 percent improvement in performance.