What's the Deal With Cycling at Altitude?

Published September 12, 2022 11:42AM

The Mountain West and Pacific Northwest offer some fabulous locations for cycling and triathlon—both racing and training. However, epic climbs and panoramic views also go hand in hand with increased elevation and altitude. You’ll enjoy the scenery and training better if you know how to work out at altitude—the facts, not just “altitude myths.”

The term “high altitude” technically refers to locations higher than 5000 feet above sea level, such as Boulder, Colorado, (5300 ft) and Flagstaff, Arizona, (6900 feet), and exerting yourself at those elevations present specific physiological challenges. While cities that host marquee tris like St. George, Utah, (2700 ft) and Coeur d’Alene, Idaho, (2200 ft) are below that threshold, you may still notice some subtle physiological effects. What kinds of “effects” are we talking about? Training at altitude affects everything from effort level to caloric and hydration requirements to speed.

Most Anticipated Effect of Cycling at Altitude: You’re Tired During Exercise

Most people instinctively understand that there is “less air” at altitude. What does this mean, exactly? “Less air” actually refers to the lower air density at altitude, which means fewer molecules occupying a given amount of space. Critically, we’re talking about fewer oxygen molecules occupying a given amount of space at altitude. That’s important, since oxygen is a key ingredient in fueling our muscles during endurance exercise.

Fewer oxygen molecules occupying, say, the average volume of a breath translates to less oxygen entering your cardiovascular system with each breath. Less oxygen entering your body means less oxygen moved per heartbeat, which in turn means that your heart has to beat more frequently than it does at sea level to deliver the same amount of oxygen.*

While cycling, then, at any given effort level your heart rate will be higher and your power lower than at sea level. Or maybe your minute-per-mile pace while running, or your 100 splits in the pool. In mental dialogue terms, this sounds like your bike computer’s data saying “this isn’t too bad” while your legs and lungs are saying “this is really hard.” If you’re a power-driven athlete, you can anticipate a 5-10% drop in power zones at an elevation of 5000 feet above sea level; this lowers to 3-5% at 3000 feet and increases to 8-13% at 7000 feet above sea level.

*This is true for a couple of weeks, until you’re acclimated to the altitude, at which point the effect is largely dissipated.

Less Anticipated Effect of Cycling at Altitude: You’re Hungrier & Thirstier

Less obvious than the effects driven by lack of oxygen is the altitude’s effect on your metabolism. Especially during your first few days at altitude,* your metabolic rate increases. This leads to greater daily caloric requirements overall—specifically during exercise—and in particular an increased reliance on carbohydrates as a fuel source.

Upping your daily food intake and on-bike nutrition sounds simple enough, but you also need to take into account that the lower oxygen levels result in fewer resources being sent to your gastrointestinal system to digest food—especially during exercise. Your best bet, given these factors, is a multi-pronged strategy for training at altitude: Begin carb-loading before arriving, don’t ignore your pre-workout fueling, rely on proven fueling strategies (that don’t upset your stomach) for riding or running, and whatever you do, don’t get behind on fueling mid-session.

*Similar to the effect on your cardiovascular system, the effect of altitude on your metabolism dissipates as you acclimate.

In addition to increased caloric requirements, your body also experiences increased hydration requirements. Due to the lower air density at altitude, your breathing becomes shallower and more frequent in order to maximize the amount of oxygen brought into your cardiovascular system. This change in breathing pattern requires more fluid intake to balance your needs.

Additionally, high altitude environments are also typically dry climates. The lack of humidity results in a lower perceived – but not actual – sweat rate, which in turn often results in under-hydrating. This can be compounded even further if the high-altitude environment isn’t necessarily hot. As with fueling, you want to pre-hydrate (with electrolytes) prior to arriving at altitude, stay on top of your hydration and electrolytes once arriving, and whatever you do, don’t get behind on hydration mid-session—the consequences can be more severe than at sea level.

(Side metabolic note: High altitude also lowers your tolerance for alcohol, so keep that in mind during your post-workout libations or post-race celebrations.)

Least Anticipated Effect of Cycling at Altitude: You’re Hotter

Until you’ve stood under the sun on a blue-sky day in a high-altitude city, it’s hard to really understand just how hot the sun gets. I mean, one mile closer to the 93-million-miles-away sun doesn’t sound like such a big deal. But thinking just about those miles within the earth’s atmosphere, it does mean there are a lot fewer air molecules between you and the sun, and when that sun hits your skin you will absolutely notice their absence.

To compound the issue, the hotter-than-usual-sun will then radiate up from road surfaces in a hotter-than-usual fashion. Ninety-plus degree air temps on a hot, sunny day will radiate to create temps well over 100 a few feet off of pavement and asphalt—as we saw at the St. George Ironman World Championships in May of 2022.

Your best strategy, if you have the option, is to start your training early. If timing isn’t under your control, be mindful of footwear (think light and breathable) since your feet are living within the hottest temps above the road surface. Pack the sunscreen and apply liberally and often. And seek out shade whenever possible, as the fabulous side benefit of low humidity is that warm temperatures are shockingly comfortable when you’re not in the direct sun.

Most Surprising Effect of Cycling at Altitude: You’re Faster

If you’ve gotten to the point where you aren’t sure that any of this is worth the effort, here’s your motivation: Less air density and fewer molecules presents less air resistance when cycling.

Why does less air resistance matter? Cycling speed is predominantly dictated by mass (rider + bike + things on the bike), rolling resistance (tires + road surface), coefficient of drag (aerodynamics), and air resistance. Air resistance contributes substantially enough to this equation that your speeds are notably faster at high altitudes than at sea level.

Yes, you ride faster at altitude than you do at sea level—even if your power is lower and your heart rate is higher. And doesn’t that make it all worth it?

Alison Freeman is a co-founder of NYX Endurance, a female-owned coaching group based in Boulder, Colorado, and San Diego, California. She is also a USAT Level II-certified and Ironman University-certified coach as well as a multiple iron-distance finisher.