Friday, September 25, 2009

Quick: What's 40ºC in Farenheit??'s HOT, like 104ºF hot. And, that's the theme of this post. (Yes, I'm just learning Celsius conversions; why does the U.S. have to be so difficult by using all these non-universal measuring systems?!)

Anyways, as triathletes, families, friends, etc., head out to Kona for Ironman, knowing a little about what happens to the body in a hot, humid environment can be useful--or scary, possibly. (Plus, I know some of you wanted me to talk about it, and I have a test on Thermal Physiology next week so I could use this as a study opportunity.)

First, though, on the topic of heat: OC Tri is this weekend and word on the street is that Lake Mission Viejo is apparently 85ºF. That's way over USAT's 78ºF cutoff. And, inland OC has been freakin scorching lately, with no end to the heat wave in sight, so chances that the lake cools down? Um, like zero. Which means: NO WETSUITS on Sunday. I have mixed feelings about this. A wetsuit is like my security blanket, and I'm not ready to give it up. Just think of me like this guy:

But there's no way I'd pull myself out of contention for awards/rankings (the consequence for those who choose to wear a wetsuit still). Thinking positively, I am looking forward to not dying of claustrophobic overheating in my wetsuit and to having a faster T1 time.

Heat & Exercise
Now to that nerdy exercise science stuff. We all know that it sucks to exercise in the heat and humidity. But why? Physiologically what's going on in the body that makes exercise in heat so difficult and detrimental to performance, etc? Anyone, anyone? (fyi: I love this stuff, so I might go overboard. Stop reading now if you have a short attention span or just don't care about phys.)

The following are acute responses to heat. After acclimatization (~2 wks) the body adapts to better handle heat stress; more on that at the end. Repeated training in SoCal's heat waves before Kona is obviously a good thing, then.

1) Humidity Hinders Cooling
First the humidity factor. The body cools itself by sweating, but sweating itself doesn't cool you--it's the evaporation of sweat that's key to cooling. The problem with humidity is that the more humid it is the more saturated the air is and the harder it becomes to evaporate sweat. (Relative humidity is the amount of water in the air compared to the theoretical maximum amount of water in the air.) So, you don't technically sweat more in humidity, it's that evaporation doesn't occur as well so the sweat stays on your skin, which makes it feel like you're sweating more, and, worst of all, prevents your body from successfully cooling down.

2) Blood Flow Changes
In heat, blood vessels vasodilate allowing more blood flow to the skin in order to dissipate heat (sweat, which is blood plasma). The problems are blood also needs to go to working muscles, the heart, etc, and as blood flows to skin and is sweat out you're losing blood volume. So less blood to the muscles that need it, less blood back to the heart, hurting your cardiovascular system. No bueno. This one reason why hydration is so essential to replace sweat loss.

3) Cardiac Output Suffers
Cardiac Output (Q) is the product of Heart Rate x Stroke Volume. SV is the amount of blood pumped out of the heart per beat (higher SV = good). But in heat, there's less venous return (due to increase in skin blood flow), which means SV decreases. To compensate, HR increases. So although Q is unchanged at submaximal intensities, a higher HR and lower SV isn't exactly a good thing.

It gets worse. At max, intensities, Q decreases because HRmax decreases. An Ironman athlete is hopefully not working at HRmax, but because HRmax is lower, this means at any exercise intensity you're working at a higher % of your HRmax than in a thermoneutral climate. Ouch.

4) VO2max Decreases
Changes in Q directly affects VO2, and in heat VO2max is reduced. In endurance sports, obviously the higher your VO2max, the better. (VO2max is maximum amount of oxygen that the body can take in and use for important stuff like working muscle!)

Now here's the thing: At submaximal exercise in heat, VO2 is the same, but (like HRmax) since VO2max decreases, at any exercise intensity you'll be working at a higher % of VO2max. Again, ouch.

(If you don't know your VO2max, you can use % of HRmax to estimate % of VO2max. Click here for a conversion chart.)

5) Increased Anaerobiosis & Metabolic Rate With the changes in Q & VO2max, the body's maximal aerobic (with oxygen) exercise intensities are limited. Meaning, you start relying on anaerobic (w/o oxygen) means of producing energy. We all know what it feels like to work in that anaerobic state: mega ouch! It results in more blood lactate accumulating, carb metabolism increases while lipolysis (fat utilization) decreases.

With anaerobic systems kicking in, total energy expenditure increases (i.e. you're burning more calories). Think of it this way: as temperature increases exercise costs more energy. And, for most people, this generally decreases time to exhaustion (i.e you poop out sooner than later).

Heat Acclimatization
For those who've adapted to intense heat--like in the 30º-40º C range for at least 2 weeks--there's good news:

1) You become a better sweater
Sweat sooner, sweat more and at a faster rate, less salt loss via sweat, better sweat distribution.

2) Your plasma volume increases
This means all that stuff about blood volume and skin blood flow affecting Cardiac Output and VO2max is alleviated. More plasma = more blood = good.

Alright, whew! Killer study session for me! I also have a Stats exam next week, would you like me to blog about that? Aw, c'mon... Z scores, standard deviations, probability of error... good stuff!

Lastly, for those of you who don't know Celsius conversions (like me!) here's a hint: Starting at the point where water freezes (0º C, 32º F) for every 10º increase in C there's an 18º increase in F..... 10º C is cold (50º F); 50ºC is hot (122ºF); and the body is happy at 37ºC.

Let me know if you have any questions please!!!


  1. Thanks for sharing! I'm a information nut so I loved this post. If you need any help with stats, let me know, I'm a brain for quantitative methodology. I have more stats books than I do running and tri books (which says a lot) :)

  2. Nerdy, technical and scientific! HOT (pun intended ;))I LOVED it. Thanks for sharing the knowledge! Stoked to be your guinea pig this next week. Woohoo!

  3. Your conversion seems like way too much work to me. I prefer :)

  4. Yes. I live in Florida. It's freaking hot here :) Wet suits? Only if you're racing in April or November!!!

  5. That's a great load of info! You're going to do great in your classes for sure.

  6. Well written. I am sure you'll ace your test. Hope things went well this weekend.