A study in heat stroke maybe

Yesterday was hot. As in Environment Canada heat warning hot. Maybe it was stupid of me but I went for a long ride anyway. It seems like there’s some form of weather warning every week now. For thunderstorms, tornados, heat, cold, smoke and if I heeded them all, I’d never ride.

So I rode. I was rolling just after 6am in a bid to avoid the hottest part of the day but I had planned to be riding for between five and six hours, so I knew I wasn’t going to avoid it all. I filled two 750ml standard issue bidons — one with water and one with electrolyte and carb powder. I figured I would need to refill about half way, so I checked on my route map for water sources.

And off I went. When I left, the temperature was around 20C. It was humid so I imagine the ‘feels like’ was higher. I noticed this because my forearms were glistening with sweat, quite early on in the ride, and never really dried off. Apparently high humidity is a contributing factor to heat stroke because it keeps your sweat from evaporating which makes it harder for the body to cool itself.

I rode in the prescribed wattage (Zone 2 — your all-day pace) and was fairly mindful of the need to drink. More so than normal because of the anticipated high of 30C with a 37-42C humidex. But drinking ‘enough’ is always a struggle for me. At least I had a target: empty both bottles by Plantagenet. But I was rolling well, feeling comfortable and strong, so I revised that to ’empty both bottles by Casselman’, some 20km further along the route.

And that happened. I drank gradually and by the time I hit Casselman, 3:43 into the ride, I had two empty bottles. I ducked into a grocery store (all I could find in a hurry) and bought a 1.5 litre bottle of water. Deep regret about that. But I had to refill. I dropped an electrolyte tab into one of the bottles, filled them both, drank the remainder, and rode off.

Looking back at my heart rate data it may already have been too late. The rest of the ride wasn’t remarkable. I remember feeling like my legs still had lots of energy, holding Zone 2 power was still easy. But still I had this odd sensation of feeling like I was working hard. And my right foot was sore. I was alert, lucid and able to ride. Though I was looking forward to being finished. Situation normal, am I right?

But when I got off the bike, my physical state got worse: my breathing was shallow and rapid. I started to feel dizzy. Brain fog started to push in on my conscious thoughts. It was all I could do to put my bike away, stagger to the bathroom, peel off my kit and shower. I leaned on the wall so as to avoid falling over. And there I stayed for quite some time.

It wasn’t the first or the worst bout of heat stroke I’ve had. In August 2022, I rode 145km during a similar ‘heat event’ (average temperature, 28C, max 35C). I rode four and a half hours on two bottles, when I might have more happily have had four or five. When I got back I could barely walk and had to sit down in the shower to avoid falling over. I’m not actually a masochist. I was trying to see how effective these new-to-me electrolyte tabs are and whether or not putting them in all your water bottles would allow you to consume less liquid.

I think I can safely say the answer is ‘no’. Or at least, ‘not substantially less liquid’.

So this time around I was determined to drink more. And I was armed with a new tool. My Garmin 1040 calculates how much water I’ve lost through sweating based on the temperature and my work rate. Unfortunately it only does this at the end of the ride. You can set it to chime regularly to remind you to eat or drink, but to me those sorts of things are easily ignored and switched off.

For yesterday’s ride, I drank four bidons — 3000ml — where Garmin estimated my sweat loss at 4804ml. So I was two and a half bidons short.

Apparently your heart rate rises 10 beats per minute for each 1C your body temperature rises. And if it ever gets to 40C, bad things can happen.

For me, in the last hour of a long zone 2 ride, my heart rate is usually around 137 to 140bpm, whereas yesterday I registered an average heart rate of 151bpm for hour five and 149 for hour six.By then I realized I needed to ease off on the pedals and dropped my average power from 179 watts to 156. So that ‘scans’ as they say. The journal article I cite studied people doing the hadj. Moving slower, but quite likely with similar levels of religious devotion as your average roadie.

My main takeaway from this is: on heat wave days, it’s a bottle an hour, regardless of what’s in it.

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