On very hot days, especially if heat is combined with high humidity, your heart rate can be very high. I remember one time I was doing very low Z2 to upper Z1 and my heart rate was in the 150s. Granted, this was after a race and I was getting back from the venue (75 km one way).
The question is: what do you want to train. With Z2/endurance rides, one primary aim is to make your cardiovascular system more efficient. Your heart rate measures the load of your cardiovascular system. So yes, I would stick to heart rate targets to keep the load on your body low and avoid accumulating unnecessary fatigue.
If it is that hot/hot and humid, I would also avoid doing hard intervals or so outdoors in the middle of the day. Do them early in the morning when temperatures are more bearable. Or in the late evening if you prefer. Or do them indoors.
And make sure you have enough cold liquid with you. You could put one bottle in the freezer and fill the other with ice. (Just make sure to not overfill the bottle, ice is less dense than water.)
Are you racing? Or just building fitness for enjoyment?
If you’re racing, and your races require heat adaptations…then train your body to adjust.
Otherwise, wake up early, go late, train next to an air conditioner, ect. I wouldn’t lower your power outside, id lower your duration and progress from there. Or just know that heat is an added stress to your training and plan your rest/recovery accordingly.
Even here I would caution: training is all about balancing intensity and volume with recovery. Heat puts extra strain on your body. Yes, you can get acclimatized, but it will still put additional strain on your body and result in you not being able to dig as deep. Use with caution and use that deliberately.
Not trying to be contrarian, however heat adaptations let me go deeper and put out more power. Repeatedly and reliably, season after season. It’s not always 96F / 36C every time I ride. Embrace the dry heat if you have it, and see if the same works for you like it does for so many out here.
Heat adaptations come naturally if you regularly ride outdoors in the summer, and it is hot or hot and humid where you live.
To get adapted during Z2 rides, pacing by heart rate (e. g. sticking to <= 140 bpm) has the advantage that it puts a comparable load on you and you will get quicker with time. If you always pick the same route, you should be able to see that comparing your times. But make sure to give your body time to adapt.
The other key word you used, dry heat, is important. Dry heat tends to be much easier on me. Cooling is alright in the dry heat if you drink enough. But in Japan, heat is usually humid, and humidity can play a number on you. E. g. when you climb, you are moving more slowly through the air and cooling gets worse. I remember having to pack a second pair of socks to change into mid-ride. In the South it was so bad that even walking had me drenched in sweat in no time, and I had to wait until sundown to go running.
Make sure you’re getting enough electrolytes in your bottle. I’ve made an effort this spring to be more consistent using, and increased the amount of, sodium in my bottles, and it has made a huge difference in how I feel when I get off the bike and more importantly how I feel that evening and the next day. I still sweat like crazy on the bike.
Also, as to heat adaptation, I say embrace it and get it done, especially if you live in those environments (I’m in FL). I learned a few years ago, that getting out there and getting adapted to the heat over a couple weeks time pays dividends for the rest of the summer, on and off the bike. If yo do it, you won’t have the same dread about the heat on your Sat morning ride, you’ll be more comfortable inside on the trainer, and you won’t be (as) miserable doing other outside activities. I noticed the last couple days that I’ve been feeling cold in the house (a/c set at 74) so I bumped it up to 76 yesterday, and was perfectly comfortable, even sleeping last night).
And be careful with Oreo Cookie’s cold water prescription. If you get into heat trouble, dumping ice water into your stomach can backfire on you. I’ll add some ice to my 2nd bottle so it’s (only) cool and refreshing in that 2nd hour. Also, I’ve tried freezing water in insulated bottles and froze too big a chunk and it wouldn’t thaw fast enough to keep up with my needs when the time came.
If you are riding the same route at the same speed let’s say Z2. Is the muscle adaptation any different at temperature of 25C than 40C?
Or another way to look at it ; if on the trainer and I heat up the room to 40C will it affect the muscle adaptation?
Haven’t read anything about improved “muscular adaptions” (aka. peripheral adaptions) due to heat training.
The only improved adaptions I have heard about in this regard are central adaptions like increased blood volume (a bit like altitude training) and of course improved tolerance to physical activity in heat.
I lived in NM for a couple of years and would regularly ride in the middle of the day at 100F. Because it was dry, I didn’t even sweat that much compared to a humid environment.
Others in my club would ride at 6/7am and were always avoiding the heat like the plague.
But once you get used 100F, 85F feels like a cool breezy day.
It’s still aerobic training if you stay well below threshold. Targeting zone 2 is fine but I wouldn’t worry about normal drift. To me the solution is not to use watts and the higher heart rate but to just go a bit slower. If it’s super hilly in your area, get an easier gear on your bike so you can stay aerobic and not go above threshold when you don’t want to.
I live in central TX and I do a good bit of riding/training in the heat. Heat acclimation helps, but at some point you get into a math problem where you are producing more heat than your cooling system can dissipate. At some point, the body is smart enough to shut down (hopefully before things go totally sideways). It’s like a truck climbing a mountain pass. Your radiator might have enough capacity to keep up or you may have to back off the power to reach equilibrium. When riding, that may be in Z2, might be tempo, might be threshold. So many factors besides temp and power, including speed through the air, humidity, radiant heat from direct sun, etc. The best you can do is adapt the best you can and then listen to your body. Watching HR doesn’t work for me personally, it’s always off the charts with heat and/or caffeine. An effective management tactic I’ll use when racing is to make hay while it’s still early/cool when I know it’s going to be a hot day. I find that regardless of how many matches I’ve burned early, the heat will become the limiter as the day progresses. So, use those watts while you can.
Muscle adaptations, probably not. But the heat is another stress that will add to your fatigue. Heat adaptations also has it’s own benefits like increased plasma volume. So it probably won’t be a multiplier to your normal gains but it has it’s own pros/cons you need to balance with the rest of your training.
For myself, I don’t worry, just go out and ride by perceived exertion. In the Saturday workout I posted above (Zone 2 and heat - #14 by WindWarrior) you can see as the temperature drops I increase power while still keeping HR well below normal for such a ride.
My garage hits 90+F and I’ve used the trainer with 2 fans and 3 fans. I’ll modify workouts if necessary. Last week I did some 30-sec jobbers outside in the heat, followed by a shorter threshold interval - that was a ‘heat friendly’ workout format. These days I just go outside and ride, however where I live outside is time-efficient, and better cooling.
Heat, as well as many other outside factors such as humidity, fatigue and even caffeine, can definitely affect heart rate, making it harder to keep your heart rate in a specific HR zone and even change from day to day. This is why at TR we are firm believers that power is the ultimate metric if you are doing interval-based training.
In short, if you’re out of zone on a hot day, you’re still out of zone. So, it’s important to really listen to your body and adjust your effort accordingly, especially when training in hot conditions. You may need to lower your intensity to stay within your target heart rate zone.