Heat is an additional stress. Zone 2 will feel harder because now your body has to also prioritize cooling itself. So you’re doing additional work (but not the raise your FTP work), coupled with RPE dramatically increasing. However, just because it feels harder doesn’t mean you’re necessarily pushing the intended physiological fitness goals (ie: z2 is still z2, it just feels like threshold efforts.) This is why it’s preferred to “optimize” training with a cool room and fans so your body doesn’t have to deal with the additional stress of offloading heat. However, the affects of the additional heat stress can be mitigated through heat adaptation (HA), similar in concept to how one would adapt to altitude. If you have to live, train and ride/race in the heat, it’s probably best to just become better heat adapted. If you “dial your effort back” then you are no longer doing the intended workout.
That is a bit dated, but some good podcasts on HA to listen to are:
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Listen to How to Get Faster with Heat Training - Ask a Cycling Coach: 103 from Ask a Cycling Coach - TrainerRoad Podcast in Podcasts. Ask a Cycling Coach Podcast - Presented by TrainerRoad: Ask a Cycling Coach: 103 – How to Get Faster with Heat Training on Apple Podcasts
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Listen to How to Train for the Heat: Part 1 with Dr. Stephen Cheung from The Sonya Looney Show in Podcasts.
The Sonya Looney Show: How to Train for the Heat: Part 1 with Dr. Stephen Cheung on Apple Podcasts -
Listen to How to Heat Train for Endurance Events with Balance Point Coaching from The Sonya Looney Show in Podcasts.
The Sonya Looney Show: How to Heat Train for Endurance Events with Balance Point Coaching on Apple Podcasts
Additional discussion can be found here: