Your experience of higher heart rate and lower power or speed when training is extreme heat is normal. It is possible to acclimate to heat enough to reduce the impact of riding in heat, but you will always perform better in pleasant weather (hight teens Celsius, 60s Fahrenheit) than in temperatures much warmer than that.
Here’s the physiology: Your body can shunt blood to different places according to where it is most needed. Your brain always gets it’s needed share. When you are resting, a bit of blood goes to your muscles just to keep them alive, and a bit goes to your gut, to keep it alive.
If you’ve eaten recently, more blood goes to your gut to support peristalsis and absorb the nutrients absorbed during digestion. The heart always gets the blood it needs to support it’s pumping, since otherwise it doesn’t pump. As you start to exercise, your muscles need more oxygen and fuel, and need to dump more CO2 and waste heat. Blood is directed to the muscles and away from the digestive system.
If you are untrained, you have enough blood volume to support your brain and your muscles, but only by almost completely shutting down any blood flow to your gut. That’s why beginners get nauseated if they try to eat while exercising. The food will sit as the muscles needed to keep it moving along won’t be doing their job.
One of the effects of endurance training is to increase blood volume, particularly by increasing the volume of plasma, the liquid part of blood. That’s why better trained athletes begin to be able to eat and digest while riding.
Blood has one more job we haven’t discussed yet: All that waste heat that is being carried away from the working muscles needs to be dissipated into the environment or the body overheats and stops functioning efficiently. To do that, the body shunts blood to the skin where radiation and evaporation of sweat can remove heat.
In order to supply blood to the skin, it has to be shunted away from muscles, so on the hotter the day, the less blood goes to the muscles at the same heart rate. To some extent, heart rate can increase so you can maintain normal power so long as that power is well below your cool-day peak, but that effect is limited. Any sustained, aerobic power that is near your LT on a cool day will be above your limit on a hotter day simply because your ability to deliver oxygen to your muscles is compromised by the need to deliver it to your skin for cooling.
Now you can probably understand why eating while riding becomes more challenging on hotter days, even for well trained athletes. Blood volume may be enough to support digestion and muscular work, but not enough to support digestion, muscular work and cooling. Endurance training in the heat will cause a further increase in plasma volume beyond that caused by endurance training in pleasant weather, but won’t be enough to completely negate the effect of heat on performance.
There is one more important effect to discuss that would prevent you from performing as well on a very hot day as on a cooler day, even if you could pump enough blood to deliver normal amounts of oxygen to your muscles at all work rates: Your brain doesn’t want to be boiled. Your brain only works well in a narrow range of temperatures, and completely fails (you die) if you get outside that range, so your brain has evolved some very effective safety mechanisms for keeping your core temperature in that good range.
One of the mechanisms works like an amplifier on signals from your cortex to your muscles. On a cool pleasant day, it passes those signals along at full strength, but as core temperature rises beyond the 37 about 38-39 degrees, it starts to tamp down the signal. That means you have “try harder” mentally to get the same signal to the legs.
You feel like your are pushing harder to generate the same speed, or you go slower for the normal effort. You can overcome this effect to some extent by making the harder mental effort, but as core temperature rises with harder work, the tamping down of signals to the muscles becomes more extreme and there is an upper limit to how hard an effort can be sustained.
So, to answer your question briefly: Your experience of being slower on hot days is normal. You can train to reduce but not eliminate the impact of heat on performance.
Training in the heat, because you are making less power, is less effective than training in more pleasant conditions, so do a bit of heat acclimatisation training, but do the majority of your training in the early morning before the heat of the day. Note that you can acclimate to heat by exposure to heat, on or off the bike, and that exposure to cool undoes the effect, so dress warmly if you’ll be in air-conditoned spaces in the summer. (That’s one reason you see pros wearing sweaters and hats on days when you’d think they really ought not to need them).
By: Scott Saifer of Cyclingnews.com