Heat stroke during exercise is a rapid uncontrolled rise in body temperature that can cause you to pass out and can even kill you. It is a medical emergency that can cause permanent organ damage, kidney failure and seizures. It should never happen to you because your body sends you plenty of warning signals as your temperature rises.  In 1965, I almost died from heat stroke in an unimportant local race in Arlington, Virginia. I passed out during the race, and I am still embarrassed by the stupidity that I showed when I ignored all the warning signs as my temperature continued to climb.
First your muscles are affected, then your lungs and then your brain.
• Muscles: As your temperature starts to rise, your muscles feel like a hot poker is pressing against them. It is normal for intense exercise to make your muscles burn, but hard exercise does not cause painful burning that feels like fire. Furthermore, the burning of hard exercise is relieved by slowing down, while the muscle burning of impending heat stroke does not go away when you slow down.
• Lungs: As your temperature rises further, the air that you breathe feels like it is coming from a furnace and no matter how rapidly and deeply you try to breathe, you can’t take in enough air. When you exercise intensely, you can become very short of breath, but the air you breathe will not burn your lungs. Burning in your lungs, not relieved by slowing down, signals impending heat stroke. When you feel that the air is so hot that it burns your lungs, stop exercising. This sign means that your heart cannot pump enough blood from your exercising muscles to your skin so heat is accumulating and your temperature is rising rapidly. Your temperature is now over 104 degrees F, and continuing to exercise will raise your body temperature even further so it will start to cook your brain.
• Brain: When heat stroke begins to affect your brain, your head will start to hurt, you may hear a ringing in your ears, feel dizzy and have difficulty seeing. Then you will end up unconscious. Your temperature is now over 106 and your brain is being cooked just like the colorless portion of an egg that turns white when it hits a hot pan.
Mechanism
During exercise, more than 70 percent of the energy used to drive your muscles is lost as heat, so your heart has to pump the heat in your bloodstream from your hot muscles to your skin where you sweat and the sweat evaporates to cool your skin to dissipate the heat. The harder you exercise, the more heat your muscles produce. Everyone who exercises, particularly in hot weather, has to sweat to keep their body temperature from rising too high.
Risk for heat stroke is increased by:
• any pre-existing illness
• heart disease
• use of various stimulants and recreational drugs such as cocaine, and some prescription drugs
• lack of fitness
• not drinking enough fluid
• exercising for extended periods without eating
• wearing excess clothing that traps heat in your body
• not listening to your body when you feel the warning signs described above
Many cases of heat stroke during exercise occur when a person suddenly increases the intensity of exercise, such as a sprint at the end of a long distance running or cycling race, or an intense run down the field in soccer.
Treatment
When a person passes out from heat stroke, get medical help immediately. Any delay in cooling can kill the person, and you may need an expert to help decide if the person has passed out from heat stroke or a heart attack.  Carry the victim rapidly into the shade and place them on their back with the head down and feet up so blood can circulate to their brain. Once it has been established that the person is not having a heart attack, they can be cooled by pouring on any liquid. As you cool them, they may suddenly wake up and talk to you and act like nothing has happened. Don’t stop cooling them, because while they are sitting or lying there, their temperature can rise and they can go into convulsions or pass out again. They must be watched for several hours after they are revived.
My Recommendations
When you exercise in hot weather, stop exercising if you start to feel any of the symptoms of heat stroke described here, and find a shady spot to recover. Stay well hydrated, but realize that too much fluid (Hyponatremia) can also be harmful.
