Dr. Craig Heller

And then during rest, that heat will leave the muscle, but it's not fast.

And certainly the heat can't leave the muscle very fast while you're working out because when the muscle contracts, it squeezes the blood vessels.

And the only way heat gets out of a muscle is in the blood.

And your muscle metabolism can go up 50 or 60 fold during anaerobic activity.

That means the heat production in the muscle goes up 50 or 60 fold.

The blood flow to that muscle cannot go up 50 or 60 fold.

So you literally have the capacity to cook your muscles.

So to keep you from damaging your muscle,

by hyperthermia, we have fail-safe mechanisms.

And one of those fail-safe mechanisms is an enzyme which is critical for getting fuel, in other words, the results of metabolism of glucose, getting that fuel into the mitochondria, which is making our major coinage of energy exchange, ATP.

So that particular enzyme is temperature sensitive.

So when the muscle temperature gets above 39 or 39.5, it shuts off.

And that essentially shuts off the fuel supply to the mitochondria.

That's when you cannot do one more rep.

The most immediate, the most immediate impairment of muscle activity, muscle fatigue, in other words, is the rise in temperature of the muscle.

Well, you will still have a fatigue curve with your upper body.

And that will be influenced by any rise in temperature that has been generated by your lower body exercise.

It's one of them.

And it's a quick one.

Because your body surface is a very good insulator.