Fact It's a source of energy.
Lactic acid, or lactate, is a chemical that your body produces 24/7, even while you're doing nothing, says L. Bruce Gladden, PhD, professor of kinesiology at Auburn University. Its purpose? To feed your muscles so you can move.
Fact It's made in different ways.
When you're spinning along comfortably, your body primarily gets energy via a slow but steady process that converts fat to fuel. But when you put the hammer down, muscles need fuel faster, so they use a speedier process that taps stored carbs, or glycogen. Both methods produce lactic acid, which helps supply energy to muscles. The more carbs the body uses, the more lactate accumulates in the muscles and blood.
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Fiction You can blame it for the burn.
Lactic acid is not what makes your legs threaten mutiny when you're climbing or going hard (try these Workouts That Help You Crush Every Climb to get better).
If anything, it delays the point at which you fatigue by providing fast energy when you're pushing into the red. The burn you feel when you hit your limit is probably the result of your nervous system sensing increased acidity from the accumulated lactate, Gladden says.
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