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Where does lactic acid come from and go to?
jkbond posted:
I''ve been trying to vary the intensity of my time on the ellipticals as I've heard that this is a good thing to do. I'll go for a couple of minutes or so at a comfortable pace of 150 strides per minute. Then for twenty seconds or so I go as fast as I can, 220-230 strides per minute, then back to 150 and repeat the process for 1/2 hour.

What I am noticing is that within 5-10 second of the time that I speed up I feel a burn in my quadriceps which is, as I understand it, a buildup of lactic acid in the muscles. This continues until the point that I slow down and within 15-20 seconds of that time my pulse rate increases substantially but the burn disappears.

I am wondering what is happening here and what the physiological process is. I don't understand where lactic acid comes from in the first place and how the "burn" disappears at the end. Is my increased breathing and pulse rate somehow changing the chemistry in the muscles? Not the biggest problem in the world but I'd appreciate any information. Is this type of cardio (alternating fast and slow over a half hour or so a good way to go?
Rich Weil, MEd, CDE responded:
Lactic acid is formed as the result of high intensity exercise like the interval training you a re doing. When there is enough oxygen to meet the demand of the exercise, like in aerobic, or "cardio" exercise, a compound called pyruvate leads to formation of lactic acid, which is then used to create energy (fuel) in the muscles. When there is a lack of oxygen and the cells are overwhelmed and can't keep up with demand for fuel, then pyruvate is converted to lactic acid.

Contrary to popular belief, the lactic acid isn't a bad thing, in fact, it can be used to create energy for the muscles. When you train your muscles with intervals, the muscle cells learn how to use the lactic acid efficiently for fuel. Very conditioned athletes use lactic acid very well, and that's why they can perform at very high levels, even when they perform high intensity exercise that depletes oxygen. It's not the lactic acid causes the burn, but rather that the muscle cells become acidic from other chemical reactions, which in turn stimulates nerve endings to cause the burning sensation. As for why the burn disappears, one of the reasons is that the lactate is cleared when you start the recovery phase of your intervals by increasing the amount of oxygen in the cells. Another reason why the burn disappears is that the lactate travels to the liver to make extra glucose. And finally, lactic acid can be removed from the muscles by the blood, and in fact, it only takes only about an hour for that to happen. The soreness that you feel 1-3 days later is mainly from swelling of the muscle as it heals. Lactic acid production and removal is a complicated biochemical process, but that's the simplified version in a nutshell.

The bottom line is that interval training is a good thing. It will help you get more fit and allow you to be able to sustain higher intensity exercise much better than if you did not train with intervals.

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