Lactate is a source of energy that your body can shuttle around. It is not a waste product, and it does nor make you sore.
Lactate: a valuable souce of energy
When you are running hard, you need a fast source of energy. Lactate is the end product of the glycolytic or anaerobic metabolism, which is a quick pathway to produce energy by breaking down glycogen or glucose without the need for oxygen. However, that is not the end of the story, because lactate can then be shuttled to other muscle fibres, your heart or your brain, and further broken down to produce more energy. Inactive muscles or the liver can even transform it back into glucose or glycogen and store it. Lactate is therefore a valuable molecule that allows you to move energy around.
Breaking down lactate requires oxygen and this is therefore called the oxidative or aerobic metabolism. It delivers much more energy than the glycolytic metabolism, but it is a slow process. That is why the lactate breakdown cannot follow the production when you are running hard. The clearance by other organs might not keep up either, and the concentration in your blood will increase. When that happens, you have reached your lactate threshold.
“Yes, but I thought lactate was created when my muscles do not have enough oxygen?” you ask. It is true that the glycolytic metabolism does not require oxygen, but the amount of oxygen is never so low that the oxidative energy metabolism has to stop. Lactate accumulates because the oxidative metabolism is much slower than the glycolytic, and the clearance cannot follow the production.
This should not change the way you train. Even though the lactate threshold is not exactly what you thought it was, it is still a good idea to do part of your training at that intensity.
Nothing to do with sore muscles
As lactate is not a waste product, you do not have to “flush it out”. It is taken out of the bloodstream and used even if you do not perform an active cooling down, and it is incapable of making you sore. Soreness usually develops in 24 to 48 hours, while lactate levels are back normal within 2 hours.
Scientists think that muscle soreness develops because fibres get injured by vigorous exercise. The injury is worse if you perform eccentric actions, whereby you contract your muscles while they are lengthened, as for example the quadriceps when you go downhill. You can therefore develop muscle soreness without increasing your lactate levels.
This does not mean that an active cooling down is a waste of time. It does indeed help, but it is not clear why. I you know the reason, please tell me.
Lactate and fatigue
During exercise, an increase in lactate levels coincides with fatigue, but it is still not clear if lactate is a cause or a consequence of fatigue.
It is possible that lactate is just the messenger. To stay alive, there has to be a balance between the chemical substances in your body. Rapidly rising lactate levels might be a signal for your unconscious brain that you could disrupt the balance by exercising too hard for too long, and that you should slow down. Your brain will then make you tired and reduce the number of muscle fibres you can recruit. This theory is still controversial, but it would explain why you cannot keep going hard by using the glycolytic system for very long.
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