If you're putting in a modest effort to your strength training exercises, then you feel the lactic acid "burn" in your muscles. You feel the burn during the exercise. Why is lactic acid there, and is it still sitting in the muscles days later when soreness is present? Keep reading...
Lactic acid only builds up when the effort is intense, unsustainable, and when the fast twitch muscle fibers are involved (A.K.A. crossing the anaerobic threshold) This is the point where your body's use of oxygen for energy exceeds the oxygen availability. Without the necessary oxygen, energy is being used and lactic acid is the result. As mentioned, this is only for activities of a highly intense nature (walking and jogging don't cross this threshold for most people).
In regards to lactic acid and delayed onset muscle soreness (what you feel the days after your workout), there is no connection. Lactic acid is long gone by the time you would feel any soreness the following day. Lactic acid produced from your workout is just about completely gone 90 minutes post workout. Just after the exercise session, it is converted into a similar version called "lactate". Lactate is then used for the following purposes:
- Oxidized to form pyruvate, which is then used for energy (aerobic metabolism).
- Converted to glucose through an energy-creating process called the "cori cycle" via the liver (the diagram is pictured at the top).
- Used as fuel for the heart.
Most importantly, when it comes to lactic acid, here's what you need to know:
- If you don't feel some kind of burning in your muscles, you're probably not working hard enough to use your largest muscle fibers, and therefore, you're probably not working hard enough to build muscle.
- When you are strength training and feel the "burn", don't quit! That burn and lactic acid will go away soon after, so push through it for a little longer and really exhaust your muscles.
- Finally, any soreness you could feel days after working out can come from a number of causes, but lactic acid is not one of them.