[Link]

Although Xen's reply is more or less correct, the matter is a lot more complicated.

Lactic acid is quite harmless in itself. Indeed, as Xen mentioned, it's a substance that is pumped into the muscles to let the performer know that the muscles aren't getting enough oxygen. It's a sign of the body to say "Hey, hello up there, I can't go on like this for much longer!" This especially happens in high performance sports, where the body's limits are explored further and further each time you perform (sports like rowing, speed-skating, long distance running, cross-country skiing etc.).
More in-depth: the whole picture is, technically speaking, an issue of oxygen. The body's muscle system works on oxygen, and when you're performing the amount of oxygen in your muscles slowly decreases, up to a point where the body starts making lactic acid. Now this in itself is not a problem, because the muscles will continue to work anyway. For example, in an olympic rowing regatta, or any regatta for that matter, rowers will be starting to feel lactic acid in their muscles after a mere 600 meters (with another 1400 to go!). How do they get to the finish line? Because they've learned to get used to the feeling of burning muscles, because they know the body can endure much much more than it indicates. That's why many high-performance sports train not only the body's physical shape (which is the aerobic system), but also the limits of that physical shape (the anaerobic system): all to perform the best they can.

Delayed Onset Muscle Soreness, however, is contrary to Xen's belief not a good thing. Muscle soreness, in all it's glory, is in fact nothing more or less than tiny ruptures in the muscle - most likely these ruptures have occurred due to insufficient warming up or cooling down, or not taking a shower after your exercises. It's not devestatingly bad, however, but it's best to avoid the feeling so to say.
Xen's right to say that the way to train muscles is to test them to their limits. When you're hauling around weights in the gym, you should be able to do 12 (or so) repetitions, but the last two should be quite hard, you should be having difficulties to complete your exercise. Do not fall into the trap most people fall in: it is NOT, and I repeat, NOT more effective to use heavier weights to boost your muscles, on the contrary. You're probably better off using lesser weight and doing more repetitions, anyway.

To get things straight (just an overall conclusion): pain is essentially never a good thing, even though I claimed that the body is capable of doing things it indicates are impossible. Never, ever, ignore your body's calls. Never.

[ 12-31-2005, 10:42 AM: Message edited by: Link ]