If I took something that blocked the pain and continue to lift weights, what might happen?
You would severely injure yourself. When you lift weights, you create extremely small tears in the muscle tissue. The tears scar over and the new scar tissue is stronger and more resilient which allows you to lift more and heavier weights which create new tears, etc., etc., etc. This is also why you are supposed to wait at least a day before exercising any one muscle group again.
Tendons…ligaments…these can hurt, too.
There are several reasons why you might hurt. Normal muscular fatigue (lactic acid buildup) is painful, but not harmful. Injuries (muscle, tendon tears) are obviously harmful. Bodybuilders have the saying “No, pain, No gain”, and try to workout past the point of muscular fatigue pain, so that the muscle is completely exhausted. Taking painkillers might work, but it would also make you an irredeemable wuss.
Lactic acid dissipates with seconds of the end of exercise. Inner Stickler gave the correct example, that you’re causing actual structural damage to the muscle itself.
Microtears in the muscle are not harmful. They are what stimulate the muscle to grow. If you experience sudden pain while doing an exercise, you probably have injured your self in some way. If you have pain the day after doing an exercise that is normal “muscle soreness” and is not harmful. I’ve never heard of any damage being caused by exercising a muscle to complete (painful) exhaustion. But, you are going to be really sore the next day!
Please, please be careful when lifting. It is very easy to injure yourself by overtraining. Do too many sets or too many weights, and you’ll be so sore for the next week that you won’t be able to lift at all, wasting a whole week’s worth of exercise that you could have gained by doing lighter weights and switching off muscle groups. Definitely don’t use painkillers, you need to be perfectly in-tuned with your body’s pain so you can judge which muscles are getting worked and when you can work them again.
That said, if it’s really a problem, take something and make a note not to do that workout again. Pain is useful as a gauge for various things, but if you’re really hurting afterwards, don’t be stupid.
Let me emphasize that this is for after you lift weights. Naturally you wouldn’t take anything in the middle, and most likely you won’t need to at any point. It’s just that if the soreness makes it significantly more difficult for you to perform basic stuff for more than a few hours afterward, there’s no point in just being in pain. Of course, that doesn’t happen often; usually it’s more like for an hour or two, the muscles aren’t good for much, and the next day, using them for moderate activity isn’t going to be fun.
A while back, after pretty much ignoring my legs, I did a very heavy and thorough workout, and felt pretty good afterwards. The next day, I was so sore that I could barely walk. I lumbered around like Frankensteins Monster for quite a while.