when it hurts?

let's say for example your doing sit ups and it starts to hurt you know. do you keep going through the pain even when its unbearable? i havn't kept going when it starts really stinging 1) because im still trying to find something to inspire me at that exact moment ;D it's when it really hurts that its hard to find something to make you do more 2) when i was at P.E the teacher said something to do with the heartrate.im not sure of teh exact numbers but it was something like in between 150 - 180 beats per min is the training range. if you go above or underneath that it isn't doing much good. is this true? or should i just keep training until i can barly move?
thanks!

U should never over do it your body should tell ya when youve had enough as for the sit ups when my abs get real tight I streach then try to relax them give it a couple of minutes then im usealy right for me its like my abs stay contracted that might be the pain hope that helps

Yeah theres a point when my abs "cramp up" as well. But a slight burning sensation is nothing, its just showing your muscles are being worked. You should go slightly over that. I find it easier just to focus on numbers. Do 15, see how that feels, then 30, etc etc.

When the muscle starts to burn its the bodys way of telling the mind that its running out of its juice (glucose), and it will soon have to start to breakdown nearby energy supplys to continue (fat, or protein if its in your system, which it shouldn't be entering right now, since you already ate around 2 hours prior to your work out... right? ;D) when you start to feel tired that it because your brain starts to try and shut the body (muscle) down to prevent any more damage - thats what your doing when you work your muscles, every time you work out to some point you are 'ripping your muscle down', the brain just wants to keep the body in good running or and maintain energy... the brain makes alot of choices for us without us knowing... some people complain about lower back pain while doing situps, and crunches - but there is always a difference from pain and the normal burn that you should get. Don't listen to your brain, do more - push your abs hard if its burning. Your body will repair itself when you sleep, and they will come back better.

Thats why we do sets, with reps to let the body 'recover' and rip the muscle abit more. Heart rate will effect the condition of the body while doing the entire exercise. I believe that you should try to keep the heart-rate rather high while working out, high intensity workouts seem to be better for good conditioning, now if your trying to bulk up on the other hand longer waits inbetween sets to lower your heart rate and help recovery are normal.

"No pain no gain" is wrong. Pain is different from burn. Make sure its the muscles burning from being worked hard, not from being pulled from injury. Most people take 24-48 hours to heal correctly after working a muscle (different muscle groups heal alot faster then others.) eat protein to heal.

Hope I helped ya bud. ;D