Pain Ain't Weakness Leaving the body!
Face book has had a lot of exercise postings including one that claims pain is weakness leaving the body. Perhaps it is meant as a spiritual metaphor, but it is lousy physiology. First, one must understand what exercise/training really is: controlled injury.
Take any system (muscular, cardiovascular, pulmonary) and you will find that it has its limitations. The combined limitation of, say, your running muscles determines how fast and/or how long you can run. When you train you slightly exceed that limit (not to the point of pain). That is the first step. The next, and equally important step, is to allow the insult (training) to mend. If you wait long enough and have not overloaded the muscle too much it will heal stronger than before and you will be able to work that muscle harder and/or longer.
If, however, you overload any system too much or (as surprisingly frequently happens) you do not allow a long enough rest interval before exercising again, the system will start to break down. The ultimate in this area is the over training syndrome seen in runners. It can take up to a year off for a healthy young individual to recover if they rest.(I have, sadly, seen this in my cardiology practice in young school athletes pushed by their parents).
Now we have lots of information, formulas and other things to let us know how hard we are stressing our symptoms. Heart rate for one (and our breathing pattern) can tell us when we go from aerobic to anaerobic levels of exercise. Another is pain.
Most stimuli (hot, cold, pressure, etc.) if applied in a high enough amount will be interpreted by our evolved brains as pain. It is not weakness leaving the body...it is your body begging you to stop!
So next time you decide to go for a 10 mile paddle and at 1 mile into it you just don't feel like it, trust your guts and go home. You haven't recovered from your previous training. Going to the gym to "work out the soreness" from yesterday's training? That's like walking on a broken leg until it heals. Doubt me? I'm sure the autopsy will show I am right.