Everyone always says "no pain no gain" . when is pain healthy?

No pain no gain: It is mainly to reiterate the fact that we have to make an effort to achieve the goals. In medicine the gainful pain probably is the one you get with therapy to train your body regain the lost function or improve the functional capacity

Exercise: Exercising to the point of pain or while pain is present is harmful to muscles,tendons, and ligaments. Mild soreness after a workout is one thing but significant pain is another matter. Also allow a day of rest of those sore muscles before resuming exercise if them. A physical trainer would be best to advise you.

Exercise: The phrase "no pain no gain" relates to the motto that hard work during exercise can increase your life with enormous benefits. Also when you weight train (anaerobic) you break down muscle fibers in the form of small micro-tears that occur at the cellular level in those fibers (catabolic metabolism). Then it repairs those tears and makes them stronger. So in the end Pain is good

No and no: no pain no gain applies only to those people who need to add muscle as fast as possible--and those usually are young athletes or bodybuilders. The Royal Canadian Air Force proved unequivocally in 1960 that to become fit, pain is unnecessary, and to be avoided. A moderate exercise program of 12min/day 5 days/wk for women and 11 min for men enables people to become fit at their own speed