Second wind is a phenomenon in distance running, such as marathons or road running (as well as other sports), whereby an athlete who is too out of breath and tired to continue suddenly finds the strength to press on at top performance with less exertion.
The feeling may be similar to that of a runner's high, the most obvious difference being that the runner's high occurs after the race is over. Some scientists believe the second wind to be a result of the body finding the proper balance of oxygen to counteract the buildup of lactic acid in the muscles. Others claim second winds are due to endorphin production, while still others believe it to be purely psychological.
Documented experiences of the second wind go back at least 100 years, when it was taken to be a commonly held fact of exercise.
The phenomenon has come to be used as a metaphor for continuing on with renewed energy past the point thought to be one's prime, whether in other sports, careers, or life in general.