Second wind is a phenomenon in distance running, such as marathons or road running (as well as other sports), whereby an athlete who is out of breath and too 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.