What about lactic acid buildup? I can't post URL's but I found this information on WebMD dot com:
"Lactic Acidosis Symptoms
When a person's exercise intensity crosses the lactate threshold, the activity rapidly becomes much more difficult and unpleasant. Muscles ache and burn, the heart pounds, and a person feels starved for air. The muscles performing the exercise become extremely fatigued. These symptoms increase if a person continues to exercise above the lactate threshold. In a brief time, the person is physically unable to exercise any longer at that intensity.
Experts disagree as to whether it is lactic acidosis itself that causes these symptoms or if they result from other chemicals or processes that occur during vigorous exercise.
Lactic Acidosis Treatment
Lactic acidosis due to exercise is usually not harmful and no treatment is necessary. Natural processes make exercising above the lactate threshold impossible for more than brief periods.
Unlike lactic acidosis caused by serious medical conditions, the kidneys and the liver usually can clear the extra lactic acid that accumulates from vigorous exercise."