Glycolysis involved in glycolysis. The burning sensation in muscles during hard exercise can be attributed to the release of hydrogen ions during the shift to glucose fermentation from glucose oxidation to carbon dioxide and water, when aerobic metabolism can no longer keep pace with the energy demands of the muscles. These hydrogen ions form a part of lactic acid. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen reached high concentrations in the atmosphere between
Aerobic conditioning able to diffuse more oxygen into their blood than they previously could. Although exercising at lower intensities will improve aerobic conditioning, the most rapid gains are made when exercising close to an individual's anaerobic threshold. This is the intensity at which the heart and lungs can no longer provide adequate oxygen to the working muscles and an oxygen debt begins to accrue; at this point the exercise becomes anaerobic. Anaerobic training intensity for most individuals will be <85-92% of maximum heart rate. Once improvement in aerobic conditioning is apparent, for example in metabolism and oxygen uptake, the body will progressively