Lactate is always accompanied by protons (H+), due to the breakdown of ATP in muscle. even though it consumes one during each lactate dehydrogenase reaction. When muscles are contracting intensely they require a lot of energy from the breakdown of ATP. This causes a huge release of protons (H+) within the muscle (known as acidosis). That proton accumulation is what causes “muscle burn” and nausea since the pH of the muscle and blood are decreasing (i.e., becoming more acidic).