When muscles are overworked, what happens in ATP production? What toxic byproduct is made?

For glycolysis to continue, NAD+ must be replenished in animals, this occurs through lactic acid fermentation. Total production of 2 ATP per glucose molecule. Lactic acid is the toxic byproduct. Glucose -2 NADH and 2 ATP 2 Pyruvate -2 NAD+ end result - 2 Lactic acid