ATP, therefore, must be constantly replenished. That's where creatine comes to the rescue. The resynthesized ATP can then be used again to fuel more muscle contractions. The more creatine in the muscle, the more times ATP can be quickly regenerated. When creatine is present the muscles don't have to switch to a second energy system called glycolysis.
Lactic acid is a by-product of glycolysis.
Lactic acid inhibits glycolysis and blocks the resynthesis of ATP through the combination of ADP and creatine. By delaying the accumulation of lactic acid, creatine allows the muscles to continue working with less pain and fatigue. Admittedly, this is a bare-bones simplified explanation. The bottom line is that more creatine in your muscles allows them to contract harder and longer, especially in activities involving short bursts of explosive energy, such as weightlifting, bodybuilding and sprinting. Nothing works for everyone and creatine is apparently no exception.