Final answer:
The lactic acid system is slower than the PCr system due to the involvement of more complex chemical reactions required for ATP production during glycolysis, contributing to a slower rate of energy release.
Step-by-step explanation:
The lactic acid system is slower than the phosphocreatine (PCr) system because it involves more complex chemical reactions. The PCr system can supply the energy needs of a working muscle at a very high rate for about 8-10 seconds by directly utilizing stored phosphocreatine. Once PCr is depleted, muscles turn to glycolysis to generate ATP. Glycolysis involves breaking down glucose into pyruvic acid, which is then converted to lactic acid in the absence of oxygen, contributing to muscle fatigue. This process is slower because it requires about 12 chemical reactions, unlike the PCr system.