Are the three energy systems used sequentially?

I believe that sometimes people think about the energy systems in a systemic manner. From my understanding, this is incorrect. For example, your prime movers such as the quadriceps muscles might be using the ATP/PCr system primarily during a sprint, but also the anaerobic glycolytic and aerobic systems to a lesser degree. This will change based on exercise duration, as the energy substrates are exhausted. However, postural muscles are likely getting most of their energy from the aerobic energy system. In addition, your heart works exclusively with the aerobic energy system. Think of what could happen if you had metabolic end products building up and inhibiting contraction of your heart.

So try to think of things at the muscle level and not systemically when thinking about the energy systems. At any point, our muscles are working mostly from aerobic energy. Only with exertion beyond the capacity of the aerobic system will other energy systems be recruited to a significant degree.