Which of the following energy systems is providing the majority of the energy requirements at 5 seconds?

The ATP-PC system, also known as the phosphagen system, is the primary source of energy for high-intensity activities lasting up to about 10 seconds. At the 5-second mark, this energy system provides the majority of the energy required. It relies on stored ATP (adenosine triphosphate) and phosphocreatine, which are readily available in muscle cells and allow for rapid energy production without the need for oxygen. This makes it ideal for short bursts of high-intensity effort, such as sprinting or heavy lifting.

During the first few seconds of high-intensity exercise, the ATP-PC system is dominant because it can generate energy quickly without the delay associated with the utilization of other energy systems. In contrast, while the lactic acid and glycolytic systems become more important as the duration of activity increases (typically after about 10 seconds), they do not contribute significantly at the 5-second mark. The aerobic system is responsible for energy production during low to moderate intensity over longer durations and would not be the primary contributor in this short and intense effort. Thus, the ATP-PC system is the correct answer as it meets the immediate energy demand most efficiently at this short time frame.