Final answer:
After 45 seconds of sustained activity, the primary energy system used is the anaerobic glycolysis system, which relies on glycogen to produce ATP, leading to the production of lactic acid as a by-product. Creatine phosphate provides energy initially but gives way to the glycogen system, which lasts up to 90 seconds.
Step-by-step explanation:
The primary energy system used after 45 seconds of sustained activity is the anaerobic glycolysis system, also known as the lactic acid system. This system uses glycogen stored in the muscles to produce ATP, which provides energy to muscles during high-intensity, short-duration activities.
Glycogen is used without the need for oxygen, which is why it's referred to as anaerobic. While this process is efficient for shorter events like sprints, it does produce lactic acid, which can cause muscle fatigue and soreness.
Initially, during high levels of exertion, creatine phosphate is used, but this lasts only for about 8-10 seconds. After creatine phosphate is depleted, the body starts to rely on glycogen, which can provide energy for up to 90 seconds.
Aerobic metabolism takes over for sustained low-intensity activity, using fatty acids as the primary source of energy. However, for the time frame of around 45 seconds of sustained activity, it's the breakdown of muscle glycogen that is the predominant source of energy.