Which energy system is predominantly engaged during circuit training aimed at muscular strength and endurance?

• A. Aerobic energy system.
• B. Anaerobic energy system.
• C. Oxidative phosphorylation only.
• D. Creatine phosphate solely.

Answer: (B)

Circuit training that targets muscular strength and endurance relies on rapid ATP production during short, intense work bouts. The main energy source in these moments is anaerobic metabolism, which includes the phosphagen system for immediate energy in the very short bursts and anaerobic glycolysis for slightly longer efforts. These pathways kick in because the work intervals are brief and performed with high intensity, so oxygen isn’t needed right away to supply the ATP. The aerobic system does contribute, especially during rest periods and as total workout duration lengthens, but it isn’t the primary driver of the high‑intensity bursts typical of this training. The creatine phosphate system alone can’t sustain energy for longer bouts beyond about ten seconds, and relying on oxidative phosphorylation only would be too slow for the rapid repeats in circuit training.