ATP-PC (Phosphagen) System

The ATP-PC system (also called the phosphagen system) is one of the two anaerobic pathways. Two compounds are stored directly within the muscle and are ready for immediate use to supply energy to the cell:

1. ATP (adenosine triphosphate)
2. PC — phosphocreatine, a phosphate group bound to a creatine molecule

Stored quantities are substantial: roughly 4 to 6 moles per kg of muscle of ATP and 15 to 17 moles per kg of muscle of PC are on hand for immediate demand. When PC is broken down to creatine and inorganic phosphate (Pi), the released energy is used to recombine ADP and Pi back into ATP; the rebuilt ATP can then be split again to ADP + Pi.

The defining feature is that this energy is stored and ready to go with no waiting. Although the total amount of energy available is extremely limited, the system has a very large power capacity (a large amount of energy supplied per second). In an all-out bout, the ATP-PC source is exhausted in 30 seconds or less. The stored nature makes it ideal for very short events (roughly 0 to 45 seconds): a maximal lift, shot put, high jump, 40-yard dash, throwing and jumping events, sprints, short sets in weight training, and stop/start sports with short rests between bouts.

Because it is anaerobic, no oxygen is required during use, and it produces no fatiguing by-products. Replenishment of the depleted stores, however, depends on the aerobic system (see Connections) — which is why heavy breathing continues after a hard effort.

• Two stored compounds: ATP and PC (phosphocreatine = phosphate bound to creatine).
• Stores: ATP ~4–6 moles/kg muscle; PC ~15–17 moles/kg muscle.
• Anaerobic; very rapid ATP production; very high power output; very limited capacity.
• Exhausted in ≤30 seconds in all-out exercise.
• Best for short-duration, high-power events (~0–45 seconds).
• No fatiguing by-products; replenished via the aerobic system.