Phosphagen system

The phosphagen system, also called the ATP-PC system, is the fastest way muscle regenerates ATP.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} Creatine phosphate (CP, or phosphocreatine) donates a phosphate group to ADP in a single reaction catalyzed by creatine kinase.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} This system supplies most ATP for short, high-intensity efforts such as jumping and sprinting, and is active at the start of all exercise, regardless of intensity.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 42.}

CP stores are small, so the system can’t remain the main energy supplier during long, continuous efforts.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} Muscle CP is normally four to six times higher than ATP, and fast-twitch fibers hold more than slow-twitch fibers.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 62.} CP can fall 50 to 70% in the first 5 to 30 seconds of high-intensity exercise and nearly empty during an effort to exhaustion.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 71.}

In practice

• The NSCA’s interval table pairs efforts of 5 to 10 seconds at 90 to 100% of maximum power with work-to-rest ratios of 1:12 to 1:20.^{5Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 76.}
• The NSCA’s program design guide describes inconsistent findings on whether training improves the system’s efficiency and suggests this may help explain the effectiveness of creatine supplements.^{6Hoffman, NSCA’s Guide to Program Design, p. 142.}
• The guide also cites a study in which two weeks of daily 15-second maximal cycling sprints raised creatine kinase by 44%.^{6Hoffman, NSCA’s Guide to Program Design, p. 142.}

How long does phosphocreatine take to recover?

The NSCA’s strength and conditioning text gives full ATP resynthesis within 3 to 5 minutes after exercise and CP resynthesis within 8 minutes, largely through aerobic metabolism.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 71.} ISSA gives 15 to 25 minutes for full phosphocreatine replenishment after maximum effort, with 50 to 60% restored within 15 minutes.^{7International Sports Sciences Association, Strength and Conditioning, p. 120.} It says a more conditioned client can replenish it in 5 to 10 minutes.^{7International Sports Sciences Association, Strength and Conditioning, p. 120.}