Energy systems

Energy systems are the phosphagen, glycolytic and oxidative pathways that remake ATP to fuel muscle work.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} The body stores only about 80 to 100 g of ATP, so it must be replenished constantly.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} All three systems are always active, with their relative contributions determined mainly by intensity and secondarily by duration.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 71.}

The phosphagen system makes ATP fastest, while fat oxidation has the largest total capacity.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 71.} In the NSCA’s rough timeline for all-out effort, phosphagen leads for the first 6 seconds, then shares the load with fast glycolysis up to 30 seconds.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.} Fast glycolysis dominates from 30 seconds to 2 minutes; the oxidative system joins it at 2 to 3 minutes and takes over past 3 minutes.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 69.}

In practice

• Apply specificity to conditioning: a client trying to beat the throw to first base in softball needs repeat sprints and lower-body power work that tax the phosphagen system, while a client preparing for a 20-mile hike needs long sessions that tax the oxidative system.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 53.}
• A football lineman shouldn’t prepare for the sport with long-distance runs.^{5National Strength and Conditioning Association, NSCA’s Basics of Strength and Conditioning Manual, p. 11.}
• Longer rest intervals allow fuller ATP resynthesis by the phosphagen system.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 53.}
• Repeated moderate- to high-intensity bouts that challenge the glycolytic system improve the lactate threshold.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 53.}