Where the ATP comes from
Every muscle contraction costs one molecule of ATP. Your body stores almost none of it. Instead, three systems regenerate ATP at different speeds and durations. Knowing which system fuels which activity is the difference between programming that works and programming that doesn't.
ATP-PC (phosphagen) system
- Duration: 0–10 seconds at maximum intensity
- Fuel: stored ATP + creatine phosphate (PC)
- Power output: highest
- Recovery: 2–5 minutes for near-full replenishment
- Training examples: 1RM max lifts, 100m sprint, single high jump, Olympic lifts, plyometric singles
Glycolytic (anaerobic glycolysis) system
- Duration: 10–60 seconds at high intensity (some sources extend to 2 min)
- Fuel: muscle glycogen → pyruvate → lactate
- Power output: high
- Recovery: 1–3 minutes
- Byproduct: hydrogen ions (the burn), lactate (NOT lactic acid — common myth)
- Training examples: 400m sprint, hypertrophy sets (8–15 reps), Tabata, finishers
Oxidative (aerobic) system
- Duration: 2 minutes to hours
- Fuel: carbs, fats, and (in long durations) protein, all processed through the Krebs cycle with oxygen
- Power output: lower, but sustainable
- Recovery: depends on duration and depletion
- Training examples: 5K run, hour-long Zone 2 ride, any steady-state cardio, recovery between sets
Programming consequences
If you're training strength (ATP-PC system): rest 2–5 min between sets. Anything less and you're under-recovered for the next max effort. If you're training hypertrophy (glycolytic + some ATP-PC): rest 60–90 seconds. The metabolic stress is part of the adaptation. Longer rest = less hypertrophy stimulus. If you're training muscular endurance (glycolytic + oxidative): rest 30–60 seconds. You're teaching the body to clear waste products and keep working. If you're training aerobic capacity (oxidative): keep the heart rate elevated for the entire session. Zone 2 work (60–70% max HR) is the gold standard.Common programming mistakes
1. Resting 30 seconds between heavy singles. You're now training glycolytic capacity, not max strength. 2. Resting 4 minutes between hypertrophy sets. You're losing the metabolic stress stimulus. 3. Calling everything 'HIIT'. True HIIT = work intervals at 90%+ HR with structured rest. Most 'HIIT' classes are just glycolytic conditioning.
What to know cold
- ATP-PC: 10 seconds, max power, 2–5 min rest
- Glycolytic: 10–60 seconds, high power, 1–3 min rest
- Oxidative: 2+ minutes, sustainable, continuous
- Rest periods match the system you're trying to train