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ATP–PCr system (phosphagen)

The fastest way a muscle can pay for work. Every muscle stores a tiny amount of ready-made ATP plus creatine phosphate, a molecule that instantly donates its phosphate to rebuild ATP the moment it is spent. It powers the first few seconds of any all-out effort — a heavy single, a jump, a sprint start — and then runs dry.

Relative power output over time power → 0s10s30s1m2m5m+ATP–PCr — this pageGlycolyticOxidative

Every all-out effort recruits all three systems at once — this shows which one dominates as the seconds tick by.

The metabolic pathway — where the energy comes from

FATS · triglycerideCARBOHYDRATEPROTEIN Fatty acids + glycerolGlucoseAmino acids Glycolysisnet +2 ATP · fast Pyruvate Lactateno O₂ → fatigue ↑ Cytosol (sarcoplasm) — no oxygen needed↓ Mitochondria — needs oxygen (O₂) β-oxidation Deamination→ into glycolysis or Krebs Acetyl-CoA Ketone bodies Krebs cyclecitric-acid cycle Electron transport chain ⚡ ATP Creatine phosphate → ATPinstant · 0–10s · skips the mill
The metabolic mill — how each fuel is broken down and fed into ATP. Lit path: the ATP–PCr (phosphagen) system.

Duration: 0–10 seconds · Intensity: Maximal / explosive · Fuel: Stored ATP and creatine phosphate (PCr) inside the muscle · Oxygen: None required (anaerobic, alactic)

How it works

  1. ATP is split (ATP → ADP + Pi) and that released energy drives the muscle contraction.
  2. Creatine phosphate immediately donates its phosphate to ADP (catalysed by creatine kinase): PCr + ADP → ATP + creatine.
  3. ATP is regenerated in a fraction of a second, so power stays maximal — but PCr stores are small and deplete in ~8–10 s.
  4. During rest, PCr is rebuilt using aerobic energy (most restored within 3–5 minutes).

What it powers

Byproduct: None that causes fatigue (no lactate, no acid) — this is why it is called the 'alactic' system.

Recovery: PCr resynthesis is aerobic: ~50% back in ~30 s, near-full in 3–5 min. This is why heavy-strength and power work uses long rests.

Training: Trained with maximal, brief efforts and long recoveries (heavy lifts, plyometrics, short sprints). Creatine monohydrate supplementation enlarges the PCr store, extending this system slightly.