Full mechanism, in text

How it recharges muscle energy

1 · Rest · three pools

A muscle cell holds three energy reserves at once: for about 2 seconds, phosphocreatine (PCr) as its backup, and slow-starting glycogen, fat and mitochondria.A muscle cell holds three energy reserves at once:

· pool: direct energy currency, only enough for ~ 2 s of max output — extremely small, must be resynthesized constantly
· PCr (phosphocreatine) pool: ATP's "millisecond-scale backup," about 3-4× the ATP store, enough for ~ 8-12 s of max output
· Glycogen + fat + mitochondria: long-term energy supply, but slow to start up (seconds-to-minutes scale)

Key asymmetry: ATP is "cash in the wallet," PCr is "the change drawer next to the ATM," and mitochondria are "the bank" — the three differ in reaction speed by orders of magnitude.

· Total creatine: 120-140 g, 95% in skeletal muscle, of which ~ 2/3 is PCr, 1/3 free creatine
· reaction at rest is in equilibrium: PCr + ADP ⇌ ATP + Cr — biased toward PCr (ADP is extremely low at rest)
· An ordinary diet (1-2 g/day) only fills muscle PCr to 60-80% saturation, leaving 20-40% headroom — this is the physical basis on which supplementation can work

2 · Burst · CK rebuilds ATP in ms

In an all-out burst is used up fast, and rebuilds it from PCr within milliseconds, so short bursts run almost entirely on PCr.Seconds 0-2: muscle fibers contract at maximum intensity, myosin ATPase rapidly consumes → ADP + Pᵢ.

If you relied on ATP alone, you would crash in under 2 seconds — but PCr saves the day:

(CK) reaction:

PCr + ADP → ATP + Cr

· Reaction speed: millisecond scale — 1000× faster than mitochondrial oxidative phosphorylation
· The phosphate-bond energy is slightly higher than ATP (-43 vs -30 kJ/mol), so the equilibrium favors ATP regeneration
· CK lives in both myofibrils + outer mitochondrial membrane, converting PCr reserves to ATP "locally"

This is why the energetics of short bursts (lifting / sprinting / jumping) are almost "pure PCr":

· 0-2 s: 70% ATP / 30% PCr
· 2-6 s: 10% ATP / 80% PCr / 10% glycolysis
· 6-10 s: PCr already declining, glycolysis starts to take over

Key: the ceiling of performance in this window does not depend on "how big the muscle is," but on "how deep the PCr pool is" — which is why supplementing creatine can directly raise this line.

3 · ~10s · glycolysis takes over

After about 8-12 seconds PCr is nearly gone and glycolysis takes over; between sets it is rebuilt aerobically, and a bigger pool means better repeat sets.After ~ 8-12 seconds: the PCr pool is near empty, and maintenance falls to glycolysis (anaerobic) to fill the gap:

· Glucose → 2 ATP + 2 lactate + 2 H⁺ (much less efficient than the ~32 ATP from oxidative)
· Lactate is not the culprit of fatigue — H⁺ accumulation lowers intracellular pH, and that is where the burn comes from
· But acid is not the main thing that jams the muscle: in mammals, acidification probably does little to weaken the contractile proteins directly (Allen 2008); peripheral fatigue is mainly impaired calcium release from the sarcoplasmic reticulum

This phase:

· 10-30 s max output: 60% glycolysis + 30% remaining PCr + 10% oxidative
· 30-90 s: almost entirely glycolysis, performance declines steadily
· After 2-3 min: mitochondrial oxidative takes over completely (aerobic)

Why this step matters:

· During between-set recovery, PCr is rebuilt under aerobic conditions by oxidative phosphorylation — 30-90 s of rest restores ~ 50-70% of PCr
· Bigger PCr pool → faster refill → better repeat capacity in the next set — this is the core mechanism by which creatine improves "multi-set training," not just single-burst power

The Branch 2003 (96 papers, 100 randomized, placebo-controlled, blinded human trials): effect sizes were small but significant — ES 0.24 for tasks of 30 s or less and 0.19 for 30-150 s; effects were larger for repeated bouts, upper-body and laboratory tasks, smaller for field tasks, and running and swimming showed no improvement. It reports effect sizes, not percentages. A little more in each set, accumulated over several sessions a week, is the route by which it turns into strength and muscle over months.

4 · 4 weeks · PCr pool +20-30%

After about 4 weeks of creatine, the muscle PCr pool is 20-30% deeper; single max efforts barely change, and the gain is the extra repeats in each set.5 g/day × 4 weeks (slow saturation) or 20 g/day × 5-7 days then 5 g/day maintenance (fast saturation): both routes reach the same total muscle creatine level after 4 weeks.

Muscle PCr measurements (muscle biopsy, per dry mass):

· Baseline (omnivorous): 75-85 mmol/kg dry mass
· After saturation: 95-105 mmol/kg dry mass — ↑ 20-30%
· Vegetarians get no creatine from food, start lower and gain proportionally more at saturation

Performance cascade after saturation:

· Short bursts: the PCr buffer extends ~ 2-4 seconds, single-burst max output barely changes, but the effect shows more on repeated bouts (Branch 2003: small but significant effect sizes)
· Between-set recovery: PCr refills faster, compounding multi-set training
· Indirect hypertrophy benefit: lifting a bit more / doing one more set → accumulated hypertrophic stimulus. Chilibeck 2017 pooled 22 trials in older adults (721 people, with resistance training for 7-52 weeks): lean mass rose about 1.4 kg more than with placebo
· More intracellular water: not subcutaneous edema but swelling muscle cells — part of the early weight gain is water weight

"Fill the tank" model:

· A saturation ceiling exists — eating > 5 g/day, the excess is excreted in urine, no infinite accumulation
· Cycling not needed — a 1990s myth, explicitly rejected by ISSN 2017
· Stopping for some weeks returns PCr to baseline — but the real hypertrophy gained from training does not vanish immediately

In the end, creatine doesn't "make you stronger"; it "deepens the burst-system bucket" — those 1-2 extra reps you can do (compounded over months) is what it actually does for you.

Full mechanism, in text

1 · Rest · three pools

2 · Burst · CK rebuilds ATP in ms

3 · ~10s · glycolysis takes over

4 · 4 weeks · PCr pool +20-30%

Creatine · the PCr-ATP buffer

Full mechanism, in text

1 · Rest · three pools

2 · Burst · CK rebuilds ATP in ms

3 · ~10s · glycolysis takes over

4 · 4 weeks · PCr pool +20-30%

Creatine · the PCr-ATP buffer