FSHP Academy
Exercise Physiology — all lessons

Muscle Physiology

Muscle physiology: excitation–contraction and the sliding filament modelConcentric, eccentric and isometric muscle actionsMotor units, recruitment and rate codingNeuromuscular adaptations to training

Energy Systems

Energy systems overview: how the body makes ATPThe ATP-PC (phosphagen) systemAnaerobic glycolysis and lactateThe aerobic oxidative system

Cardiovascular Physiology

Heart anatomy and the cardiac cycleStroke volume, heart rate and cardiac outputBlood components and oxygen transport

Aerobic Fitness and Intensity

Aerobic fitness concepts and adaptationsVO₂ maxLactate thresholdHeart rate zones and RPE-based intensity control

The ATP-PC (phosphagen) system

The fastest energy pathway — phosphocreatine, the creatine kinase reaction, how quickly it depletes and recovers, and how this governs sprint, strength and power training.

Intermediate Evidence: Strong evidence 3 min read Reviewed 2026-10-31 Status: draft

Learning objectives

  • Write and explain the creatine kinase reaction
  • Describe phosphocreatine depletion and resynthesis kinetics
  • Explain why rest intervals determine quality in sprint and strength work
  • Evaluate creatine supplementation in relation to the phosphagen system

Before this lesson

Why this matters

Sprints, jumps, throws and heavy lifts last seconds. Their quality depends on phosphocreatine — and on how long you let it recover.

The science

PCr + ADP + H⁺ ⇌ ATP + creatineCatalysed by creatine kinase; near-equilibrium, extremely fast.

Phosphocreatine acts as an immediate ATP buffer. Muscle PCr stores are a few times larger than ATP stores but still small, and they fall sharply within seconds of maximal effort [hargreaves2020]. The reaction also consumes a hydrogen ion, briefly buffering acidity.

Recovery kinetics

After exercise, PCr is resynthesised using ATP from aerobic metabolism. Recovery is rapid initially and then slows. After a 30-s maximal sprint, Bogdanis and colleagues found PCr was still not fully restored after 6 minutes, and the recovery of power output closely followed PCr recovery [bogdanis1995]. After shorter efforts, recovery is quicker, but short rests (<60 s) leave PCr only partly restored.

Practical application

Creatine supplementation

Supplemental creatine monohydrate increases muscle creatine and PCr stores. The ISSN position stand concludes it improves high-intensity repeated performance and training adaptations and is safe in healthy people at recommended doses (commonly 3–5 g/day, or a loading phase of ~20 g/day for 5–7 days) [kreider2017]. Initial weight gain (~1–2 kg) mostly reflects water stored in muscle.

Common mistakes and misconceptions

Case study

A sprinter’s coach notices times worsening across 6 × 60 m with 90-s rests. Extending rest to 4 minutes restores consistent times, aligning the session with its speed-development purpose.

Further learning: video lessons

Video links are only marked “verified” after a human has watched them. Unverified entries open a YouTube search for that channel and topic rather than a guessed link.

Introductory needs verification — opens a YouTube search

The phosphagen system

Khan Academy

You will understand: how PCr regenerates ATP

Why: Clear biochemistry basics.
Practical demo needs verification — opens a YouTube search

Rest intervals for speed and power

Science for Sport

You will understand: applying PCr recovery to session design

Why: Coaching-focused application.

Knowledge check

Knowledge check · 0/5

1. The creatine kinase reaction produces…

Answer B. PCr donates its phosphate to ADP forming ATP; creatine is released.

2. PCr resynthesis after exercise depends mainly on…

Answer B. Resynthesis requires ATP from mitochondria, so aerobic fitness speeds recovery.

3. In Bogdanis et al. (1995), after a 30-s maximal sprint, PCr recovery was…

Answer B. PCr recovered substantially over minutes but was still not complete after 6 minutes, and power recovery tracked PCr.

4. Why do maximal strength sets often use 3–5 minute rests?

Answer B. Longer rests restore more PCr and maintain force output across sets.

5. Creatine supplementation is best described as…

Answer C. The ISSN position stand concludes creatine is effective and safe in healthy populations at recommended doses.

Short-answer & application questions

  1. Design a repeated-sprint session for team-sport athletes and justify the rest intervals.
  2. Why might better aerobic fitness help a sprinter recover between repetitions?
  3. Explain to a client why creatine causes initial weight gain.

Key takeaways

  • PCr + ADP → ATP + creatine (creatine kinase) — a single, very fast reaction buffering ATP.
  • PCr falls sharply within seconds of maximal effort and is resynthesised aerobically after exercise.
  • Recovery is fast initially but incomplete after short rests; repeated sprints with short rests lose power.
  • Creatine supplementation increases muscle creatine and PCr and improves repeated high-intensity performance.

Connected topics

physiology

Anaerobic glycolysis and lactate

sports nutrition

Evidence-based supplements (creatine, caffeine and others)

strength conditioning

Speed and agility program design

strength conditioning

Plyometrics and explosive power training

References

  1. Hargreaves M, Spriet LL (2020). Skeletal muscle energy metabolism during exercise. Nature Metabolism, 2(9):817–828. doi:10.1038/s42255-020-0251-4 · PubMed 32747792 Narrative review verified
  2. Bogdanis GC, Nevill ME, Boobis LH, et al. (1995). Recovery of power output and muscle metabolites following 30 s of maximal sprint cycling in man. Journal of Physiology, 482(2):467–480. Classic study check pending
  3. Kreider RB, Kalman DS, Antonio J, et al. (2017). International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition, 14:18. doi:10.1186/s12970-017-0173-z Position stand check pending

Educational content only — not medical advice. Consult a qualified clinician for diagnosis or treatment.