ATP: the muscle's energy currency
The energy used directly for muscle contraction is released when ATP is split into ADP and phosphate. Muscles store only a few seconds' worth of ATP, so the body continuously regenerates it through three energy systems.
The three energy systems
- ATP-PCr (phosphagen) system: phosphocreatine (PCr) donates a phosphate to ADP to remake ATP instantly. It is the fastest but dominates only maximal efforts of about 10 seconds (100 m sprint, weightlifting).
- Glycolysis (lactic system): glucose and glycogen are broken down to pyruvate without oxygen, producing ATP and, at high rates, lactate. It matters for intense efforts lasting tens of seconds to about two minutes (400 m run).
- Aerobic (oxidative) system: mitochondria use oxygen to fully oxidize carbohydrate and fat (and protein if needed). It is slower but has a huge capacity, sustaining long activities such as marathons and walking.
How the systems interact
Older textbooks showed the systems switching on one after another, but Gastin (2001) concluded that all three contribute in almost every activity, and that anaerobic and aerobic contributions become equal in maximal exercise between one and two minutes, probably around 75 seconds, earlier than traditionally thought.
Lactate threshold and EPOC
- Lactate threshold: the intensity at which blood lactate begins to accumulate sharply, used to set endurance training intensity. Lactate is not merely a fatigue product; it can be reused as fuel.
- EPOC (excess post-exercise oxygen consumption): oxygen uptake stays elevated after exercise because of PCr resynthesis, lactate clearance and recovery of temperature and hormones.
Aerobic and anaerobic exercise and health
Aerobic exercise improves cardiorespiratory endurance and fat oxidation, while anaerobic work (resistance training, high-intensity intervals) builds strength, power and muscle mass. Combining them is known to help prevent metabolic syndrome, for example by improving insulin sensitivity (McArdle et al., 2015).
See also: Periodization Theory (Training)
References
- Gastin, P. B. (2001). Energy system interaction and relative contribution during maximal exercise. Sports Medicine, 31(10), 725–741.
- McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise Physiology: Nutrition, Energy, and Human Performance (8th ed.). Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.