Exercise Physiology
The three energy systems
ATP-PC (phosphagen) supplies maximal effort for roughly the first 10 seconds; fast glycolysis dominates from about 10 seconds to 2 minutes; oxidative metabolism takes over beyond roughly 2 minutes and is the only system that uses fat meaningfully. All three run simultaneously - the question is which predominates. Aerobic training increases stroke volume, capillary density, and mitochondrial volume, and lowers resting heart rate.
Acute responses and regulation
During exercise, cardiac output rises through both heart rate and stroke volume, blood is redistributed to working muscle, and ventilation increases. Hormones shift with intensity: catecholamines and cortisol rise (catabolic), while insulin falls to allow glucose mobilization. Thermoregulation depends on sweat evaporation; high humidity blunts evaporation and raises core temperature risk.
Traps and a rule of thumb
Two myths recur in item stems: that lactate causes soreness (delayed-onset soreness reflects mechanical damage and inflammation, not lactate), and that there is a magical low-intensity “fat-burning zone” where total calories do not matter. Also be wary of assigning a whole session to one system. Rule of thumb: 10 seconds, 2 minutes, then forever - ATP-PC, glycolysis, oxidative. Worked example: a client performs a 6-second maximal sled push (ATP-PC), then 60 seconds of hard cycling (glycolysis), then a 30-minute steady ride (oxidative, with glycolysis still contributing).