Biological energy systems

All muscular work is powered by adenosine triphosphate (ATP), but muscle stores only a few seconds' worth. The three energy systems exist to regenerate ATP at different rates. They are not sequential switches — all three run at once, and the mix shifts with the intensity and duration of the activity.

The phosphagen system uses stored creatine phosphate and the enzyme creatine kinase to rebuild ATP almost instantly. It has the highest power output and the smallest capacity, dominating efforts of roughly the first 6 to 10 seconds: a heavy single, a shot put, a short sprint. Glycolysis breaks down glucose or muscle glycogen to pyruvate, supplying ATP at a moderate rate for efforts lasting roughly 30 seconds to 2 minutes. The fate of pyruvate is set mainly by the rate of energy demand rather than by oxygen availability: when ATP demand is high, pyruvate is converted to lactate (fast glycolysis); when demand is low enough, pyruvate is shuttled into the mitochondria (slow glycolysis). The oxidative system uses the Krebs cycle and electron transport chain to metabolize carbohydrate and fat aerobically. It is the slowest to produce ATP but has by far the greatest capacity, and it dominates at rest and during prolonged submaximal work.

Understanding this continuum drives program design. Work-to-rest ratios, set duration, and rest intervals are prescribed to target the system that fuels the sport — near-maximal loads with long rest to train the phosphagen system, or longer intervals with incomplete recovery to stress glycolytic capacity. Substrate choice also shifts: fat contributes a larger share of fuel at low intensities, carbohydrate at high intensities.

Bioenergetics is a heavily weighted section of the NSCA Certified Strength and Conditioning Specialist (CSCS) exam. Expect questions on which system predominates for a given duration and intensity, the primary substrate for each system, ATP yield, and how to set work-to-rest ratios that train a specific pathway.