Scientific Foundations
Energy systems
ATP is resupplied by three overlapping systems. The ATP-PCr (phosphagen) system dominates brief maximal efforts up to about 10 seconds; anaerobic glycolysis supplies most ATP from roughly 10 seconds to 2 minutes; oxidative metabolism dominates beyond about 2 minutes. These systems overlap rather than switch on and off, so questions often ask which system contributes most, not which one is exclusively active.
Muscle and neural adaptation
Henneman's size principle states that motor units are recruited from low- to high-threshold, so heavy or fast efforts progressively recruit Type II fibers. Early strength gains are largely neural (improved recruitment, rate coding, coordination); measurable hypertrophy takes weeks of consistent overload. Type I fibers are fatigue-resistant and oxidative; Type II fibers produce more force and power but fatigue faster.
Cardiovascular and metabolic markers
VO2max reflects central oxygen delivery and utilization; lactate threshold reflects the sustainable fraction of that capacity and often predicts occupational endurance better. Lactate is a fuel and byproduct, not the direct cause of fatigue; acidosis comes largely from ATP hydrolysis and buffering. Endurance training lowers lactate at a given workload and raises threshold intensity.
Training principles
Apply overload, specificity (SAID), reversibility, individualization, and recovery. Adaptations are specific to the demands imposed, so a ruck-based program improves rucking more than it improves sprinting.
Worked example
A 30-second maximal casualty-drag effort: ATP-PCr covers the opening seconds, anaerobic glycolysis dominates the majority, and oxidative metabolism contributes a small but real share. If asked the primary system, answer glycolysis; if asked whether oxidative contributes, answer yes.
Rule of thumb: 0-10 s phosphagen, 10 s-2 min glycolytic, over 2 min oxidative.