Exercise Physiology and Anatomy
Bioenergetics and the cardiorespiratory response
The ATP-CP (phosphagen) system fuels roughly the first 5–10 seconds of maximal effort; anaerobic glycolysis covers roughly 10 seconds to 2 minutes and yields lactate and hydrogen ions; oxidative phosphorylation dominates longer, lower-intensity work. Aerobic capacity is described by the Fick equation: VO2 = cardiac output × a–vO2 difference. From rest to about 40–60% of VO2max, rising stroke volume and heart rate both increase Q; beyond that point stroke volume plateaus, so further increases in Q come from heart rate alone, while the widening a–vO2 difference supports continued oxygen uptake. Because heart rate rises nearly linearly with %VO2max through the middle range, HR is a practical prescription anchor.
Muscle and chronic adaptation
Type I fibers are oxidative and fatigue resistant; Type IIx fibers are fast and glycolytic; training shifts IIx toward the more oxidative IIa. Endurance training increases plasma volume, stroke volume, capillary density and mitochondrial content while lowering submaximal heart rate. Resistance training mainly increases neural drive and muscle cross-sectional area.
Common traps
Lactate is a fuel and byproduct, not the cause of the burning sensation-hydrogen ion accumulation drives the acidosis. The 220 − age formula is a population estimate with roughly ±10–12 bpm error, not a measurement. DOMS peaks 24–48 hours after exercise and is not caused by lactate. EPOC reflects more than simply "repaying" an oxygen deficit.
Worked example (Fick equation)
Cardiac output 20 L/min and a–vO2 difference of 150 mL O2 per liter of blood give VO2 = 20 × 0.150 = 3.0 L/min.