Exercise Physiology and Energy Systems
The body resupplies ATP through three integrated systems, and exam questions test whether you can match an activity's intensity and duration to the dominant system.
Phosphagen (ATP-PC): immediate, roughly 0-10 seconds, no oxygen required, uses stored ATP and creatine phosphate. Think a 1RM lift, a 40-yard dash, a power clean.
Fast glycolysis: about 10 seconds to 2 minutes, glucose and glycogen to pyruvate and lactate, fast ATP with hydrogen ion accumulation contributing to fatigue. Think a 400 m sprint or a high-rep set.
Oxidative system: dominant beyond about 2-3 minutes, uses carbohydrate and fat, requires oxygen and mitochondria, slow but enormous capacity. Think a 5K or a 45-minute ride.
All three run at once; the question is which predominates. Aerobic fitness is captured by VO2max, the maximum rate of oxygen use. Lactate threshold is the intensity above which blood lactate rises faster than it can be cleared.
Traps: lactate is not the cause of delayed-onset soreness (mechanical damage and inflammation are); oxygen deficit and EPOC are not the same thing; fat oxidation is highest at low-to-moderate intensity even though total calorie burn is lower.
Worked example (Karvonen): target HR = ((HRmax - resting HR) x intensity) + resting HR. For a 40-year-old with a resting HR of 70 training at 60%: HRmax is about 180, reserve is 110, 110 x 0.60 = 66, plus 70 = 136 bpm. Percent-of-HRmax methods give different numbers, so check which formula the question uses.