AQA A-Level PE Energy Systems Practice Exam

The main concept is the point during increasing exercise intensity when blood lactate begins to accumulate above resting levels, signaling a shift toward greater reliance on anaerobic metabolism. This moment is what we call the lactate threshold. Up to this point, lactate production and clearance are balanced, so blood lactate stays near baseline. Once threshold is passed, lactate builds up faster than it can be cleared, contributing to fatigue and limiting sustained performance.

OBLA, or onset of blood lactate accumulation, is related but refers to a specific fixed level of lactate (often around 4 mmol/L) at which accumulation becomes noticeable. It’s a useful marker, but it isn’t the general definition of the threshold itself. The Cori cycle is the liver’s conversion of lactate back to glucose, a related metabolic process but not a description of when lactate begins to accumulate. Calorimetry measures energy expenditure, not lactate dynamics.

So, the best description here is the lactate threshold—the exercise intensity at which lactate begins to rise in the blood above resting levels.