Oxygen consumption during exercise (maximal and submaximal oxygen deficit).

The idea

Oxygen consumption rises at the start of exercise because aerobic respiration cannot meet ATP demand immediately. The oxygen deficit is the difference between the oxygen actually consumed and the oxygen that would have been consumed if aerobic metabolism had been fully engaged from the start. During submaximal exercise, oxygen consumption rises until it plateaus at a steady state, where ATP demand is met aerobically and the deficit remains small. During maximal exercise, oxygen consumption rises until it reaches maximal oxygen consumption, or VO2 max, and the deficit is larger because anaerobic systems must contribute more ATP. After exercise, excess post-exercise oxygen consumption, or EPOC, repays the deficit by restoring phosphagen stores, clearing lactate and replenishing oxygen stores.

What earns the marks

• Oxygen deficit is the shortfall between actual oxygen consumption and the oxygen that aerobic metabolism would have required from the start of exercise.
• At the onset of exercise, anaerobic systems such as ATP-PC and anaerobic glycolysis supply ATP before aerobic respiration accelerates.
• Submaximal exercise allows oxygen consumption to plateau at a steady state, so the oxygen deficit is relatively small.
• Maximal exercise drives oxygen consumption to VO2 max, and the oxygen deficit is larger because anaerobic contribution is greater.
• EPOC is the elevated oxygen consumption after exercise that helps restore the body to its pre-exercise state.
• EPOC supports replenishment of phosphagen stores, lactate clearance and restoration of myoglobin and blood oxygen stores.
• Trained performers reach steady state faster and have a higher VO2 max, which reduces the relative oxygen deficit at a given intensity.

Common mistakes and how to put them right

• Confusing oxygen deficit with EPOC.

  Put it right: The deficit occurs at the start of exercise, while EPOC occurs after exercise.

• Thinking oxygen deficit only happens in maximal exercise.

  Put it right: A deficit occurs whenever exercise begins, but it is larger in maximal work.

• Believing steady state means no anaerobic contribution at all.

  Put it right: At steady state, aerobic metabolism meets most ATP demand, but the initial deficit still had to be covered anaerobically.

• Treating VO2 max as the oxygen consumed at any intensity.

  Put it right: VO2 max is the highest rate of oxygen uptake achievable during maximal exercise.

Exam tips

• Sketch or describe the oxygen consumption curve: rapid rise at the start, plateau at steady state in submaximal work, continued rise to VO2 max in maximal work, then slow decline during EPOC.
• Use the term steady state when oxygen consumption plateaus and ATP demand is met aerobically.
• Link the size of the oxygen deficit to exercise intensity: higher intensity means a larger anaerobic contribution and a larger deficit.
• Explain EPOC functions with specific examples, such as restoring phosphagen stores and clearing lactate.