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7.4.3Topic 7 · Run for your Life

Intensity of Exercise — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 23 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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7.4.3 Intensity of Exercise — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how exercise intensity affects respiration, lactate production, oxygen debt and fitness measures such as VO2max.The intensity of exercise determines how muscle cells make ATP.At low intensity (walk…

  • lactate threshold
  • excess post-exercise oxygen consumption
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Oxygen deficit

Section 1 of 8Intensity and energy supply

The intensity of exercise determines how muscle cells make ATP.

Aerobic
At low intensity (walking, jogging) oxygen supply is enough and respiration is mainly aerobic, using fat and carbohydrate.
Lactate
As intensity rises, the demand for ATP outpaces the rate at which oxygen can be delivered,
  • so muscle cells use additional anaerobic routes: phosphocreatine for the first seconds,
  • then anaerobic glycolysis producing lactate.
Even during heavy exercise some aerobic respiration continues, so both pathways contribute.
IntensityMain energy sourceFibre type usedDuration possible
Low (walking)Aerobic, fats and carbohydrateSlow twitchHours
Moderate (steady run)Mainly aerobic, using glycogenSlow twitch, some fast30 to 90 min
High (sprint)Phosphocreatine and anaerobic glycolysisFast twitchSeconds to a few minutes

Section 2 of 8Lactate and the oxygen debt

In anaerobic glycolysis, pyruvate is converted into lactate to regenerate NAD.

Lactate dissociates to lactate ions and H+, lowering the pH of muscle cells.
Fatigues
This inhibits enzymes of glycolysis and affects the binding of calcium ions to troponin, so the muscle fatigues.
Lactate threshold
The lactate threshold is the exercise intensity at which lactate begins to accumulate faster than it can be removed.
Trained athletes have a higher threshold because their muscles have more mitochondria and capillaries.

After exercise, breathing and heart rate remain elevated because extra oxygen is needed to deal with the lactate. This extra oxygen is the excess post-exercise oxygen consumption (the "oxygen debt"). It is used to:

Lactate and the oxygen debt
convert lactate to pyruvate in the liver and oxidise it, or convert it to glycogen;
resynthesise phosphocreatine and ATP;
restore oxygen to myoglobin and haemoglobin, and support a raised metabolic rate and body temperature.

Section 3 of 8VO2max and fitness

VO2max is the maximum volume of oxygen the body can use per minute per kilogram of body mass during intense exercise (units cm3 kg-1 min-1).

1
It reflects the combined efficiency of ventilation, the heart, the blood and the muscles.
2
A higher VO2max means more aerobic ATP can be produced, so it is a good predictor of endurance performance.
3
Typical values are about 35 to 40 for untrained adults and over 70 for elite endurance athletes.
4
It is influenced by genetics, age, sex and training.

Training adaptations include a higher stroke volume and cardiac output, increased capillary density in muscle, higher myoglobin and number of mitochondria, an increase in vital capacity and stronger respiratory muscles.

VO2max and fitness
High-intensity interval training can raise the lactate threshold, whereas long steady sessions raise the efficiency of aerobic metabolism.
Measures of fitness include recovery rate of the heart rate after a standard exercise, and the speed with which lactate in blood returns to normal.

Section 4 of 8Interpreting data

A graph of oxygen consumption against time during exercise shows a rise to a plateau (steady state) at moderate intensity, and a gap between the oxygen needed and the amount taken up at the start (the oxygen deficit).

Interpreting data
After exercise the oxygen consumption is still above rest for some time.
Blood lactate against intensity is a flat line that suddenly rises at the lactate threshold.
When asked to compare two athletes, use figures from the graph and relate them to adaptations such as mitochondrial number, capillary density, stroke volume or fibre type.

Remember that percentages of maximum heart rate are often used to define intensity: roughly 50 to 70 percent is light to moderate, 70 to 85 percent is vigorous and above 85 percent is near maximal. Recovery is faster in fitter people because lactate is cleared more quickly and the oxygen debt is repaid sooner.

Section 5 of 8Fuels and the practical side

The fuel used also changes with intensity.

Fuels and the practical side
At low intensity a large proportion of the ATP comes from fat,
  • which is rich in hydrogen and provides much ATP per gram but needs more oxygen per unit of ATP;
    • at higher intensity carbohydrate is used preferentially,
  • because it releases ATP faster per unit of oxygen.
This explains why endurance athletes store glycogen before an event and why a body runs out of fuel (hitting the wall) when muscle and liver glycogen are used up.
The respiratory quotient rises towards 1.0 as the share of carbohydrate increases and can exceed 1.0 during heavy exercise when extra CO2 is released from the buffering of lactate.

Section 6 of 8Exam-style questions

  1. Explain why a sprinter keeps breathing heavily after the race has finished. [3]3 marks

  2. Explain why lactate accumulation causes muscle fatigue. [3]3 marks

  3. Athlete A has a VO2max of 72 cm3 kg-1 min-1 and athlete B has 48. Suggest two reasons for the difference. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Lactate thresholdIntensity at which lactate accumulates faster than it can be removed
Oxygen debtExtra oxygen used after exercise to restore normal conditions
Oxygen deficitShortfall of oxygen uptake at the start of exercise
VO2maxMaximum volume of oxygen used per minute per kg of body mass
Muscle fatigueReduced ability of muscle to contract
Training adaptationLong-term structural or functional change due to exercise
PhosphocreatineStore that regenerates ATP quickly without oxygen

Section 8 of 8Exam technique

1
Saying lactic acid is a waste product that is permanently lost: it is converted back to pyruvate or glycogen.
2
Saying muscle cramps are caused only by lactate.
3
Saying anaerobic respiration starts only after aerobic stops: both operate together.
4
Confusing VO2max (oxygen use) with vital capacity (lung volume).
5
Describing 'oxygen debt' as oxygen the muscles lack and cannot recover.

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