Anaerobic Threshold, Fuel Switching, Lactate, Ventilation

What does RER indicate?

RER tells us the balance of fat vs carbohydrate use; RER < 1 means aerobic fat use, RER > 1 means carbohydrate use via anaerobic glycolysis.

What is the anaerobic threshold?

The point where metabolism shifts from primarily fats (aerobic) to carbohydrates (glycolytic), shown by rising RER.

What fuel is used in low-intensity exercise?

Fats via beta-oxidation, which requires large oxygen amounts and supports long-duration activity.

What is the typical RER during low-intensity work?

RER < 1 because fat metabolism uses more oxygen relative to CO₂ production.

How are fats broken down?

Through beta-oxidation, a slow, oxygen-demanding process that supports sustained exercise.

What happens at low-moderate intensity exercise?

Fuel becomes a mix of fats + carbohydrates, with RER rising but staying <1.

Why are carbohydrates used more at moderate intensity?

They require fewer chemical reactions, produce pyruvate quickly, and need less oxygen per CO₂ produced.

What happens to pyruvate during aerobic metabolism?

Pyruvate → acetyl-CoA → Krebs cycle → ATP + CO₂.

Why does RER rise as intensity increases?

More carbohydrate is used because it produces ATP faster than fat.

What happens during high-intensity exercise?

Glycolysis speeds up, hydrogen ions accumulate, and anaerobic metabolism increases.

What does high hydrogen ion accumulation mean?

It signals anaerobic glycolysis, leads to acidity, and pushes pyruvate to convert to lactate.

How is lactate formed?

Pyruvate binds hydrogen ions → lactate (when aerobic system is overloaded).

What is the Lactate Threshold?

The point where lactate production exceeds lactate clearance, indicating reliance on anaerobic metabolism.

What does rising lactate in blood indicate?

That aerobic metabolism can't meet energy demand and anaerobic glycolysis is dominant.

What happens to ventilation as exercise intensity increases?

Ventilation rises gradually, then spikes rapidly at the ventilatory anaerobic threshold.

What drives increased ventilation at higher intensity?

CO₂ production increases from buffering hydrogen ions during glycolysis.

What is the Ventilatory Anaerobic Threshold?

Point where ventilation increases disproportionately to VO₂, signalling shift to anaerobic metabolism.

What causes RER to exceed 1?

Excess CO₂ from buffering acids in glycolysis and increased carbohydrate metabolism.

What does RER > 1 always indicate?

Carbohydrate-dominant, anaerobic energy production.

Why does carbohydrate metabolism produce more CO₂?

Glycolysis + buffering of hydrogen ions generate extra CO₂.

What happens to breathing at vigorous intensity?

Breathing becomes harder and faster to remove CO₂ and meet metabolic demands.

What is the main fuel during vigorous exercise?

Carbohydrates (they produce ATP faster than fats).

Why can't fats be used at high intensity?

Fat metabolism is too slow and oxygen-demanding for rapid ATP supply.

What are the signs of reaching anaerobic threshold?

Spike in ventilation, RER approaching or >1, rising lactate.

Why is lactate NOT the cause of muscle fatigue?

Lactate is a fuel; hydrogen ions (acidity) cause fatigue.

What happens when pyruvate can't enter mitochondria fast enough?

It binds hydrogen ions and becomes lactate.

Why does ventilation spike before exhaustion?

To remove rising CO₂ produced during anaerobic metabolism.

What systems dominate before vs after threshold?

Before: aerobic, fat-dominant. After: anaerobic, carbohydrate-dominant.

Why is anaerobic threshold important for physiotherapists?

It determines safe exercise intensity and helps prescribe training zones.

How do athletes improve anaerobic threshold?

Interval training + tempo training to increase ability to tolerate and recycle lactate.

What does poor aerobic fitness look like during threshold testing?

Higher breathing rate earlier, early rise in RER, early lactate accumulation.