IB Sports, exercise and health science A.2.3.4 Lactate Inflection Point and Its Significance in Metabolism Notes

Lactate inflection point marks the highest sustainable intensity (HL Only)

Beyond this point, lactate accumulates in the blood, leading to fatigue and a decline in performance.

Why LIP predicts sustainable endurance intensity

1. LIP determines the maximal sustainable exercise intensity.
2. Athletes with a higher LIP can maintain higher intensities for longer without lactate accumulation.
3. LIP is a better predictor of endurance performance than VO₂ max for events lasting more than 15 minutes.

LIP as a Percentage of VO₂max

1. LIP is usually reported as a percentage of VO₂max. It tells you how much of the aerobic ceiling an athlete can use sustainably. The fitter the athlete, the higher the percentage.
2. Untrained adults: LIP sits around 50 to 60% of VO₂max. Above this they cannot last long without breathing hard and slowing down.
3. Moderately trained: LIP rises to roughly 65 to 75% of VO₂max.
4. Elite endurance athletes: LIP often reaches 80 to 90% of VO₂max. They can hold paces close to their absolute aerobic limit for long periods.

Lactate accumulates when production exceeds clearance

Lactate is produced continuously during glycolysis

1. Lactate is produced continuously even at rest and during low-intensity exercise. The body clears it as fast as it is produced at low intensity, keeping blood lactate near 1-2 mmol/L.
2. It is continuously produced, even at rest and low-intensity exercise, but the body efficiently clears it through oxidation, gluconeogenesis, and tissue transport.
3. Lactate accumulates when production exceeds clearance.

Lactate is a Usable Fuel, Not Waste

1. Old view (wrong): lactate was taught for decades as the cause of muscle fatigue and burning.
2. Modern view: lactate is not a metabolic dead end. It is a usable fuel that the body actively transports and oxidises.
3. Lactate shuttle: lactate produced in fast-twitch fibres is shuttled to nearby slow-twitch fibres, the heart, and the liver.
4. Heart: oxidises lactate directly back to pyruvate and feeds it into aerobic metabolism.
5. Liver: converts lactate back into glucose through gluconeogenesis (the Cori cycle).

Lactate is cleared by oxidation, gluconeogenesis, transport, and buffering

1. Oxidation in muscle fibres: Type I (slow-twitch) muscle fibres use lactate as a fuel source.
2. Gluconeogenesis in the liver (Cori cycle): Lactate is transported to the liver and converted back to glucose.
3. Transport to other tissues: Lactate is shuttled to different muscle fibres and the heart for energy.
4. Buffering mechanisms: hydrogen ions (H⁺) from metabolic acid are neutralised by bicarbonate (HCO₃⁻) to prevent acidosis.

Critical power is closely linked to the lactate inflection point

1. Critical Power (CP) is another physiological threshold closely linked to LIP.
2. It represents the maximal sustainable power output without a rapid rise in lactate.
3. Any exercise intensity above CP results in exponential fatigue onset.
4. W-prime (W′) reserve: above critical power, work depletes a fixed energy reserve called W′. Below CP, the reserve refills.
5. Why team-sport athletes care: every sprint costs from the W′ reserve. Players who recover their W′ quickly (high aerobic fitness) can sprint repeatedly without performance falling off. Players who recover slowly fade in the second half.

CP measures sustainable power, whereas LIP measures lactate accumulation

1. LIP focuses on blood lactate accumulation, while CP focuses on power output sustainability.
2. CP represents the highest power an individual can maintain over time before fatigue sets in.
3. If exercise intensity exceeds CP, fatigue occurs rapidly, whereas staying below CP allows for prolonged performance.

Training, muscle fibres, and physiology influence LIP

LIP is influenced by training status, muscle fibre type, cardiovascular efficiency, metabolic adaptations, and genetics.

Threshold, HIIT, and endurance training can raise LIP

Threshold training, HIIT, and endurance training can raise LIP by improving lactate clearance, buffering capacity, and oxygen utilisation.