capacities & thresholds

Your power/pace at the Anaerobic Threshold, has long been understood to be the most important determiner of endurance performance. Whilst this is certainly true in many (but not all) racing scenarios, what is typically less well understood, is the importance of your Aerobic Threshold, and how both thresholds are almost entirely composed by the relative strengths of your Anaerobic and Aerobic Capacities.

 

Anaerobic Capacity (AnC)
The Glycolytic energy system, results in the formation of Pyruvate and Hydrogen. Under anaerobic conditions (when insufficient oxygen is present) the pyruvate and hydrogen are then turned into a substance called Lactate. We can therefore use blood lactate measurements as a proxy for the amount of pyruvate that is being produced through anaerobic pathways. The Maximum Production Rate of Lactate (VLamax), measured in mmol/l/s, can then be used as a measure of your Anaerobic Capacity.

 

If we don't have access to blood lactate measurements, or software such as Inscyd which can accurately estimate lactate accumulation, (as you will see in the images below) it is very possible to monitor changes in VLamax, by simply monitoring changes in the fractional utilisation of your Aerobic Capacity. If a greater fraction of VO2max is being maintained at both thresholds, then it implies you have reduced your VLamax. If AnT and AeT are now at a lesser fraction of VO2max, it implies you have increased your VLamax. These changes can also be verified by a sprint assessment, as an increase in glycolytic capacity will correlate with an increase in sprint power.

 

Although a 1 minute duration has commonly been used to assess the strength of the anaerobic capacity, approximately half of the energy produced over this duration comes from aerobic pathways. Therefore, it is unsuitable by itself to monitor changes in VLamax. In the THP system, we use a 15 second sprint assessment to verify these changes.

 

Aerobic Capacity (AeC)
Your VO2max, measured in ml/kg/min, is the maximum volume of oxygen that you can utilise per kilogram of bodyweight, per minute of exercise. It is therefore a measure of your Aerobic Capacity. When glycolysis occurs in the presence of oxygen, instead of pyruvate combining with hydrogen ions to form lactate, it is converted into Acetyl Coenzyme A before entering the Krebs cycle. Any lactate that had been formed due to insufficient oxygen being present, can also be cleared by the aerobic system, mainly through being transformed back into pyruvate.

 

The rate at which your aerobic metabolism can clear lactate, is determined by your Aerobic Capacity. If it is not possible to assess VO2max through direct measurement, it is very possible to estimate VO2max, and far more importantly - monitor changes in VO2max. We can do this by simply performing repeat assessments over a duration we associate with the theoretical maximum power output that will illicit a peak in oxygen uptake. Individual differences aside, we will typically see this power output fall somewhere between your best 4-6 minute power. In the THP system, unless we are using laboratory or Inscyd testing for an athlete, we use a 5 minute assessment to monitor changes in VO2max.

 

Anaerobic Threshold (AnT)
Your Anaerobic Threshold (aka ... Maximum Lactate Steady State, Lactate Threshold, LT2, VT2, FTP, Threshold Pace) is the maximum power/pace output that you are able to maintain without lactate accumulating in your blood. As your anaerobic metabolism determines how much lactate you produce, and your aerobic metabolism determines how much of this lactate you combust - your anaerobic threshold is almost entirely composed by the relative strengths of your anaerobic and aerobic capacities.

 

As you will see in the images below, if your VLamax increases relative to your VO2max, it will suppress your anaerobic threshold by producing more lactate than your aerobic metabolism is able to deal with. Conversely, if your VLamax decreases relative to your VO2max, it will elevate your anaerobic threshold by producing less lactate. If we are not using laboratory or Inscyd testing to assess the anaerobic threshold, it is very possible to accurately estimate AnT by performing a time-trial. At THP, we recommend calculating AnT from your output over a TT duration between 20 and 30 minutes. We believe this duration range strikes an appropriate balance between reducing uncertainty and increasing repeatability.

 

Aerobic Threshold (AeT)
Your Aerobic Threshold (aka ... LT1, VT1) is the location on the intensity spectrum, where the reliance on carbohydrates to fuel energy production, exceeds the number of ATP produced from fat. It is therefore sometimes referred to as 'the crossover point' in substrate utilisation. It is situated above, but correlates very well with, your Maximum Rate of Fat Oxidation (FATmax). The stronger your aerobic capacity is, the less your metabolism will have to rely on carbohydrates as a fuel source, therefore the greater your ability to use fat.  As fat is in almost unlimited supply within even the leanest of athletes, your power/pace at the aerobic threshold, is perhaps the most significant determiner of endurance performance, in events that last for multiple hours. As you will see below, your AeT is also composed by the tug of war between your capacities.

 

If we are not using laboratory or Inscyd testing to assess the aerobic threshold, it is very possible to estimate AeT by performing a 'Talk-Test', as demonstrated by Carl Foster et al., 2008. As exercise intensity increases below AeT, you will reach a point where you notice a distinct increase in your breathing rate. Whilst you will still be able to string words together at AeT, talking in complete sentences will no longer be comfortable. This increase in breathing rate, is your body's response to the increasing reliance on carbohydrates to produce energy. As blood lactate accumulates at a faster rate, you are forced to breathe faster in an effort to clear this lactate, and exhale the extra carbon dioxide being produced.

Fig. 1
Representation of the relative intensities associated with the Aerobic Threshold (AeT), Anaerobic Threshold (AnT), Aerobic Capacity (VO2max) and Anaerobic Capacity (VLamax). Location of the Anaerobic Threshold is determined by the point at which lactate production equals lactate clearance. Above this threshold lactate is always accumulating.

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Fig. 2
Representation of the scenario when an athlete's VO2max remains constant, but the VLamax increases or decreases. From baseline positions (A), if the theoretical athlete increases their VLamax (B), lactate clearance can no longer be achieved at such a high output. The Anaerobic and Aerobic thresholds therefore both decrease. If the athlete reduces their VLamax from baseline levels (C), lactate clearance can take place at a higher output, therefore elevating both AnT and AeT. Note how the output associated with VO2max remains constant at all times, and it is the fractional utilisation of this capacity which is changing. 

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Fig. 3
Representation of the scenario when an athlete's VLamax remains constant, but the VO2amax decreases or increases. From baseline positions (A), if the theoretical athlete's VO2max decreases (B), lactate clearance can no longer be achieved at such a high output. The Anaerobic and Aerobic thresholds therefore both decrease. If the athlete increases their VO2max from baseline levels (C), lactate clearance can take place at a higher output, therefore elevating both AnT and AeT. Note how in this scenario, the fraction of VO2max that can be maintained remains constant, but the the output at VO2max is changing.

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Fig. 4
Representation of the worst and best case* scenarios for an endurance athlete. From baseline positions (A), if the theoretical athlete's VO2max decreases, and their VLamax increases (B), output at AnT and AeT is reduced severely, due not only to a lower VO2max, but also to a smaller fraction of this reduced capacity providing lactate clearance. In the best case* scenario for an endurance athlete (C), the athlete decreases their VLamax and increases their VO2max. This significantly increases output at both AnT and AeT, because it not only benefits from a higher output at VO2max, but also from a greater fractional utilisation of this elevated capacity.

*Please note: this is only a 'best case' scenario, if the athlete doesn't require a higher VLamax in order to make a race winning acceleration. The specific demands of an event determine the theoretical optimal level of VLamax. It is also important to note, that even for a 'steady-state' event that doesn't require a 'high' VLamax - the VLamax must still be 'sufficiently high' to fuel the aerobic metabolism.

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Fig. 5
Representation of the scenario where no change occurs in the output at AnT or AeT, despite changes to both VO2max and VLamax. From baseline positions (A), if the theoretical athlete's VO2max and VLamax decreases (B), the output at AnT and AeT can remain stable. Despite the reduction in VO2max, the fractional utilisation of this reduced capacity has increased. If the athlete increases both their VLamax and VO2max (C), output at AnT and AeT can also remain stable. In this instance, the increase in VO2max is offset by a reduced fractional utilisation of the elevated capacity.

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