The term threshold is probably the most misunderstood term in running.

It’s commonly referred to as the Anaerobic threshold yet if you go above it you can still be aerobic. It is sometimes referred to as the Lactate Threshold , Ventilatory Threshold or Functional Threshold. It can be defined in a number of different ways, and measured via different testing protocols. The different definitions and testing protocols usually end up at a similar measure of threshold but they are not the same.

It is a complex topic that is still being debated about in academic and coaching circles today. For this course, we will aim to give you a deeper understanding of what threshold means and how you can use it for coaching ultra runners without turning it into a biochemistry lesson.

There are two main thresholds that we refer to,  the first is the transition from easy to moderate intensity and the second is the transition from moderate to hard intensity.

This forms the basis of the 3 Zone model of intensity , Zone 1 is below the first threshold , zone 2 between the first and second and zone 3 above the second.

We will talk in more detail about training zones in another video but it’s hard to talk about thresholds without mentioning zones as thresholds form the boundaries of zones.

Is Threshold a distinct boundary or a transition ?

Many runners ( and coaches ) believe in the idea of a distinct threshold. Eg  if your  anaerobic threshold has been determined to be  170 HR then when you are running at 171HR  you are in zone 3 and 169 is zone 2.

But is it really as clear cut as that ?

To understand this lets talk about the second threshold first  , the boundary between zone 2 and 3 .

There are many different names for the thresholds that attempt to define this including

– maximal lactate steady state
– critical speed
– critical power
– lactate threshold
– ventilatory threshold
– respiratory compensation point
– gas exchange threshold

The definitions of these also vary and include

– intensity one can hold for 60 minutes
–  the speed/ HR/ power  that corresponds to 4mmol of blood lactate
–  the speed/ HR/ Power  at which there is rapid/distinct change in blood lactate levels
– the highest intensity at which lactate production and elimination are in equilibrium
– the point at which CO2 production increases rapidly

The duration at which one can run at threshold is also debatable with some saying 60 minutes some 40  minutes and some saying it depends on the athlete.

So with some many definitions where does that leave us?

First of all there is a lot that all these definitions have in common  – they all attempt to define a point at which the following happens

– Lactate is being produced faster than the body can remove it.
– Hydrogen ion production increases at a rate faster than the body can neutralise it leading to an acidosis in the muscles.
– Respiratory rate increases significantly.
– A greater reliance on the anaerobic system using glycolysis to obtain energy without oxygen.

The problem lies in
A) how accurately do these definitions demarcate the transition between moderate and severe exercise
B) how accurately can they be determined

There is a belief that lab testing can accurately determine your threshold and therefore your training zones but your results will depend on which definition of threshold the lab uses and which testing protocol they use. Different definitions and different testing protocols will lead to different results.

For example compare these two charts

They both show blood lactate vs pace
The first one has twice as many data points as the second but every second point is the same. Ie the red dots on the first graph correspond to same points on the right graph.

If we used the second graph to determine threshold we get a different result compared to the first graph- on the second graph its at the 4th data point , on the first graph its the 8th data point.

Obviously  the more data you have the better accuracy you get  BUT when we are dealing with athletes exercising to exhaustion we are limited to how many data points we can take.  Do we test blood lactate every minute ? Every two minutes , do we increase speed of the treadmill or the incline , do we increase every 2 minutes  , 3 minutes  ? The answers to these will change the results of the test.

So theoretically if you sent an athlete to several different testing laboratories they could come back with different results.  (For a more detailed discussion on lactate see the lactate video )

These results would all be similar but not the same.

With so many different definitions  and different testing protocols all of which lead to slightly different threshold values we have two possible scenarios

1. One of those definitions and testing protocols  is correct and all others are incorrect
2. It is impossible to define the boundary with a high degree of precision , instead we should think of threshold as a narrow band  and  there is a  transition from one zone to the next

The current research is far from concluding one  particular definition is more accurate than another and even less clear on what testing protocol is delivers the most accurate results.

So the only logical conclusion is that ones threshold which delineates the boundary between one zone and another is more a transition than a distinct boundary.  This means ones threshold cannot be defined to a single heart rate or pace or power.

The chart below illustrates this concept

The purple line is blood lactate levels  plotted against pace or heart rate or power in a graded exercise test to exhaustion

The orange band shows the transition from moderately hard intensity to very hard – ie zone 2 to zone 3   and is what we think of as Anaerobic/ Lactate/ Functional Threshold

The green band shows the transition from easy intensity to moderate intensity Zone 1 to zone 2

Boundary or transition why does it matter ? 

I frequently see athletes worrying about exactly what their threshold is so they can make sure they stay in the correct training zone. In reality, there is no noticeable difference in training adaptions between slightly above or below whatever they have determined their threshold to be for two main reasons.

1. Threshold has a range ( a narrow range admittedly but it’s not a distinct point)
2. Even if threshold could be determined to a precise intensity,  most athletes are relying on software to determine threshold which introduces a relatively large source of error.  The accuracy of any software relies  on the data you feed into it and very few athletes truly understand what data is needed for the software to have any chance of accurately determining threshold.

Threshold as a transition

There is this notion that above threshold we accumulate fatigue rapidly and will be forced to slow down quickly.  If we explore that concept we find it not to be true.

For example lets say an athletes threshold was determined to be 4min k pace  and that the duration they could hold that for is 40minutes/ 10km.

If they took off at 3:55 pace would they blow up quickly as they are above threshold ? Obviously they wouldn’t make the 10km at that pace but they would probably make it to 7 or 8km before they were forced to  slow down.  If they started at 3:50min k  pace likely they could get to 5km.

The point is there is no sudden large increase in fatigue that comes from running just above threshold . An athletes  8km race pace is a touch quicker than 10km  pace  and their 5km is a touch quicker again. Going a touch faster than threshold does reduce the duration at which you can sustain that pace but not massively.

Yes the duration at which you can sustain paces above threshold for drops of quicker than at paces below threshold but its not like if you go just above your threshold and 5 minutes later your legs will rapidly become acidic and force you to a halt.

The same goes for training adaptions, going just above or just below threshold does not make any measurable difference in how the body adapts.

This doesn’t mean determining ones threshold is a waste of time though.

So why bother with threshold  at all ?

If we are going to give our athletes a metric of intensity to guide their training we have to use something and threshold is the best anchor point we have.
The other two anchor points are VO2 and HR max which studies have shown are less effective in helping delineate the moderate and hard  domains of exercise.

Eg this one

“However, there is little evidence to support the validity of most commonly used methods, with exception of CP and CS, to delineate the heavy and severe domains of exercise.”
(https://www.researchgate.net/publication/343321238_An_Examination_and_Critique_of_Current_Methods_to_Determine_Exercise_Intensity)

“VO2max and Wmax are also often used in studies to determine exercise intensity in subsequent interventions. However,this approach is flawed, as there are interindividual differences in the percentage of VO2max and Wmax at which boundaries between different exercise intensity domains occur. Thus, different physiological responses between participants can be observed when anchoring exercise intensity to fractions of VO2maxor Wmax (40). If the CP/CS is determined as part of the classification process, these values can also be used to anchor exercise intensity in any subsequent intervention.”

(https://www.researchgate.net/publication/358686503_Viewpoint_Using_VO_2max_as_a_marker_of_training_status_in_athletes_-_can_we_do_better)

EI definition based on percentages of peak HR and peak VO2 may misclassify the effective EI …… A ventilatory threshold–based rather than a range-based approach is advisable to define an appropriate level of EI.
(https://onlinelibrary.wiley.com/doi/pdf/10.1111/sms.14007)

Another reason to use threshold rather than HR max is  threshold is directly related to one’s fitness.  This  means  our prescription of exercise intensity can be the same regardless of fitness. For example  an easy run at 80% of threshold  will feel the same relative intensity regardless if it is an elite athlete or someone with minimal running fitness.

Whereas an easy run of 70% of HR max will feel much harder for the unfit runner vs an elite. Threshold is typically 80-90% of HR max so if both runners are running at 70% then the elite runner is running 20% slower than their threshold vs the unfit runner running at 10% slower.

How to define threshold ?
While debate continues as to which  threshold is best  what really matters is what can we do on a practical level to give us a measure from which we can prescribe intensity from.

One thing to keep in mind when discussing thresholds  is the idea that experienced athletes know where their thresholds are without testing. Before the age of heart rate monitors and lactate testing runners instinctively knew where that transition was between easy and moderate and moderate and hard.

Researchers have been trying to quantify that using what they have at their disposal . But lactate levels, changes in CO2 levels etc , are the body’s response to running not the driving force behind it. To run faster we don’t increase our lactate levels and then notice our speed increasing.

The idea that you need to precisely determine  lactate levels or respiratory exchange ratios to determine your threshold so you know how fast you can run is a backwards way of thinking of it .  How fast you can run determines your lactate levels not the other way around.  For sure lactate levels  and ones ability to process lactate ( amounts other things ) limits performance but performance comes first. The knowledge of lactate concentrations may help one understand why performance is what it is but the primary metric is performance.

Researchers and Coaches are moving towards the idea that using a measure of threshold based on performance is more valid as it relates to what the athlete can actually do.  When racing,  how high your heart rate is,  or how low your lactate levels are does not get you on the podium. Fastest wins. Hence critical speed or critical power are  becoming the thresholds of choice. As a trail runner critical speed is of much less value hence my recommendation that power be an athletes go to metric.

But let’s look at all the options and you and your athlete can determine which works best for you.

Before we do that I think it’s important to have a working definition of threshold. and to do that we need to discuss  how long one can spend at threshold.

A common misbelief is that threshold is the highest intensity or pace you can maintain for an hour.  The chances that there is a physiological threshold that corresponds exactly to 60 minutes are about zero.  In fact the idea that there is any standard duration that one can exercise at threshold for is also flawed.
Its far more likely that the time one can maintain threshold for depends on ones fitness.

A more practical definition of threshold is .. “ the highest intensity one can maintain in a quasi-steady state for 30-75 minutes  without a notable decline“ and this is what is commonly referred to as Functional Threshold

The quasi-steady simply means that physiological variables dont change much from beginning to end, eg blood lactate , pulmonary oxygen uptake.

The duration at which one can sustain ones Functional Threshold  is called Time to Exhaustion  (TTE)

Why such a big range  in TTE ?
If you consider a 30 minute  park runner with a sub 60 minute  half marathoner you can start to understand why TTE varies so much . They are both probably running at same relative intensity but the sub 60 half marathoner can a) hold it for longer and b) is much faster.

As much as we would love to have exact definitions  for threshold you can hopefully see that the human body doesnt work that way!

Having discussed that threshold is a transition lets know talk about how to derive a number for it so we cause it to prescribe training using our 4 main metrics – RPE, pace, heart rate and power

RPE
Being the subjective measurement one can argue it’s the least accurate of the 4 intensity metrics however experienced runners can dial in their pace without even looking at their watch. With many runners not having power metres, with wrist based heart rate monitors having reliably issues and pace being useless on hills it may be the only metric you have to use with your athlete.

A good baseline to refer to for most athletes is a 10km race ( assuming it takes between 30-60 minutes  )

For runners closer to 30-35 ish  minutes 10km intensity is  higher than threshold.
For 35-45 minute runners its around threshold
for 45+ runners its a touch easier than threshold.

Of course how one feels at the start of a 10km is significantly different to how one feels at the end so if giving a session say 6 x 5 minutes  at threshold then the first rep should feel like the start of a 10km not the end !

Power

To determine Threshold power the best method  is to perform an all out 2-3 minutes effort and a 10-15 minutes effort and then use an online calculator or specific software to calculate threshold.  There are a number of different calculators around and will discuss this in more detail in the power video.

Heart rate

A common method is to do an all out 30 minutes  effort and record your average heart rate for the last 20 minutes . That heart rate is said to be your threshold , its likely a touch over your threshold but depends on how fresh your legs are, with fresh legs is a touch higher than threshold , on legs with normal levels of training fatigue is probably very close to threshold.

Pace

Whilst pace has limited value for trail runners it is still of value for flatter speed sessions so it can be helpful to understand what an athletes threshold pace is.
Same as for RPE , a recent 10km race time is a good starting point. If you are a 35-40 minutes  10km runner your 10km pace is likely same as threshold.
A park run is something many runners do and you can use that to give you an estimate of Threshold  using a factor of 0.96 to 0.99 to convert park run pace to threshold hold pace.

Eg a 20 minutes  park runner  runs at 4 min ks   divide by 0.96 = 4:16 minutes  k threshold hold pace
A 30 minutes  park runner runs at 6 min ks  divide by 0.99 – 6:03 threshold pace

This is an estimate only but will get you in the ball park.

Critical Velocity is another means of doing it and involves having 2 maximal efforts in the 2-20 minutes  range and then using a calculator such as this one (https://running-universe.com/critical-velocity-calculator/)

Critical velocity is not the same as most definitions of threshold but it to al intensive purposes represents the same thing. For a detailed discussion see here here

The 1st Threshold

The first threshold the delineates the border between easy intensity and moderate intensity is sometimes called LT1 , VT1 or the aerobic threshold ( AeT)

As with the 2nd threshold it too is a transition rather than a distinct point.

Defintions include

– lowest intensity in which there is a sustained rise in blood lactate levels above resting values
– the first significant increase in breathing as exercise intensity increases
– the point at which the slope of the ventilatory curve changes
– the point at which fat oxidation is the highest

There are a number of ways to determine it outside a lab including

50-75% of max Heart rate
80-85% of 2nd Threshold
180- age

As you can start to understand there is even less precision and more variation in determining  the  AeT  than there was with the AnT. For example the intensity at maximum fat oxidation varies considerably depending on an athletes diet and if they consume calories before or during runs – ie not related to their fitness.

Given the majority of training should be performed under the  Aerobic threshold it’s important to be able to give athletes some means of working out what intensity that corresponds to.

The easiest method is the talk test which may sound unscientific but in practice is quite accurate. The aerobic threshold is the highest intensity at which one can have a normal conversation.

A simple test is to get your athlete to recite some song  lyrics they know well whilst running at different paces and get them to tell you when they couldn’t make it through the song without needing to catch their  breath . Thats your aerobic threshold.

In practice I have seen this ranges from 80-88% of threshold power.

Key Take Homes

At the Aerobic threshold one transitions from easy intensity to moderate intensity.

At the Anaerobic threshold one transitions from moderate intensity to very hard intensity.

The are many definitions of both, and whilst they are similar they are not the same.

Lab testing does not give a definitive determination of ones threshold as it depends on the testing protocol and the definition the lab is using

The thresholds should be thought of as a transition from one intensity to another not a distinct border.

A practical determinant of threshold based on performance is a) likely more accurate and b) all that is needed in terms of an anchor point from which to determine training intensity.

Using Anaerobic threshold as an anchor point for intensity is more valid than using max heart rate or Vo2 max.