Is ‘Threshold’ the most misunderstood sports physiology term? It certainly does cause confusion and in many cases, a runner’s perception of threshold is different. Marc Laithwaite takes a look and strips it back-to-basics.

What is threshold?

To start this conversation, we need to first point out that there are many different types of threshold and this is where the confusion begins. To give a few examples:

1. Lactate threshold

2. Aerobic / Anaerobic threshold

3. Ventilatory threshold

4. Functional threshold

There’s lots of magazine articles which outline the benefits of calculating your threshold and how you can use it for training purposes, but many of them are poorly written and incorrect, so here’s our low down.

Lactate Threshold

You can complete a lactate threshold test by cycling or running, gradually increasing your pace and taking finger prick blood samples at regular intervals to measure lactate in the blood. As the exercise gets harder, the lactate levels will increase.

There are technically 2 lactate threshold points. The first one is very early in the test, when your lactate levels start to rise above their resting levels. In practical terms, if you can hold a full conversation whilst riding your bike, then suddenly you feel that your breathing rate rises slightly, this is your true ‘lactate threshold’. This occurs very early and generally the heart rate at your lactate threshold will be the border of zone 1 and zone 2, so relatively comfortable.

As the test continues your breathing will get harder and harder and then you’ll reach a second lactate threshold. Up until this time your lactate has been steadily rising, but this is followed by a sudden and sharp kick upwards. As the test continues, your lactate will continue to rise sharply and you’re on borrowed time… you will be stopping soon as the lactate accumulates in your muscles. This second and sharp ‘kick up’ or ‘deflection point’ is what we tend to incorrectly refer to as ‘lactate threshold’. This is the figure that most people have completed the test for, technically, this is the ‘lactate turn-point’ or ‘Onset of Blood Lactate Accumulation’ (OBLA).

So why should I measure my lactate threshold / lactate turn-point?

Well, your lactate threshold is a very good marker for many endurance events. Optimal Ironman bike pace tends to be very close to lactate threshold for many people (for training zones we use, it’s the border of zone 1 / 2, but this is not the case for other calculated zones).

The lactate turn-point is the measurement that most people really want to find out. When does my lactate start to rapidly accumulate, what’s the running speed / heart rate / power output at that point? Many magazine articles generally state that your hard / sustained exercise pace coincides with lactate turn-point, but in practical terms, that’s incorrect.

Let’s give an example of a runner who completes a lactate turn-point test and we calculate the heart rate at lactate turn-point to be 165bpm. If we ask this runner what their heart rate generally is during a 5-10k race, they will generally give is a figure 5-8 beats higher than the lactate turn-point. Initially this is confusing, as most people think that the heart rates will match. In terms of calculating heart rate training zones, we will therefore have to guess by adding 5-8 beats to the lactate turn-point, to calculate a ‘threshold heart rate’.

So why do a lactate threshold / lactate turn-point test?

Lactate testing does provide some information, but it can also be relatively limited in it’s use. One of the key things it does provide is a bench mark. If you repeat the test and the turn-point occurs at a later speed or power output, then your fitness has improved. From a coaching perspective, if we want to use the test results to provide coaching advice and training zones, then it’s not the best option for us to choose.

So what’s the other options?

Aerobic/Anaerobic and Ventilatory thresholds can be calculated by measuring expired gases and breathing rates during testing. These tend to fall much more accurately as predictors or training intensities. Functional threshold, is a completely different concept.

We don’t need to take blood (as we do for lactate) to measure these thresholds. They can be calculated by measuring the air going in and out of your lungs. Primarily, there are 3 things we measure, how quickly you’re breathing, how much air moves in and out and what the air is made up of e.g. oxygen and carbon dioxide.

The role of carbon dioxide

Carbon dioxide is a waste product produced by muscles and other tissues. It’s pretty toxic so when we produce it, we need to get rid of it. Your body has sensors which detects when carbon dioxide level increase, it triggers your breathing rate to speed up so you can exhale it.

When you run harder, you need more oxygen so you breathe harder; yes?

Technically yes, although the main trigger is carbon dioxide. When you start to run, you produce CO2, this triggers breathing and heart rate to go up. The harder you work, the more CO2 you produce, this triggers breathing and heart rate to increase further.

How do we use this to calculate thresholds?

This is very simple. We can measure the increase in breathing rate when you exercise and we can measure how much oxygen your body absorbs. When you breathe faster, you do it for 2 reasons: to get more oxygen in and to remove waste carbon dioxide. If your breathing rate goes up but your body doesn’t absorb any more oxygen as a result, then you must be breathing faster for the other reason… to get rid of carbon dioxide.

Calculating thresholds

During a cycle or run testing session, there are 2 key thresholds. The aerobic threshold occurs quite early, this is the point when your breathing rate increases above rest. The best example of this is being able to exercise and hold a full conversation, then as the pace increases, you notice a change in your breathing and can’t hold a full conversation. This threshold occurs quite early during an exercise test.

As the exercise test gets harder, your breathing rate increases steadily to match. Eventually you hit a second ‘anaerobic threshold’ point where your breathing starts to rapidly increase. During a 10k / 5k running race at your fastest pace, your breathing will be fast and hard, but it will remain ‘stable’. If you push the pace just a little too much, it becomes ‘unstable’ and you start to hyperventilate. The only way to change this is to slow down and regain control of breathing.

These 2 thresholds can be measured during a Vo2 max testing session, by using a mask and gas analysis machine. Their description sounds similar to lactate thresholds but we find that the heart rate calculations are generally higher than during a lactate threshold test. Measuring thresholds as above tends to be more accurate for most athletes, when lactate threshold tends to calculate lower than expected and is therefore less practical.

Ventilatory threshold

The test we’ve outlined above involves the measurement of breathing rate to identify changes, for this reason, they are often referred to as ventilatory thresholds (VT). Next time you are riding with a friend and approaching a hill, listen to their breathing (and your own) and you can identify the 2 thresholds. Start at an easy conversation pace and climb steadily, the conversation will soon stop at VT1. Continue to climb and increase the pace and your breathing will become more laboured but still under control. For the last few minutes, ride at a pace which is harder than you can sustain, you’ll sense and hear your breathing rapidly increasing beyond control, this is VT2. On a long, hard climb, most people know where their VT2 is, and instinctively ride/run a few beats below it, to ensure that they don’t ‘blow!!’.

Now we’re talking ‘functional threshold’ which is a term more commonly used amongst cyclists in particular.

What is functional threshold?

The clue is in the name ‘functional’. The lactate / anaerobic / ventilatory thresholds are all valuable physiological markers but what’s their practical use? A cyclist riding a time trial really only needs to know one thing, how much power can they sustain for the full ride. The more power they can sustain, the faster they will complete the course.

There is often a misunderstanding with regards to lactate / anaerobic threshold. If you visit a lab and have your lactate threshold measured, that doesn’t tell you the power or speed you are able to sustain for a long period of time. To find the answer to that question, there is a more simple / practical / functional approach. Simply get on your bike and ride as hard as you can for an hour, then you’ll know the answer.

Functional Threshold Power

The FTP is a real ‘buzz term’ in cycle coaching and I’m sure most cyclists and triathletes will have heard it mentioned by someone at some point! FTP is quite simply the highest amount of power you can average for an hour. To complete this test, you need a power meter on your bike or turbo trainer.

Riding for an hour on the turbo is a killer!! So to get round that, most people complete a 20 minute test and take the average power reading. If you then calculate 95% of that figure, that’s your predicted FTP (average power for hour). For example:

1. Bob completes a 20 minute maximal test and averages 250 watts.

2. 95% of 250 = 237.5, this is Bob’s FTP and what he should be capable of holding for an hour.

Is this just for cycling or can it be used for running and swimming?

The issue with running and swimming is that you don’t have access to power data, so you can’t calculate the figures as you can with cycling. Having said that, the functional threshold is really just a ‘practical test’ to calculate what you can hold for a period of time.

Swimmers will often complete a ‘critical swim test’ which is basically the same thing. The test is simply swimming as far as you can in 20 minutes. For running, the same applies, you could complete a 15-20 minute test and measure distance on the running track.

So why are functional tests popular?

They’re popular because they are very practical. If you want to find out how fast you can complete a 25 miles time trial, the figure which is most likely to give you the answer is how much power can you average for an hour. Doing a lab test to calculate your lactate threshold or anaerobic threshold is valuable and useful for many reasons, but it will not give you the answer to the above question. Ultimately, functional tests tell you how fast you can swim/cycle/run for a set period of time and that’s the best indicator of race performances.

About Marc:

Sports Science lecturer for 10 years at St Helens HE College.

2004 established The Endurance Coach LTD sports science and coaching business. Worked with British Cycling as physiology support 2008-2008. Previous Triathlon England Regional Academy Head Coach, North West.

In 2006 established Epic Events Management LTD. Now one of the largest event companies in the NW, organising a range of triathlon, swimming and cycling events. EPIC EVENTS also encompasses Montane Trail 26 and Petzl Night Runner events.

In 2010 established Montane Lakeland 50 & 100 LTD. This has now become the UKs leading ultra distance trail running event.

In 2010 established The Endurance Store triathlon, trail running and open water swimming store. Based in Appley Bridge, Wigan, we are the North West’s community store, organising and supporting local athletes and local events.

Check out the endurance store HERE