Critical Power: A Useful Concept for Functional Fitness Athletes?

Critical Power: A Useful Concept for Functional Fitness Athletes?

Download the spreadsheet to find out your figures here.

Critical Power: A Useful Concept for Functional Fitness Athletes?

What is Critical Power?

Critical power is a concept that all athletes will be intuitively familiar with. Your ‘critical power’ is the highest pace that you can sustain when exercising, where if you go just a little bit harder things start to get hard really quickly. Imagine you were asked to do three maximum effort tests on an Assault Bike. The aim in all three trials is to complete as many calories as possible within the time period – one is 3 minutes long, one is 8 minutes long, and the third is 12 minutes long. The testers are kind enough to give you enough time to fully recover between each one. Let’s say you complete 54 calories in 3 minutes, 123 calories in the 8 minute trial, and 174 calories in the 12 minute trial. The results will have the following pattern:

Critical Power Graph

On the y-axis we have a measure of ‘power’, in this case the number of Calories burned each minute. On the x-axis we have time. As you might expect, you can produce a far higher ‘average power’ over the 3 minute duration compared to the 12 minute test. But, eventually that line starts to level out, and the point at which it becomes flat is called the “critical power” threshold. It’s a threshold that separates ‘steady state’ and ‘non-steady state’ work. That is, you can exercise below the critical power threshold for a relatively long period of time (20 mins plus) in a sustainable manner (e.g., without lactate levels building up to intolerable levels), but as soon as you go above that line, you’re on borrowed time - - you’re eating into your ‘anaerobic battery’, which has a finite size and once it’s been used up, it takes a long time to recover. This is known as W′ (pronounced “W Prime”). Now, you can either tap that battery out very quickly by going well above your ‘critical power’ threshold (e.g., the green box in the figure above; this athlete could only sustain working at a rate 50 cals/min for ~30 seconds), or you could use that battery up much more slowly by working just above your critical power threshold (e.g., the orange box; this athlete could work at a rate of 17 cals/min for about 5 minutes). Either way, once that battery has been used up, the only way to recover is to bring you power output back down below the critical power threshold. Having a higher critical power threshold is clearly beneficial for functional fitness racing – it means you can work at a sustainable pace for longer, and recover more quickly between bouts. Functional fitness racing is an intermittent, high-intensity sport, and so knowing your critical power numbers can be extremely useful. Here’s how:

Identify strengths, weaknesses, and areas for improvement

  • It’s useful to know whether you’re an ‘aerobic’ or ‘anaerobic’ athlete. Whilst those terms are not strictly correct (as all the energy systems are at play at all times), it’s still a useful descriptor. You could have two athletes with the exact same 2000 m row time. However, one of them might have a higher critical power threshold, but a smaller battery (i.e., is more ‘aerobic’), whilst the second athlete has the reverse; a lower critical power, but a much larger ‘battery’ (more of a ‘power’ athlete). The first athlete has a higher ‘sustainable’ pace, but doesn’t have much capacity above that threshold. The second athlete cannot hold as high a pace in a sustainable way, but has a bigger ‘sprint’ capacity, and so can ‘empty the tank’ from much further out. The chances are that athlete 1 would prevail over longer distances, whilst athlete 2 would win over shorter distances (where critical power is less important). Knowing which one you are can help to guide your training (e.g., whether to focus on aerobic or strength/power development).

Inform your competition strategy

  • Knowing your critical power threshold can be extremely useful when it comes to pacing workouts. Take the Open workout, 19.1 (15 min AMRAP; 19 wall balls, 19 calorie row). The ideal pace for the row would be just above your critical power threshold; that will enable you to row a fast enough time (for you) whilst still preserving enough of your W′ battery for the rest of the workout. Knowing your specific critical power pace therefore comes in very handy to avoid going too hard too soon.

Predict your performance over any time/distance

  • Once you know your critical power and W′ battery numbers, you can use them to predict your performance over a huge range of times/distances. Never rowed a 5 km before? You can plug your values in from two shorter duration tests and get a pretty good guestimate of your predicted ‘best possible time’ for that 5 km distance. How many calories might you expect to burn on an Assault Bike in 10 minutes? Again, if you have your critical power and W′ battery numbers, you can get a good idea of what to shoot for.

Prescribe intervals

  • Perhaps the most useful thing about knowing your critical power and W′ battery numbers is that you can use them to create individualised interval sessions. For instance, if you’re doing 1000 m row intervals, you can find out the exact pace you need to row at the use up 60% of your W′ battery during that interval. This will ensure the pace is appropriate and sustainable across all the sets you intend to complete, by taking into account both your aerobic and anaerobic capacities.

Prescribe training zones

  • Once you know your critical power threshold, you can use it to create your own personal training zones, e.g.:

Training Zones Graph

Track changes in fitness over time

  • By measuring your critical power threshold regularly (e.g., every 6 weeks), you can track changes in your fitness levels in an objective and clear way.

How to measure your critical power and W′

You can calculate your critical power/speed and W′ values from a minimum of just two tests, although 3 is preferable. You want the shortest test to last at least 3 minutes, alongside a longer one lasting ~12 mins. If you’re doing a third, it should ideally last ~8 mins. The tests can either be ‘for time’ (e.g., a 1 km and 4 km row time-trial), or for distance/cals (e.g., maximum distance/cals in 3 minutes and 12 minutes). For accurate data, the tests need to be maximum efforts. You might already have recent ‘PB’ attempts over suitable times/distances, and so can plug those numbers in straight away. The two tests can be completed on the same day, so long as you leave enough time for full recovery (at least 30 mins). You can carry out these tests for any ‘cardio’ modalities (e.g., running, rowing, ski, bike), although it’s probably best to focus on improving one piece at a time. If you haven’t completed a test over those sorts of time-frames before, there’s likely to be a learning effect, and so it would be a good idea to perform at least one familiarisation test before doing this for real. Here’s an example of a single-day testing protocol:

Warm up:

  • 5 minutes easy.
  • Rest 5 minutes

Test 1:

  • Run/row/ski/bike for maximum distance/cals in 3 minutes (record distance/cals completed)
  • Rest at least 30 minutes

Test 2:

  • Run/row/ski/bike for maximum distance/cals in 12 minutes (record distance/cals completed)

That’s it – now you simply plug those numbers into this spreadsheet to find out your critical power and ‘battery’ size. Note, time is entered in seconds here.

Example Results Image

This athlete’s critical velocity on the rower is 4.01 m/s, which works out to a 500 m split time of 02:05 mins. So, any time this athlete rows faster than 02:05 / 500 m, they start using up their ‘battery’, which has a size of 114 m. In order to recharge their battery, this athlete would need to row slower than the 02:05 / 500 m pace. This information can now be used to guide their training and performance.

Download the spreadsheet to find out your figures here.

Clarke, D. C. & Skiba, P. F. (2013). Rationale and resources for teaching the mathematical modeling of athletic training and performance. Advances in Physiology Education, 37, 134-152.

Sean Williams
Senior Lecturer in Applied Statistics and Research Methods

My research interests include include sports injury prevention, training load monitoring, growth and maturation, heart rate variability, and meta-analyses.