If you look up the synonyms of ‘accuracy’, ‘precise’ is often top of the list. If you think about your own use of language, you probably drop either word into a sentence when you’re trying to say that something is ‘exact’. In general life, throwing ‘accurate’ or ‘precise’ into a sentence is neither here nor there. However, it isn’t so lightly scattered around by engineers, who will be quick to properly define these words for you:
Accuracy is how close you are to a known (true) measure.
Precision is how close the measured values are to each other.
As sports science advances so does our understanding of the body and how to optimise performance. By tracking heart rate, pace, distance, cadence and, the key measure of this particular article, power output we can eke the most out of ourselves, knowing we’re working in the correct zones, recovering properly or pushing ourselves to the very very limit. So, in that sense, when it comes to data, athletes need to be as pedantic as engineers.
When I say ‘athletes’, I don’t just mean the professionals, the Olympians, the record holders. I mean anyone who is striving to better themselves; those who train specifically in pursuit of a goal. If you care enough to be reading this then I probably mean you.
And so to the point: Why does accuracy matter? Furthermore, where does precision come into it? Well, when it comes to power meters many manufacturers are happy to peddle a product that they openly state is ‘accurate to +/- 2%’.
I suppose it doesn’t seem much, does it?
Let’s break this down.
You’re acquiring a training aid; this item is meant to help you improve. In order to achieve your goals, you’ll have specific sessions to complete. These sessions must be performed to a certain degree of difficulty, which should have been established following a baseline test, such as a Functional Threshold Power (FTP) test.
Typically, there are 7 zones in which to train, ranging from Active recovery (<55%), Endurance (56-75%) to Anaerobic capacity (>121%) and Neuromuscular power (hard as hell). These zones equate to percentages of your average power value (95% of sustained power), obtained during the baseline test. For argument’s sake, let’s say you did an FTP test, which gave you a value of 250 W.
Your training programme tells you to ride for 3 hours in Zone 2 (Endurance), which means you need to stay within 140 W (56% of 250 W) and 187 W (75% of 250 W). Pretty specific, right?
Let’s say that you’ve unfortunately saddled yourself with one of these power meters that’s ‘accurate to +/-2%’. Off you go on your 3-hour ride; you’re fresh-legged and fancy free, pedalling away at the top end, 187 W, OR SO YOU THINK. Your almost-accurate power meter isn’t showing you your actual power output, it’s understating it by 2%, which equates to 5 W.
Remember accuracy is how close something is to a known (true) measure. The true measure in this case, is the force your muscles are creating.
Since you’ve actually been pushing 192 W for an hour (not the 187 W you should have been doing), your legs are feeling it a bit. NO WORRIES, I’ll ease back, you think to yourself. Heck, I can ease all the way down to 140 W and I’m still in the right zone. So, ease back you do, but that pesky power meter is still slacking, it’s not being accurate. It’s saying your power output is 140 W, but it’s actually 135 W. You’re now pootling along at 54% of your average power, which is your recovery zone.
So where does precision come into it?
The way the strain gauges (the part of the power meter that measures the force you’re creating) are housed in the majority of power meters leads to problems when the conditions you ride in change. As temperature fluctuates, the material around the strain gauges expands and contracts, which affects their ability to measure your output, so the 192 W you were doing a few weeks before (and that was reported as 187 W) is now being reported to you as 180 W. It’s different by 7 W. Not only is it wrong according to your body…it’s wrong according to itself.
You will find these issues (amongst others) with every power meter on the market, apart from the Verve InfoCrank. It was commissioned by the Australian Institute of Sport to be used to ensure the accuracy and precision of its athletes’ power data. Now it is the trusted power meter of British Cycling, fitted to all of their athletes’ training bikes.
The Verve InfoCrank is externally and independently tested by British Cycling according to methods of ISO5725 and Eurolab 2006, using a load frame with calibration traceable to the National Physical Laboratory. The graph demonstrates how two randomly selected InfoCranks reported errors in accuracy of <1% in power when compared with each other and when compared against true (ten known values). The same level of accuracy was found for instantaneous torque. To emphasise the point, this is <1% vs. the +/- 2% of other power meters (which actually represents a swing of 4%).
If you want to train efficiently and make every session as worthwhile as possible, the instrument you use to monitor your output needs to be as accurate and precise as possible.
You need a tool that you cannot blame.