Here Logan Griffin of Spoken Coaching takes us through the murky world of power meters and the metrics, numbers and statistics that are available to us through them. What does it all mean and how much time should we give to them? Sit back, relax and let Coach Griffin iron out all the confusion!
Power meters, in a sense are really a simple tool, they just transmit a number, usually every second, of how many watts you’re producing. It’s what you do with that data and the other information you combine that with which is where it begins to get a little complicated and with all the acronyms, percentages and ratios it can be overwhelming when you first start using one for training and racing.
Through looking over many athletes ‘data’ every day and also learning how to explain what different metrics mean to new riders I’ve given some tips below about what to look at when analyzing a training or race file.
The second by second power numbers that come out of a powermeter is not all that useful as it really does jump around a lot, even a very smooth rider will struggle to hold power within a 20 watt range. Because of this we will usually look first at average power for different sections of a race or training session.
Also whilst riding most riders will have the 3 second rolling average to keep an eye on their current output rather than 1 second again just to smooth things out a bit.
FTP and CP
Before we go any further with some post ride analysis it is important to know a few things about the rider you’re analyzing. The first thing is to know is the rider’s FTP (functional threshold power). Without diving down the rabbit hole let’s just assume that we know the riders FTP as this is really the first thing you should figure out when getting started with a powermeter.
The next thing to know is what some of the rider’s CP or Critical Power values are. CP is the maximum amount of power you can average for a certain time, eg, CP60 is what a rider could average in a 1 hour time trial (similar or the same to FTP), CP5 is what the rider could average for 5 minutes all out. Knowing a good range of these numbers helps when looking at data as we can see how close a rider is getting to some of these numbers in a particular section of a ride or race. For example if a rider’s CP5 is 400 watts and on a hill in the race they averaged 390 watts for over 5 minutes it’s safe to say that they where pretty close to their limit in that particular moment.
Normalized power (often shortened to NP, not the same as but very similar to ‘Average Weighted Power’ in Strava) is often a more useful way to get an understanding of how hard a particular section of riding was if that section wasn’t a smooth effort. Think of a road race or criterium. In this case a straight average is often not a good representation of effort as it drops quickly with freewheeling and doesn’t take into account things like twice as many watts is a lot more than twice as hard, for example.
NP is similar to average but though some tricky calculations gives more weight to surges and higher power outputs whilst nearly taking out some of the freewheeling. The best way to think about this is that it gives you a ‘this is how hard this would have been if you had ridden it steady’ number and is a much more accurate representation of the perceived and physiological effort required to do an uneven effort.
This also ties back to the point above about CP, if we look at a hard 1 hour section of a race file and see that the NP of a rider was very close to their CP60/FTP we can understand why they may have really struggled in the latter part or after that particular effort.
IF (It’s common abbreviation) is a great way to get a quick snapshot of the level of an effort whether that be a long training ride or a short section of a race. It continues on from what we looked at above as it is simply the normalized power or a particular section of data divided by your FTP. To put that into context if your FTP is 200w and you average 100w for a recovery ride the IF would be 0.50. Again with a 200w FTP say you climb a short hill at 300w your IF for that section of data would 1.50. This is great to give a quick snapshot of how difficult a particular section of a race or ride was but also serves as a way to compare data of different riders as an IF of 0.5 for one rider will feel and have a similar physiological response to 0.5 in another rider. Now riders who are stronger in different areas, better at vo2 efforts or sprinting for example will challenge this rule a little bit but it’s a great first step for comparing efforts across different riders.
Power to Heart rate ratios
This is one of the areas often overlooked by riders when they first start using and training with power. It’s very tempting to just think your heart rate monitor isn’t much use now you’re riding with power but that’s not the case and combining power and heart rate data and looking at the relationship between them can be a great way to track changes in fitness particularly at lower intensity/aerobic levels.
You can do this yourself by simply dividing the NP of a ride or section of data by the average heart rate of that section. This will give you a number that is often referred to as EF or efficiency factor. I like to explain it as a how-many-watts-per-heart-beat number.
This number can really be useful when comparing similar rides at different times. I stress the word similar rides as the relationship between heart rate and power is not linear so comparing a recovery ride to a threshold time trial doesn’t really work. On that same note there is really no value in comparing two different riders EF’s either as it is effected by many factors such as sex, weight, maximal heart rate, fitness etc.
What I am looking for here is this number to get bigger, say when a rider starts back after a break from training they do a 1.5h endurance ride they normalize 200w and 125bpm, that’s an EF of 1.60. After a few weeks of endurance zone training they do a similar ride but now average 220w and 120bpm giving EF of 1.83. That is the improvements across time that we are looking to see and using EF is a great tool for tracking the aerobic fitness of a rider throughout the season.
Not something often looked at in a post ride analysis but interesting none the less, you may remember from physics class that power multiplied by time is equal to energy. Skipping a little math … for a rider on a bike this translates to you putting 3.6 kilojoules, per 100 watts, per hour through the pedals. So if you average 150w for 2 hours you have produced 1080 kilojoules of energy.
That may seem like a large but not so useful number but we can take this further, we know that a human on a bike is about 20-25% efficient, meaning that only about 20-25% of the energy we eat will make it to the pedals. Since a calorie is about 4.2 Kilojules to get a good estimate of how much energy we actually used on a ride (or how much we need to eat to simply replace it) we can simply take that 1080 and convert it to 1080 Calories burnt.
This can be useful to riders who are wanting to keep control of their weight and can be used as a planning and also as a reviewing tool. One thing you will pick up on when you start looking at more intense rides and races is that it is almost impossible to replace the energy you are burning on the bike with bars/gels etc and really highlights how important fuelling beforehand and replacing energy burned afterwards is.
Training Stress Score
TSS for short (and similar to Suffer Score in Strava) is what really unlocks the value in training with a power meter as it allows a coach to get numerical feedback of fatigue and plan training to hit goals through a season at peak form.
The amount of TSS you will accumulate in a ride is all based on your threshold with the baseline being that one hour at FTP is worth 100 points. This doesn’t mean that you can get 200 points in two hours though as you can’t hold the same intensity for that time.
As a quick reference here are about the amount of points you would get from a few different sessions. 1h easy recovery ride – 30TSS, 4h steady endurance ride – 200TSS, 1.5h ‘spirited’ bunch ride – 100TSS, 1h 5x5m intervals on the trainer 80TSS.
The Performance management chart
After your analysis of that one ride the next step is to look at how that ride fits into your overall training program. When using Training Peaks we can use the Performance Management Chart (PCM) to do this. The PCM tracks TSS from each ride and from this calculates 3 things; CTL – Chronic Training Load, ATL – Acute Training Load, and finally TSB – Training Stress Balance.
The first of these – CTL – gives you a great indicator of your current level of training load. It’s calculated by taking an exponentially weighted rolling average of your last 42 days of TSS… Or simply how hard you’ve been training for the past few months. So if you went out and accumulated 100TSS every day you would have a CTL of 100. More realistically though a rider might have a week that includes rides of varying length and intensity so that is why we look at the CTL as an average including big days, 0’s for days off etc.
ATL is similar to CTL however only includes the last 7 days of training so really gives a snapshot of what your current level of fatigue is. This number can be dropped quickly during a taper phase and correlates very well with how fresh and race ready a rider will feel.
The final value on the PCM is TSB which is the difference between CTL and ATL, so for example if you have trained well and built your CTL up to 100 then had a good 2 week taper and dropped your ATL to 60 you’ve got a TSB of +40 and will likely be in a good place to perform well.
Now the PCM is a great tool to try and predict form but it is just using numbers and calculations and can’t take into account many of the ‘human’ components to form like: have you been sleeping well? Do you have a cold? Have you been spending and extra 15 hours a week at work? This is where the rider/coach relationship and communication is needed to take what we have planned with the PCM and then make specific adjustments based on each rider.
Hopefully I have at least helped to scratch the surface of what you should be looking at with your power meter data. It’s a constantly evolving area and there are new things to measure and ways to interpret them popping up all the time. But you’ll find that as an athlete once you know the basics you can keep learning by looking at your own data, and just discussing with your coach things you want to know more about.
If anyone has any further questions about what I’ve talked above, any other metrics or even wants some more specific information about their own data feel free to get in touch with us at [email protected] or spokencoaching.co.nz and myself or Blair are more than happy to answer any questions you have.