Training with a Power Meter

Training with a Power Meter

This week Coach David took the time to chat about training with a power meter.

“With the recent extended Covid-19 measures in place, the move towards indoor bike training is perhaps even more acute. Now, then, really is the time to set some concrete goals in order to stay motivated on the bike and come out of this hiatus in a better shape than we started. For me, indoor bike training is a great opportunity to dig deep and develop my power on the bike in the hope I can become a faster rider.

Power can be measured either directly by a smart trainer or power meter, or indirectly using speed and cadence sensors linked to some third party training software like Zwift.  Either way, the key metric for you as a triathlete is your Functional Threshold power (although it’s worth noting that coaches are starting to take a much wider appreciation of a whole spectrum of your power output).

Functional Threshold Power (FTP) used to be the gold standard and it is sort of pretty much is in as much as it represents an intensity at which certain physiological conditions start to kick-in.  Here comes the science (skip to the bit on FTP protocols if not interested)…

You have a number of energy systems that you use to create power: 1) the Phosphocreatine system  (very short-lived and explosive – 15 seconds of energy for sprints only – ignore for now), 2) the Glycolytic system (often known as the anaerobic system), which turns sugar/glucose to energy anaerobically i.e. without the need to use O2, and 3) the Oxidative or aerobic system that turns sugars and fats to energy in the presence of O2.

Still with me?

It’s the glycolytic and oxidative systems that we’re interested in, and really, at all exercise intensities, we’re using a mixture of both.  There is no sudden switch between the two, but instead, the exercise intensity dictates the relative contributions of the two systems (as it does also the amount of fat and sugar we’re using in the aerobic system, but that’s another story). At lower intensities, the aerobic system dominates because the energy demand is such that the body has time to move and use O2 in exercising muscles.  The aerobic system is way more efficient in terms of how much energy we can get from every molecule of fuel, but it’s a relatively slow process that uses lots of biochemical pathways and enzymes.  As the intensity ramps up, these processes can’t keep up with the energy demand, and the glycolytic system progressively steps in to take up the slack.  The glycolytic (commonly known as the anaerobic) system, effectively bypasses the need to use oxygen and just snaps the fuel molecules in a crude and not particularly efficient way, but it yields quick energy.  The by-product of this process is…. lactic acid.

Lactic acid – an aside: Lactic acid is so, so misunderstood. All the initial animal physiology experiments into fatigue saw the level of lactic acid (or rather lactate, in which it is quickly converted) increase in direct correlation with muscle fatigue.  So, in the absence of any other evidence, lactate got the blame for that burn in our legs and eventual muscular exhaustion we feel when working hard. To be clear, it’s not lactate’s fault.  Lactate is actually a vital part of the anaerobic system. We can use it either directly for fuel (less efficient than glucose, but fuel none the less), or it’s transported back to the liver through a process called the lactate shuffle to be reconstituted back into glucose (or rather its stored equivalent – glycogen) for later use.

So to be clear, lactate is not the enemy.  It is the Hydrogen ions (H+) that are also produced as part of the anaerobic pathway that give rise to the acid burn in our legs. It’s the H+ ions that cause a drop in Ph (i.e. a rise in acidity) in our muscle cells that adversely affect their ability to contract, and also works as a negative feedback to limit further glycolysis and limit further drops in Ph.  Fortunately, for physiologists, lactate levels directly correlate to the level of H+ ions in the muscles, so by measuring blood lactate we can see indirectly how much H+ we’re producing and figure out when the anaerobic energy system is really dominating.  As such, blood lactate is a proxy measure for H+.

Why is this important? Well at the exercise intensity at which we see lactate start to rocket, we know H+ is doing the same and we know that it can’t be sustained.  The negative feedback loop will kick in and we will rapidly fatigue after a few minutes.

The exercise intensity at which we see that happen is your Functional Threshold Power. Up until that point, your aerobic system is dominating with only a little help from your anaerobic system. Fortunately, until FTP the level of Lactate production (and therefore H+ production) is gradual, and the body can clear the lactate and buffer the H+ to avoid the burn. At FTP though, the energy demand is such that the anaerobic system needs to ramp up and the level of H+ production rapidly increases overloading the body’s ability to buffer it (and similarly, shuttle or use the lactate), leading to the exponential increase in Lactate/H+ levels and the resultant muscular fatigue.  This point is known as the onset of blood lactate accumulation (OBLA), maximum lactate steady state (MLSS), or plain and simply (although somewhat incorrectly) the anaerobic threshold.

So, what does this mean for a power meter user? Well theoretically, if I know my FTP then I can pace myself just below that power in the knowledge that I shouldn’t step over the anaerobic threshold, explode and fade. In terms of training, I’m trying to lift my FTP higher and higher in the hope that I can push and sustain more watts, getting faster without slipping beyond my anaerobic threshold and fatiguing. I try and lift my FTP by repeatedly training very close to it, (and sometimes above it) to try and produce adaptations that either help clear the H+/lactate quicker, or supply oxygen to my cells and use it more effectively so I can stay predominantly aerobic for a higher power, i.e. delay the onset of blood lactate accumulation.

That’s why FTP is useful!

But note, FTP is really only a useful metric when we’re looking at efforts up to an hour. Indeed the sports science definition of FTP is the sustainable power for one hour. It’s kind of arbitrary I admit, but it makes it a useful metric for the standard length of workouts! It helps identify a tangible boundary between aerobic and anaerobic energy systems that we can use in training. When we get into longer rides, fatigue becomes less a factor of the dominant energy system in use, and more about the availability of fuel and contractile efficacy of muscles.  An ironman athlete for example, should rarely if ever operate above the anaerobic threshold and therefore never have an issue with H+ production, but will chronically fatigue nonetheless, despite having baseline levels of lactate in their blood/muscles.

Hope that didn’t bore you, but I think it’s kind of cool to know what’s happening in your body when we talk about abstract numbers like FTP, etc.

So, the protocol for an FTP test.

The gold standard is an hour time-trial at max sustainable power, but this is difficult to pace correctly and it’s very, very uncomfortable. So most protocols now use a shorter test and approximate FTP from that. Normally, it’s a 20minute effort slotted into an hour or so ride to allow warm up and also some efforts to try and eliminate excess anaerobic contribution (inc drain down creatine phosphate stores):

– 20 mins warm up pretty easy
– 3 x 1min all out sprints with 1min easy pedalling between
– 5 mins easy
– 5 mins hard
– 10 mins easy
– 20min time trial
– 10 mins easy

You’re looking for a consistent pace in the time trial. If you go out hard and then fade, then the test is invalid. Equally, if you go out easy and ramp up then it’s not really valid.  So, sometimes it takes a few efforts to get right.  It helps if you have an idea of what your FTP might be as a target. Say, for example, you believed your FTP was in he 250-260 watt region, I’d coach you to go out at the top end of this (you’ll see why in a moment), give it 5 mins then ask, ‘can I do this for another 15mins?’,  if no, take it down 5w, if yes, bring it up 5w. Ask yourself the same question at 10 and 15 mins.  DON’T SPRINT AT THE END.  If you’ve paced it well you won’t be able to! We then have a look at the data, look at your average power over the 20-minute effort and then use 95% (to account for the fact its 20mins and not 1hour) as your FTP. This is only an approximation though. Some riders will make a larger anaerobic contribution to a 20min test, and so if were do to an hour time trial would find their true FTP a little lower (say, 93% of their 20 min power, which in my mind is more representative across the board, but hey, standard protocols say use 95%). Nb, if you’ve an indoor trainer with zwift etc, you can pair your power meter either vie Bluetooth or ANT+ with a dongle and do the test virtually – often better as you don’t have traffic lights, etc to contend with. You will, however, find a different FTP for indoor and outdoor rides, but we can factor that into training too.

Once we’ve got your FTP, we can apply a structured training regime to it…. Working at different intensities above and below FTP, and for different intervals times, produces physiological adaptations that are pretty well understood, and all of which help develop our aerobic engine, gradually lifting our aerobic capacity and further lifting our FTP…. MAKING US FASTER!

For example, increasing our blood plasma level is a very rapid adaptation that occurs when we do short sharp intervals at 120% or so of FTP. Increased blood plasma improves the flow of blood through our capillaries and therefore improves the delivery of O2 to our muscles cells. At the other end of the spectrum, we increase our muscle cells’ ability to store glycogen and therefore facilitate aerobic output by undertaking longer intervals at something like 80-90% of FTP. Somewhere in the middle, repeatedly working at threshold for extended periods helps upregulate a bunch of genes in our muscle cells leading to more mitochondria, which in turn means we can produce more aerobic power. None of these adaptations, however, are produced singularly at these specific intensities; they all occur across the spectrum of aerobic effort. It’s just that they seem to be produced preferentially at particular points of intensity, and with a power meter we can purposefully target all these adaptations with structured training.

In part 2….

power profiling (baseline 2- another test!).
other FTP protocols.
power duration curves.
analysing the data.
structured training.

Part 2

In the last blog we talked about the usefulness (and potentially some limitations) of using FTP to baseline your fitness.  For triathletes, time trialists and wanna-be climbers, FTP remains very relevant because these activities are primarily aerobic endurance efforts, of which FTP is a good marker.  However, FTP remains only one part of your ‘power profile’ as a rider.  If you’re thinking about using your power meter to train to become a more effective rider across the board, then it’s worth exploring your capacity across this entire profile. This involves looking at your capacity to sprint, figuring out what your power at VO2max is (roughly a 1-3 min max effort just as we do running on the track with our 400m repeats) and some slightly longer intervals, too.  From this, we build your power duration curve. Here’s an example (not mine) from Strava:

The x-axis is an exponential scale (sort of) of duration and the y-axis is power.  We can see from the graph what an individual’s max power for any given duration is.  So this individual can hold something like 800w for 15 secs, 320w for 1min, 290w for 20mins and so on.  Note, you can’t really compare this directly with another individual, because it isn’t normalised for weight, and it’s the power:weight ratio that is key, certainly for climbing.  So, for instance, some of the bigger guys in the Metasport training group might have a very impressive PD curve with something like a 1 hour power of 350w, but let’s say they weigh 90kg so their p:w ratio for 1 hr is something like 3.9w/kg.  Whereas some of the lighter guys might ‘only’ be capable of producing 230w for an hour but weigh something ridiculous like 57kg, a p:w ratio of 4w/kg, and so they will climb faster. Hence, asking someone what their FTP is largely irrelevant unless you also ask them their weight…..never a polite thing to ask in Jimmy Monkey’s after the ride! Anyway, once we’ve got your power curve we can start looking for relative weaknesses and pinpoint specific intensities and durations to train at.

We can build the accuracy of this  PD curve up over time and many rides where we do intervals at a variety of duration.  But a good protocol to start understanding your power profile would be something like:

45mins of easy riding
3 x 1 min efforts at about FTP with a fast cadence, something like 110rpm
1 min sprint test all out, sprint out of the saddle for 15 secs, sit down but keep sprinting for the next 15sec, then hang on and try not to fade too much for the next 30sec.
10 mins easy riding
5 min test – looking to maintain 115-120% of your FTP, start hard and try to maintain without fading.
10 min easy
2 x 15 secs (with 2 min rest) out of the saddle, vomit inducing big chain ring sprints.
15 mins easy riding.

This, combined with your FTP test will give a good idea of where you sit as a rider!
‘Why do I need to know this to be a triathlete’ some of you will ask? Well, the pure sprint stuff we’re not so bothered about, but as we said, sustained speed and effort is influenced by FTP, and FTP is governed by a few things, one of which is your maximal aerobic capacity or VO2 max.  Vo2 Max is that maximal 3-5 min effort intensity, and knowing this on your PD curve and then subsequently training to push it is a key element of building FTP. For many years, sports scientists thought VO2max was akin to muscle type, i.e. largely governed by genetics and not particular malleable, but, there’s a growing body of evidence to suggest that’s it’s actually quite trainable with the right exercise prescription. That, combined with some threshold and lower intensity interval work is how we not only build FTP but also help triathletes sustain longer periods at higher percentages of their FTP.

So, understanding a rider’s profile and then intelligently applying a training prescription with specific power targets and durations is really what a structured training programme with a power meter is all about – knowing where you are in terms of performance, and where you want to be and then applying the right training stimulus to get there!

Further reading:

Probably the best book on the market is ’Training and Racing with a Power Meter’, by Hunter Allen et al.  It’s quite in-depth but it goes into the science and analysis behind power meter training for elite sportsmen and women.  It’s also is the accompanying manual to power meter analysis with some very cool software called WKO4 (and now WKO5), which coaches use for analysis and developing training programs. Strava Summit can also give you some useful power analysis data, and then there’s some other free software you can use for power file analysis (Golden Cheetah is good but far from intuitive, Garmin Connect will also do a reasonable job), otherwise send any power files to me and I’ll see what I can do!

So, in conclusion, training with a power meter can help us train more intelligently, training to specific intensities to develop specific physiological capabilities.  Power meter training also helps us monitor our progress with a far greater deal of fidelity.  Seeing our power duration curve improve in terms of our ability to hold more watts for a given time is a tangible indication of improving strength and fitness, and a great validation of a structured training program. “