Watt Matters is a blog about training and racing with power and other related musings.

Aside from writing items here on occasions, I also provide cycling performance improvement services via coaching, aerodynamics testing and host a cycling tour.

If you'd like to ask me a question or suggest a topic you'd like my take on, then just head to Ask Alex and let me know!

Turbocharged Training

Turbocharged Training

There's been a bit of discussion lately on various training forums about a topic that seems to crop up every so often. It's a perennial favourite. Certainly I'm not the first to write about it and I won't be the last.

Why is my power different when training indoors* compared to when I ride outside? And what can I do about it?

* Indoor training being training done on an ergobike, or with the bike locked into a turbo trainer or riding your bike on rollers. Often performed inside the house, in the garden shed or garage, on a balcony or at the local gym or training centre.

Usually people train in such a fashion because they either haven't the time or opportunity for a ride outdoors, they might be recovering from injury and/or need the controlled and safe environment an indoor trainer provides, or the riding conditions outside are not suitable (cold, rain, snow, darkness and so on). Certainly riding indoors is a safe and excellent training alternative when heading out the front door on your favourite steed is not possible.

For many riders though, they find generating power indoors much harder than when riding outdoors and end up riding at a lower power as that's all they can do (but this is not the case for all though, and some can actually produce more power indoors than outside, although that is less common).

So if I can't generate the same power, then am I getting the same training benefit?

And if power is significantly different indoors, should I use a different FTP for indoor rides (so training levels and ride data are adjusted accordingly)?

Well the answers are not straightforward but let's explore the solution(s).

The first thing to do is to understand why a difference in power production exists. Then the second thing is to take steps to address the differences between each scenario and "bridge the gap". Finally, one then needs to make decisions about how the data from their indoor training should be interpreted.

So why is it common for power to be different?

There are four main factors at play here:

  • Cooling & air flow
  • Inertial load
  • Motivation
  • Adaptation

I'll explore each of those in a bit more depth a bit further down.

OK, so what about the training benefit and setting of FTP?

Well power is power and if you are burning kJ at a lower rate, then the metabolic adaptations relating to that will be correspondingly different. So if turning out a lower power really concerns you, then the priority is to address the factors that influence indoor power production and reduce the gap so that training can still be done within the intended training level. Then the problem goes away.

Nevertheless, "hard is hard" and "alls you can do is alls you can do", so if you are unable to address/fix the key reasons why power is less indoors, then set your training at a level that is attainable for that scenario. It's better than staying on the couch. Rather than worry about what percentage of FTP or MAP that should be, just use previous indoor workouts as your guide. That really should be the guide anyway, irrespective of mode of training you are doing.

What matters is that you do the workout at around the right intensity for the right duration, rather than the precise wattage.

What about FTP, and the calculation of TSS and the other metrics that flow from it?

This really is an issue of what you are training for and where the majority of your riding will be during the course of that training period. If the trainer only represents a minority of your ride time and your power is say 10% less on the trainer, then it only represents a small difference in the calculation of overall training load. It is simply not worth the bother to have separate FTP values and calculations. The Impulse-Response model (aka the Performance Manager and the metrics CTL, ATL and TSB ) is fairly robust.

It is about the forest, not the trees.

For example, let's examine a common hour-long training ride and say, for whatever reason, your indoor power is 10% less than outdoors: 2 x 20-min at FTP + warm up and cool down.

Outdoors, this would accumulate ~ 85 TSS and indoors (with ~10% less power), ~ 70 TSS. A difference of 15 TSS (which is about the equivalent stress of 15 minutes of endurance level riding).

So if the difference in TSS calculated from a 2x20 workout (equivalent to about 15-minutes of basic endurance level riding) is concerning you, then sit on the trainer for another 15-minutes.

If however, the trainer represents (for that training period) a large proportion of your training time, then setting FTP according to that training mode makes sense. But where such rides are only occasional, then there really is no reason to worry about minor variations in the numbers, just move onto the next day.

The same principles apply if talking about training at altitude (occasional change in altitude vs. a lengthly block at a different altitude) or different bikes (occasional or lengthly training blocks on a given bike/position).

Read on for more details on the four elements of indoor training that affect our ability to produce power indoors and how you might do something about it.



People consistently underestimate the cooling needs when training indoors. There's some weird theory that a large pool of sweat forming beneath you is a good thing. All that tells me is that the air flow and cooling arrangement is perhaps inadequate for the task. A body that is under stress and not being adequately cooled will underperform.

Keep in mind that the typical cyclist operates at around 21-22% efficiency (give or take a couple of percentage points). Cycling efficiency is a measure of the ratio of energy reaching the cranks of the bicycle as a proportion of the total energy metabolised*.  In other words, to generate 100W at the cranks, our bodies are metabolising energy at the rate of 100W / 21.5%  = 465W.  

So of that 465W, 100W is converted to mechanical energy at the cranks, with the vast majority of the balance being converted to (waste) heat, with a bit used of course to run the rest of the body's functions.

What that means is that for every 100W we put through the cranks, roughly another 360W are generated as waste heat. How much heat exactly will vary depending on the individual's efficiency level (typical range is 19-24%).

So if for instance you are doing some intervals at 300W, you are in effect pumping out the heat equivalent of a 1,000-1,200W electric heater!  Now do you see why we heat up so quickly when training hard and an effective cooling system is required?  Especially if the ambient temperature is quite warm to begin with, and particularly so if the conditions are humid.

When you hop on your bike for an endurance ride, you have a ~ 30km/h wind flowing over your whole body constantly wicking sweat away and keeping you cool(er). So why would you expect to perform as well indoors with no air flow, or the piddling excuse of a breeze that comes from a domestic fan? Get real. If training indoors is going to become a sizeable chunk of your training time, then get some decent cooling happening and have some strong air flow over you. A large industrial strength fan costs much less than a trainer or rollers, so bite the bullet and sort it. But be prepared for the additional noise.

Some people do perform indoor training in quite cold environments, so of course they might be able to get away with less air flow than others.

* there are a couple of different efficiency measures (e.g. gross and delta efficiency), but for all intents and purposes, this basic definition will suffice in this context.

Inertial load

Trainer - 1Up.jpg

is the next main differential factor when comparing indoor and outdoor training. Without going into too much detail, when we ride outdoors, we have the inertial load of a bike and rider moving at some speed, plus that of the wheels turning. If we stopped pedaling, our rear wheel doesn't suddenly slow or stop turning, we would coast for quite some time. On many trainers however, since we are not moving, the inertial load is much less and confined to the rear wheel spinning and any small flywheel that the trainer has attached to the roller. When you stop pedaling, the wheel slows and comes to a halt relatively quickly. Some are worse than others.

Trainer - KK.jpg

Now what happens is each scenario feels quite different to ride, muscle activation is different, the neuromuscular demands are different and these can be enough for some to make power production much harder. In general, low inertial load trainers tends to emphasise the "dead spots" in the pedal stroke (when the cranks are passing through the 12/6 O'Clock position), whereas riding with a higher inertial load enables one to breeze through (and not waste effort on) the dead spots and focus on the downstroke where the bulk of power is produced.

Fortunately there is a way to increase the inertial load of a trainer, and that's by having a flywheel attached to the trainer's roller (or even by adding mass to the wheel itself). How much mass is needed? Well to replicate the inertial load of a rider, it would need a very heavy flywheel spinning very quickly. Think of a 20-30kg flywheel spinning at 500-800 rpm. Yikes!!

Fortunately, for effective training, going that far is not really necessary and having enough rotating mass to help smooth out those dead spots is enough. I don't have one myself but trainers like the Kurt Kinetic Road Machine or the 1-Up trainer are an excellent example of this. They both have small but effective flywheels attached to the rolling mechanism.

These are ideal options for those that are looking to attach their existing bike to a trainer but also need some portability with their indoor unit.

The other option is a dedicated ergobike like the Schwinn or Saris indoor trainers (or other similar machines). These types of set up have the advantage of being able to incorporate a much larger and heavier flywheel than a turbo trainer. They are of course dedicated units and need a permanent place to live.

Trainer - Schwinn.jpg
Trainer - Saris.jpg

So what does one do? Well if cooling has been sorted, and power is still down, then consider the inertial load of your trainer set up. Does it have a flywheel? Can one be added? Should I look at an alternative trainer? Certainly I would recommend trying a trainer that has a decent flywheel to see how much better it is to ride.

Edit note: I added the following paragraph in March 2011 as something I'd been meaning to do for quite some time, just had forgotten to do it.

I should add that the idea of inertial load on an indoor trainer affecting pedaling isn't actually backed by evidence other than anecdotal, from myself and many others I know that have used such trainers. As an example, this link to a study extract on PubMed indicates that varying crank inertial loads has little or no effect on steady state pedalling coordination.


is a big issue in training and racing, and it is sorely tested when riding indoors. Many find training indoors mind-numbingly, excrutiatingly boring. Then there are others who really love it and are happy to spend hours tapping it out, sometimes preferring that to a ride outdoors. Each to their own.

If a lack of motivation is an issue, then it needs to be addressed, otherwise don't waste your money on a trainer you won't use. It'll just end up gathering dust in the corner of the shed.

There are many ways to overcome any motivational challenges you face:

  • Variety
    there are lots of training workouts available, so keep the variety up. Dream up some of your own!
  • Duration
    indoor riding is hard work, there's no let up or coasting, so don't make the workouts as long as you might ride outdoors. It is better to complete a shorter workout and want to come back for more next time, than to get off absolutely hating it and sitting out the next one on the couch or staying in bed.
  • Set Challenges
    set yourself targets for the session and maybe have reminders of your goal event in front of you as well.
  • Music
    this is a good one - having you favourite training tunes blasting away, or on your iPod to keep the neighbours happy.
  • Video
    what about watching highlights of your favourite stage race or one-day classic. You can be smacking it up Ventoux with the Pros. Of course there is a big market out there for indoor cycle training videos, so if that floats your boat, then go for it!
  • Computer aids
    there are lots and some of the favourites are heart rate monitors, spped and cadence measurement computers and of course my favourite - power meters. These are especially helpful so that training is focussed and performed at the right intensity.
  • Ergo controllers and
  • Virtual riding
    there are many trainers that can automatically control the resistance level of the trainer and be pre-programmed to control a workout. Some can even display video of an animated figure or some real life video to provide a distraction from the effort and help to pass the time.
Of course the most obvious answer is simply to HTFU.


is the last of the four key issues. Since there are differences in riding on a trainer to riding outdoors, some of which have been discussed already, then it stands to reason it will take some time for the body to adapt to training under different conditions. If you only ride the trainer occasionally, then you may never fully adapt to being able to generate power similar to outdoor riding.

However, if you ride on a trainer regularly and with sufficient volume, and you address the other three main factors, then you will adapt and improve your ability to produce power indoors and the gap to outdoor power will typically narrow.

What do I do?

Well I set about addressing all of the issues and descibe my indoor training set up here:

Turbocharged Training Thread on TT Forum

Have fun indoors!

Smith & Wesson

Swiss Watch

Swiss Watch