What is a Watt?


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The use of power meters to track and enhance performance is no longer just a tool for the lab or the pro-peloton, but has been steadily trickling down to the competitive enthusiast.  There’s a lot of technology that goes into them, and there’s also lots of choices.  There’s also potentially confusion on how to use the data they give and translate it into useful information that can improve your abilities.  We ask TrainSharp a few questions to help decipher all of this.

1.First off, we know power meters measure watts.  But, what is a watt?

A watt is a unit of power and a measure of the required energy to move an object across a specific distance in a specific time. In short, power is the product of force and velocity, or in bike terms the relationship between torque (pressure on the cranks) and cadence (revolution of the cranks – RPM). The reason the power meter has become such a popular training tool in recent years is down to its reliability. Speed, Heart Rate, etc. are very changeable from one day to the next, weather conditions, sleep quality, fatigue and others, all affect these metrics to different degrees. Power (as long as you have a reliable power meter!) is always consistent – 100 watts one day is 100 W the next! It is the best objective tool for training out there.

2.Now that we know the definition of a watt, it would be good to get a sense of the numbers.  For example, what’s the power output necessary to maintain 30 kph on a flat road?  How about 30 kph up a 5% grade? (assuming a 75 kg mass of rider plus bike and ignoring effects such as wind resistance, rolling resistance, etc.)

Power measurement is often misused, or misrepresented; Watts is an absolute measure, to get a true idea of what it really takes to power a rider, you need to take into account their weight (power to weight ratio, or Watts per Kilogram) – their W/Kg. For example, a 65 Kg rider pushing 300W up a climb will be going a lot faster than someone 20 Kg heavier but pushing the same power. This is because the lighter rider will be producing 4.6 W/Kg vs. 3.5 W/Kg for the heavier rider. On the flats, this isn’t so important, it’s the climbs where the lighter riders thrive.

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3.There’s a lot of terms associated with talking about power output, like FTP (functional threshold power) and NP (normalised power).  Can you help define those in layman’s terms? 

Functional threshold power (FTP) is perhaps the most commonly used term to quantify a rider’s ability and certainly their bragging rights! FTP is the highest power sustained for 1 hour. As such, it is an estimation of a riders second lactate threshold – the last point before a rider’s blood lactate shows a sudden and sustained increase. Whilst Blood Lactate testing is the best way to accurately identify a riders FTP, there are a number of tests riders can complete at home to estimate this figure. At trainSharp we recommend riders come in to complete a physiological assessment with us, as this is not only the best way to identify training zones, but it is also a great way to profile the rider’s strengths and weaknesses that we would not otherwise be able to spot through power and heart rate data.

Normalised power (NP) is derived from an algorithm that takes into account more than just torque and cadence, it is particularly useful in race situations or sportive where power output is not consistent. By taking into account glycogen utilisation, lactate production and other elements, this algorithm estimates the power that you might be able to sustain if you ride a given route at a constant power. It is particularly useful when there is a lot of freewheeling involved, out of the saddle efforts etc.

4.What’s the FTP for a fit sportive rider?  How about for a pro?

This is something that is extremely variable, depending on a riders chosen discipline; an ultra-endurance cyclist will require very different energy demands compared to a track rider. That said, from all of the riders we have tested, in general, the average sportive rider has a power to weight ratio of 3.2 W/Kg, compared to our best professional rider, which is around 6.2 W/Kg. Whilst this seems like quite a bridgeable gap, in absolute terms it shows a different story – Our example from earlier, the 85 Kg rider would need to sustain 272 W to have an FTP of 3.2 W/Kg, to push this to the pro’s power to weight, he would need to hold a stonking 527 W for an hour! This is a good time to highlight that the best way to increase a rider’s watts per kilogram is to increase absolute power, but also to decrease body weight.

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5.Why is measuring power output better than what I do now, e.g. measure heart rate?  

In an ideal world, all of our riders at TrainSharp would be using both heart rate and power meters during their training and racing. The structure that power based training brings to our athletes and the way we coach them has meant that we can deliver sessions to riders that are time crunched that will be both beneficial and fun. Having power to train to will ensure that you can correctly and efficiently target various energy systems, meaning that if we want a rider to head out on a fat burning ride, or a threshold pushing specific effort; with power, we know that they will be doing just that. That said, having Heart Rate is beneficial, you can often spot early onset signs of illness or fatigue through heart rate and power data. Combined with subjective measures and a good rider-coach relationship means that we can monitor performance and fatigue effectively. 

6.Power meters are still quite expensive and may require some expertise to install and calibrate. It seems like quite an investment for the individual rider.  How much time can one except to invest to decipher the data obtained from the power meter?  What kind of improvements could a dedicated cyclist expect to see using power-based training?

There is an ever growing market for power meters, especially recently with the larger companies such as Garmin and Pioneer getting involved in this industry. Therefore, the technology is gradually making its way into the more budget-orientated market. Ten years ago, you were looking at spending at least £2,000 to get a basic power meter, now, you can pick one up for around £600.00. Bargain. The time spent deciphering the data is a very individual thing, some riders have a very analytical mind and can spend hours looking at a power trace, whilst others do not have the time or inclination to do so. Neither approach is right or wrong. At trainSharp we work closely with the individual rider to try and provide them with the data that they want and need to improve as effectively as possible. A rider that thrives of a numbers orientated approach can have that level of feedback. Whilst a rider can still improve off of a more plan-and-go type approach if that’s what they work best to.

The rate at which a rider improves is hard to predict due to a number of factors, ideally we would see an increase of around 20 W per year to a riders FTP, however, as mentioned above, it is very dependent on the rider’s individual targets and disciplines. A road riders focus may be more inclined to improving their sprint powers, or race craft as opposed to FTP.

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7.Do you think it’s true that shaved legs can save 15 watts?

Not quite! There is certainly evidence out there that suggest smooth legs improve aerodynamics, however 15 W is perhaps a bit extreme. Shaving your legs is probably more of a tradition-type thing in cycling than anything else!

We Would like to thank Elliot Lipski for spending the time to answer our questions. Elliot is the in house scientist at TrainSharp and has a Masters Degree in Applied Exercise Physiology. Basically, he knows what he is talking about.
Here are a few other links that may also help:
By Mark Roberts
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