Why Do Watts Per Kilogram Matter?

Why Do Watts Per Kilogram Matter?

Watts per kilogram is an important measurement in determining your cycling potential.  On the surface, it's a simple measurement, but it can give a ton of information about your performance and strengths.  But what exactly can you learn about your cycling by looking at your W/Kg measurements?  In this article you'll learn what W/Kg is, how to use it effectively to plan and execute your training and a couple ways to improve your power to weight ratio.

What is “watts per kilogram” and why does it matter?

Watts per kilogram, often abbreviated W/Kg is a measure of power to weight.  It takes the amount of power you produce at various time periods and divides it by how heavy you are.  Sounds pretty simple, right?  In theory, it is.  But like everything in your cycling training, there's more to it than just a number.  Click through to learn a little bit more and figure out why W/Kg is so important.

watts per kg calculatorThis calculation is important for a couple of reasons.  First, it allows you to standardize your fitness to other riders.  Here's an example: if you put out 300 watts and your buddy puts out 250 watts, you're going to be faster, right?  Well, maybe.  If you put out 300 watts at 100 kilos and your buddy puts out 250 watts at 75 kilos, he's going to be faster in some situations.  Neglecting air resistance, your 300 watts will probably push you faster on flat ground.  When the road tilts up, however, your buddy will have you beat.  He puts out 3.3 watts per kilogram while you only put out 3 W/Kg.  That's a 10% difference, and it only becomes more pronounced as the grade increases.

Second, it's as close to an absolute performance predictor as there exists in cycling.  The pros can hold around 5-6 W/Kg at threshold.  If you're only able to handle 3 watts per kilogram, you probably don't have a future as a pro cyclist.

Third, by charting W/Kg at different time durations, you can get a good idea of the type of events you can excel at.  We'll get to this a little bit later in the discussion.

The biggest thing to consider is that W/Kg is the benchmark you're looking to improve because it is a measure both of your power production and your body composition.  While you're not measuring lean muscle mass vs fat mass, you can evaluate how changes in your body composition will affect your power production.

In what terrain do watts per kilogram measurements matter most?

Measuring W/Kg is going to be of the most value to you on hilly courses where you have to battle against gravity.  The amount of power you produce versus your weight is directly correlated to how fast you can get uphill.  If your main goals involve significant climbing, such as most Gran Fondos or hilly road races, you'll need to work on maximizing your W/Kg values.

“But coach, doesn't W/Kg matter on the flats?”  Well, no, it really doesn't.

When gravity isn't in play (basically when you're on a flat road) raw power production will generally rule the day.  Sure, you have aerodynamics in play that will determine speed vs power on the flat, but that's a completely different discussion.  Generally, the more power you can produce on flat terrain, the faster you'll go.  The same can't be said for climbs and rolling terrain, so you'll want to have the highest W/Kg possible.

Using W/Kg to plan big group rides

The other place where watts per kilogram counts is comparing riders to one another.  If you have a whole group of people riding together, how do you determine how to split them up into groups?  One of the best ways to do it is to evaluate watts per kilogram at functional threshold power to get an idea of each rider's relative fitness.  You can easily break down groups into comparatively evenly matched packs, ensuring that nobody ends up left behind.  This is a fantastic idea for all of you big group ride leaders: start the faster, higher W/Kg riders first.  Send the middle range riders out next and then follow it up with the lower tier riders.

This has an effect of creating a “sweeping effect” on the ride.  If one of the faster riders falls off the pace, they can soft pedal until the next group comes along.  Once that next group comes along, the faster rider can pull on the front for a while to get their training stress up.  This also helps the second group motor along a little faster.  The increased speed will get some extra training stress for the other riders in the group as well, challenging their fitness a little bit.  The final group on the road acts as a sweeping group.  Anyone who has fallen off the pace or is having an off day can just settle in for the ride back to the meeting place.

Using W/Kg to learn where you excel

Watts per kilogram measurements can tell you a lot about what you're good at (and not so good at).  You can use this information to further tailor your build phase training and minimize your weaknesses.  You can also use it to get an idea of where your strengths lie and how to use them.  But before you can do this, you need to do a little power profiling.

Power profiling is a tool in most power analysis software bundles that calculates W/Kg for you.  The best way to get a correct profile is to perform a solid set of testing protocols.  You need to get specific mean maximal power data from 5 second, 1 minute, 5 minute and 20 minute (or functional threshold power) durations.  One of the easiest ways to go about this is picking up my Zone 4/5/6/7 power testing profile.  If you'd rather do it on your own, you can simply do maximal efforts at those durations.  You can even pluck mean maximal power from your “28 day best” curve if you're not able to do formal testing.

Power Profiling ChartBy evaluating your W/Kg against a chart (like the one to the right) you can determine where you will excel.  If you go through a complete power testing protocol, you can plug those values into a chart and determine where your strengths are.  Will you do well in time trials or are you more of a track sprinter?  Will putting on the extra muscle needed to give you an extra 100 watts be offset by the actual weight gain of that muscle tissue?  These are all things a W/Kg chart and power profile can tell you.  I'll teach you how to profile your power in the next section.

I'll teach you how to profile your power in the next section.

Calculating W/Kg and Power Profiling

Calculating W/Kg is pretty simple.  First you need your weight in Kg.  If you need to convert, divide your weight in pounds by 2.2 to get Kg.  Then you simply divide your wattage by your weight in Kg to get W/Kg.  Most performance monitoring software will do this for you and most GPS units will calculate it on the fly for you.

The important power durations that we need to calculate W/Kg for are 5 seconds, 1 minute, 5 minutes and your functional threshold power.  By charting these figures, you can get an idea of where your strengths and weaknesses lie.  If you take a look at the chart above, you'll get an idea of what I mean by charting your W/Kg figures.  It will also show you the type of rider you are.  A high 5 second and 1-minute power will indicate you have a propensity for strong, short attacks.  High 5 minute power can indicate a longer range attacker or track type rider.  High FTP can indicate a strong climber or time triallist.

If you're having trouble testing for your critical power zones, check out my Zone 4, 5, 6 and 7 power testing tool.  It guides you through testing different maximal power durations and helps show you where your strengths and weaknesses lie.  You'll also get a threshold calculator, sweetspot zone calculator and W/Kg calculator at the 4 W/Kg profiling points.

Improving W/Kg

Since watts per kilogram is a simple ratio, there are 3 ways to improve.  You can improve your power production or you can decrease weight.  The third option is a combination of both of the two.

The simplest way to improve W/Kg numbers for most people is to drop a little weight off.  Most riders have a few extra pounds on their body to shed, and each kilogram will change their W/Kg rating for the better.  This assumes, of course, that you're not losing power producing muscle in your quest to lose weight.  The more fat is lost while maintaining lean muscle, the better your power production will be.

If you want to improve W/Kg through training, you'll simply want to work on producing more power.  You can train specific zones if you want to improve one particular W/Kg range, or you can build general fitness to improve your overall aerobic fitness.

What Happens to Your Body When W/kg Improves?

As you train and shed a few pounds, your strength to weight ratio improves. You'll probably also note a few body composition changes as well. Your body will carry more water overall, having a higher percentage of it's mass made of muscle. This happens because muscle tissue is around 70% water, while fat is only about 10% water. The benefit of this is dehydration prevention: a 3% loss in water weight can lead to around 8% performance loss. At a cellular level, your body becomes more efficient at processing oxygen and producing energy to power your muscles. Short duration, high-intensity intervals such as sprints build anaerobic energy pathways and increase endurance during short efforts. The improved oxygen processing capacity allows your body to produce more power without using anaerobic pathways. You'll be able to ride harder without running into that “red zone” that tires you out quickly.

Finally, you'll be able to evaluate the effects of your training on your performance. You can ask yourself if over time, was it worth putting on the extra 5 pounds of muscle? Consider these questions as you evaluate your data and your performances:

  • Have you seen your power output improve enough to offset the change in weight?
  • Can you sprint better with the extra weight?
  • Are you a stronger time trialist with the additional muscle putting power into the pedals?
  • Have you found your climbing suffers because you put on 2 kilos of muscle but don't have enough increased power output to stick with the front group on the ascent? Can you mitigate that loss of time on the climb with your ability to chase back on during the flat or descent?
  • Has your pedaling style changed in any way? (You can use a quadrant analysis graph to figure this out.)

Looking at these questions and evaluating changes in your power to weight ratio are some of the best ways to figure out how effective your training methods are.

If you want a little help with evaluating your W/Kg or identifying where you need to improve, fire off an email to me and I'll help you out.

About the Author:

After graduating from Ithaca College with a degree in Exercise and Sport Science/Pharmacology, I continued my education with a doctorate of Chiropractic from New York Chiropractic College. As I progressed through my education, I was able to apply the concepts I learned in the lab to my own daily workouts and goals. At the time, I was following some of the principles of traditional coaching and getting mediocre results. Frustrated, I realized that if I could apply all my physiology, chemistry, nutrition and training knowledge, I could “build a better mousetrap” not just for my own training, but for other athletes. With this goal in mind, I started Tailwind Coaching, to help cyclists [with busy lives and limited training time] become stronger, faster, fitter and healthier. And while I may not be a ex-ProTour rider, an Olympic Coach or even a prolific race winner, I am something that most coaches are not: a regular guy just like you who has a job, a family and a desire to be a stronger cyclist.