Do Zwift kilometers count?
The indoor, online cycling platform Zwift has become very popular. Riding in a virtual world with other indoor cyclists from all over the world, certainly makes those rather boring indoor training sessions more enjoyable. It combines riding in (virtual) scenery with a cyclist community and social interaction. At first, it may feel very artificial compared to outdoor group rides with real people. And, admittedly, it might take some time to change ones mindset and to embrace rather than to dismiss Zwift as a training tool.
When it comes to this mindset, there seem to be two extremes. On one side, there are the hard-core cyclists that condemn any ride that is not a) outdoors, b) with other “real cyclists” and c) includes at least one coffee stop. On the other side are the Zwift or indoor fanatics that swear by the efficiency and positive training effects of turbo training workouts and maybe even have become addicted to the Zwift platform, its community and, indeed, the virtual group rides and races. Of course, there is some truth in the arguments of both sides and neither real, outdoor nor virtual, indoor rides alone will prove to be the best recipe to becoming a better and skilled cyclist or triathlete. Combining the two and using the best of both worlds makes a lot of (training) sense.
One of the most interesting and heavily debated arguments in the real world versus Zwift cycling debate is whether km´s ridden on Zwift do count or not? How does a 100 km ride on Zwift compare to a 100 km ride through the real world?
The answer is that we can´t know. Zwift features game aspects like temporary weight reductions, aero improvements and drafting, which adds extra speed relative to your power output. Also, whether you ride uphill or downhill has an effect on your speed. In other words, there is no linear relationship between the power you put down and your cycling speed and with that, the number of km’s ridden. Interestingly, it is the same for outdoor rides; weather and road conditions influence your speed for a given power output. Drafting obviously also plays a major role. One can surely argue about the real “km value” of doing a 100 km group ride when one drafts in the peloton 90% of the time and thus saves 20-30% of power during most of the ride.
So, in effect, we can say that km´s, whether ridden indoors or outdoors, in the virtual world or in the real world are simply not a good measure of how tough a ride was.
Instead we first need to look at the work that we put in our rides. If you plot your power output over the length of your ride, the surface below the curve is your actual work done. Secondly, as different workout intensities use different energy systems and muscle fibers in the body, intensity is also a key factor. For instance, doing 10 times 20 seconds sprints at 800W during your 100 km ride might not add much to the total work done but it will surely be felt in the legs afterwards! Unfortunately, the effects of those sprints are difficult to quantify in the light of this discussion, so let´s assume we skip those high intensity bouts in our 100 km ride. In this case, the above-mentioned power-time curve and calculated work done is a good measure to compare the toughness of different rides. Parameters as Training Stress Score and Training Load build upon this principle.
Another way of measuring how hard a ride has been is to look at ones heart rate (HR) during the ride. The area under that curve is a measure of the total amount of oxygen that has been used during the ride, which in turn directly relates to how much work has been done. As heart rate is a function of more parameters than just power output (like e.g. temperature, tiredness, hydration level, number of cappuccinos etc.) one has to be careful in using heart rate as a measure. However, in the absence of a power meter it can be used to gauge the work done, especially when compared to previous, similar rides. Strava´s Suffer Score is a good example of a parameter that takes your HR over time and relates it to your maximum HR to quantify the toughness of your ride.
Having established ways to measure or estimate the work being done in a workout it becomes clear that, in terms of training effect, it does not matter at all whether you do your training indoors or outdoors. Work done is work done. Suffering is suffering.
In conclusion, the length of your ride does not tell the whole truth regardless of whether you cycling indoors or outdoors. One indoor km could have been much tougher than one outdoor km or vice versa. Perhaps more important is that regardless of being a Zwift addict or a hardcore outdoor cyclist, every ride, indoors or outdoors makes you become a better cyclist!
– Peter Vriesman, Sun Tri Sports Team