Crossing through sand requires up to 33% more energy than running on a hard and flat surface.
Many races are run on a fast track. That is, as everyone knows from experience, on asphalt roads or on a synthetic athletics track. In both cases, the surface is hard and flat. This is going well. The running resistance is minimal and the specific energy consumption ECOR in these conditions for a trained runner is 1.04 kJ/kg/km. This value includes the drag from the running wind.
Experienced runners know that it can already make a difference if uneven segments with stones or boulders are included in the course. This is especially true if you have to cross sections of forest land or even through the sand. What is the impact of the running surface on performance? How much time are you losing?
Constant power
Unlike a power meter for a bike, even the accurate Stryd running power meter doesn’t help you to run a fast time. A power meter for a bike calculates the power from the force you actually put on your pedals. The Stryd power meter determines the running power with the tiny accelerometers and wind speedometer in the gadget attached to your laces. The Stryd does not see any differences in subsoil.
Stryd works well with elevation changes in the course and with the wind on a hard surface. Your fastest time will be run if you constantly focus on your achievable power for that distance along the way. You then consciously run slower uphill, don’t arrive at the top out of breath with a red face, and go downhill with a faster than average pace. The same is true with a headwind and a tailwind. For your fastest time you run with a constant power.
Running in loose sand can require more than 25% extra energy. You won’t be able to sustain that. You will therefore have to slow down your pace and maintain a lower power value on your running watch. How much lower you will have to estimate yourself.
Additional resistance of sand, grass and forest soil
There are several publications in the literature about the specific energy consumption for different surfaces. For example, the energy consumption of a forest course is about 3% higher and of a cross course even in the order of 6% higher. These values correspond well with our experiences. The (in this case average) power to be maintained should be reduced by the same percentage.
Loss of time on a non-ideal course
In the table below, we show the results of calculations on the following cases:
- Ideal course (asphalt, synthetic)
- Forest course (mix of bike paths and grass, 3% higher)
- Cross (mix of grass and sand, 6% higher)
Of course these are schematized situations and the course will differ in practice from these assumptions, but it does give a good impression of the impacts. In the table the impact of the course on the times for our Marathon Man is calculated.
The Marathon Man is the example runner we always use in our book ‘The Secret of Running’ and in many of our articles. With his 70 kg body weight, he runs the marathon in 3:30 hours.
We see that the effect of the course for our Marathon Man can be about 1 to 2 minutes on the 10K and 2 to 10 minutes at the marathon. A marathon is not often run on a cross-track, so the differences in practice are less large.
Our book ‘The Secret of Running’ is for sale in our webshop. Also available in German as ‘Das Geheimnis des Laufens’, and in Italian as ‘Manuale completo della corsa’.