We often receive questions about how we measure power in skate skiing. It’s a very good question. Since we only measure in the poles, we do not measure the entire body’s work. However, based on treadmill tests, we have seen that what we measure is very consistent. A skier who uses the third gear has a certain proportion of force in the poles, and with knowledge of this factor, we can then calculate the entire body’s contribution. It is also perfectly possible to use key metrics and force curves for feedback.
Figure 1. Key Metrics and Power for 8 Three-Minute Intervals in Free Style with Second and Third Gear
Figure 1 shows key metrics for eight three-minute intervals in free style. Note how the frequency decreases and the impulse increases for intervals 5-7 when the skier uses second gear.
An interesting observation in Figure 1 is that power decreases significantly for intervals 5-7 despite heart rate not changing significantly. The explanation is, of course, that the power shown in the graph is only the poles’ contribution. By comparing it to external power calculated from friction, speed, and incline, we can determine the proportion of power that goes through the poles in different gears, see Figure 2.
Figure 2. Proportion of Force in the Poles
Figure 2 shows that about 75% of the force goes through the poles in third gear and about 35% in second gear. An interesting note is that the last intervals running in third gear have a somewhat higher proportion of force in the poles, which may be a result of the skier having fatigued in the legs during the three preceding intervals.
Knowledge of the proportion of force from the poles is very useful for those who want to use Skisens for intensity control in skate skiing. We can then use an algorithm to detect the skiing style and subsequently multiply by the compensation factor for the upper body’s contribution so that we obtain the whole body’s work in accordance with Figure 3. It is clear that the heart rate reflects the power well.
Figure 3. Power and Impulse for 8 Three-Minute Intervals in Free Style
For deeper insight, we can also look at the force curves. Figure 4 below shows force curves in third gear, and Figure 5 shows the same for second gear.
Figure 4. Force Curves in Third Gear at 19 km/h and 3 Degrees Incline
Figure 5. Force Curves in Second Gear at 8 Degrees and 8.5 km/h with Left Arm
The force curves in Figure 4 show very good symmetry and resemble the force curves from skating, while the force curves in Figure 5 are asymmetrical as expected in second gear. You can also see that the ground contact time is longer in second gear, which is also natural considering the lower speed. Further interpretation regarding the appearance of the force curves and how they reflect skiing technique will be addressed later, but for comparison, we also include force curves from skating at 19 km/h and 3 degrees incline shown in Figure 6 below. These can be compared with Figure 4.
Figure 6. Force Curves from Double Poling at 19 km/h and 3 Degrees Incline
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