Become a Better Skier with Ski Ergometer and Treadmill

A couple of weeks ago, Skisens launched their new app that makes it possible to connect and log training on a ski ergometer as well. In the app, you can view graphs and take intermediate times in the same way as during outdoor training and on a treadmill. The data enters the same flow as other training, everything is collected in one place and compatible with, among others, Strava. One question that many people ask is how to think about watts on a ski ergometer versus on a treadmill and outdoors.

As we have addressed several times, “real” skiing places significantly higher demands on technique to channel power in the direction of travel, but there are still great similarities in data interpretation. By monitoring key metrics on both the ski ergometer and the treadmill, you can become even more aware of which aspects of your technique you primarily need to work on.

For comparison, we conducted two ramp tests with gradually increasing power, first on the treadmill and then on the ski ergometer. On the treadmill, the incline was set to 5% with an initial speed of 9 km/h. Subsequently, the speed was increased by 1.5 km/h for each level. The levels lasted 4 minutes, with approximately 1 minute of rest in between. On the treadmill, the rest periods were sometimes slightly longer to accommodate lactate testing. Data from the treadmill test is shown in Figure 1.

Two days later, the same protocol was repeated on the ski ergometer. The resistance was set to 7.5 (drag factor 100), which the athlete perceived as fairly comparable to a 5% incline on the treadmill. During the test, the participant was instructed to maintain the same power output that was measured on the treadmill two days earlier. Figure 2 shows time series from the ski ergometer test.

When comparing Figure 1 and 2, it can be noted that heart rate and pulse develop very similarly in the two tests. The perceived exertion was also comparable. To further facilitate the comparison, Figure 3 shows average heart rate versus average power per level for Skierg and the treadmill. The pulse follows the power almost identically on the ski ergometer and the treadmill.

As mentioned above, Skisens’ measured power correlates well with power on the ski ergometer, which means that the athlete can aim for corresponding power for intensity control. However, there are some differences that are important to consider when tracking progress. An obvious difference is the development of drag force on the Skierg compared to pole force on the treadmill, see Figure 4. On the ski ergometer, the drag force increases almost linearly with power, whereas on the treadmill it is initially more or less constant and even decreases from the first to the second level.

There is a physical explanation for the differences in Figure 4. The braking force on the ski ergometer is set by a fan drum whose resistance increases with rotational speed. On the treadmill, however, the braking force is determined by rolling resistance and incline, which do not change significantly when the incline, in this case, is constant. The drop from level 1 to level 2 is a result of the rolling resistance decreasing slightly as the wheels heat up. The increase in pole force with power is due to the athlete losing the ability to direct the force at higher speeds.

The proportion of force that contributes to the direction of travel is a key metric commonly referred to as the “pole-efficiency index” η = Fav / Fax, where Fav is the average force in the direction of travel, which is equal to the sum of the braking forces, and Fax is the average of the pole shaft force. The ability to maintain with increased load requires good skiing technique. Usually, a loss occurs with increased load, resulting in the force needing to increase as shown in Figure 4.

Now that we have discussed the “pole-efficiency index,” it is natural to proceed and reason about external versus internal load. The external load is the useful work that propels the body forward on the ski trail, while the internal load is the total work the body performs. These are related by an efficiency factor. In the above discussion, the external load per unit time (power) is:

Pext​=Fav​×v=η×Fax​×v

where Fav is the average force in the direction of travel. When skiing on a treadmill, the athlete must constantly overcome Pext to avoid being carried off the belt. This means that if η decreases, Fax must increase, and thus the pole force in Figure 4 increases with power.

Pole-efficiency (η) constitutes an important part of the efficiency that relates internal and external load, and therefore the body’s internal load Fax × v follows better than it follows Pext. This is also reflected in Figure 3, where we see a close to 1:1 relationship between heart rate on the treadmill and the ski ergometer at comparable pole power. However, what we ultimately want to achieve is high Pext, that is, high propulsive power, which on the treadmill is determined by speed, incline, and rolling resistance. Therefore, it is important to know what pole-efficiency you really have, as this determines how much of the “pole-power” results in propulsion.

Here it must be clarified that the pole-power presented by Skisens uses a fixed pole-efficiency of fix = 0.6, which is a typical value for a regular skier who skis at a controlled pace. In practice, this value varies among different athletes, depending on skiing technique, and as discussed above, it also varies significantly with speed and skiing effort. However, all our tests, as mentioned above, have shown that it is a very good measure for intensity control both on the treadmill and outdoors. For those who use their power primarily for intensity control, there is no need to worry too much about their actual pole-efficiency. However, it is very important to know your pole-efficiency index for those who want to assess their performance.

To further emphasize that pole-power correlates with internal load, Figure 5 shows a test where the external power was set constant at just under 200 watts for different combinations of speed and incline. It is clear how the heart rate follows Skisens’ “pole-power” rather than the external power. As the pole-efficiency index decreases with increased speed, the body’s internal load increases.

In Figure 5, we see that pole-power at low speeds lies slightly below Pext, while the relationship is the opposite at higher speeds. This means in practice that pole-efficiency up to the breaking point is greater than 60%, and thereafter it is lower.

The exact meaning of the pole-efficiency index and what constitutes an optimal value is still being researched. Although at first thought one might think that it is a value one wants to have as high as possible, a simple thought experiment reveals that this would mean that the poles must be parallel to the ground, which is not efficient either in terms of pole grip or ergonomics. Our standard value fix = 0.6 is an average of what experienced skiers have at a relaxed endurance pace, but there are skilled athletes who are both above and below this value. The most important factor is how well you maintain the value with increased load. An athlete who has difficulty maintaining pole-efficiency with increased speed often also has difficulty reaching high speeds. As mentioned above, you do not need to know your pole-efficiency index so well to guide your training. The most relevant factor is to control by pole-power. However, pole-power is not a good measure of performance because ultimately, what we want to create is high external power. Two different athletes can have the same pole power but different pole-efficiency indexes and thus different external power. Therefore, for those who want to monitor their capacity, it is important to keep track of their pole-efficiency index and how it develops with power. A good way to test this is to use Skisens’ poles on a treadmill. Figure 6 presents the pole-efficiency index versus external power for the treadmill test in Figure 1. Note that efficiency decreases with increased power. At 16.5 km/h and a 5% incline (which corresponds to an external power of approximately 225 watts) it has decreased to just under 48%, resulting in a pole-power of approximately 280 watts. In Figure 6, we have also extracted a polynomial that describes how pole-efficiency changes with external power. This relationship can be used to convert power on the ski ergometer to the expected external power on the treadmill and snow.

If we return to the comparison between the ski ergometer and the treadmill, there are a few more key metrics to look at. Two interesting ones are frequency and impulse. The body generates force either by increasing impulse or by increasing frequency. The ability to increase the former is limited by skiing technique and physics. When the impulse can no longer be increased, the athlete must increase the frequency, which often leads to a loss of impulse and, on the treadmill, a decreased “pole-efficiency index,” which necessitates further increases in frequency. Note in Figure 1 how the athlete in the last level of the treadmill test makes a marked increase in frequency, which also leads to a collapsed pole-efficiency index and rapid exhaustion.

A third key metric of interest is ground contact time (or pole time on the Skierg). Reduced pole time is one of the main reasons why impulse often decreases with increased speed. Figure 7 below shows impulse and frequency versus ground contact time when using the ski ergometer and the treadmill. A clear difference is that impulse on the ski ergometer increases almost linearly with increased pole time. This is difficult to replicate on the treadmill because one cannot maintain the angle of the poles in the same way as on the ski ergometer. This is also why, at low power on the ski ergometer, it is effective to use a very low frequency. In Figure 7(a) below, it is clearly visible how frequency on the ski ergometer begins to differ significantly from that on the treadmill when the ground contact time exceeds 450 ms. When using the Skierg, you should aim to keep the ground contact time below the value where this breakpoint occurs. When comparing impulse on the ski ergometer and the treadmill, it is obvious that impulse is higher on the ski ergometer. Higher impulse on the treadmill can be achieved by increasing the incline. By lowering the resistance on the ski ergometer, you can achieve a lower impulse, but it often becomes somewhat unnatural to have too low resistance. It is better to accept that using a ski ergometer is most similar to skiing uphill and choose a resistance between 6 and 10 for athletes weighing over 70 kg. Lighter athletes can use a lower resistance.

The next level of comparison is to look at how the pole strokes appear on the treadmill and the ski ergometer. Figure 8 below shows the force curves from the treadmill and the ski ergometer for the same athlete at a speed of 15 km/h. The curves are significantly different. An obvious difference is that on the ski ergometer, you do not get the first peak (impact-pole force) that is achieved on the treadmill and outdoors when the poles are braked against the ground. However, this has less practical significance because that peak contributes little to propulsion. What is more significant is that the height of the second peak is flattened and that the curve is more extended instead. The curve from the ski ergometer also has a clear double push with high force at the end of the pole stroke. This is a characteristic commonly found among athletes who are really strong uphill. However, creating this on rollerskiing and snow is not easy because it is difficult to maintain pressure over the poles as you move forward. On the ski ergometer, however, it is easier, but it is important to learn to utilize that technique outdoors as well if you are to benefit from it in actual skiing. Athletes who have a double push on the ski ergometer but not on the treadmill rarely achieve as high an external power on the treadmill as they do on the ski ergometer.

In summary, it can be stated that the ski ergometer is an excellent tool for tracking capacity through concrete key metrics (watts). It is also an excellent tool for intensity control, which is one of the most important ingredients behind good training planning. However, a challenge is that despite its excellence as a training tool, the ski ergometer has a major drawback. It does not stimulate skiing technique. On the contrary, there is a risk that it can have an opposite effect. Those who use the ski ergometer extensively tend to develop a technique that is successful on the ski ergometer but does not lead to success on the ski trail.

Skisens offers several tools that help skiers monitor their technique when training on the ski ergometer. Through the key metrics of frequency, ground contact time, and impulse, as well as for the more dedicated, the force curve, one can increase the chances that progress on the ski ergometer also leads to progress on the ski trail. Exact measurements are somewhat individual. To gain better insight into what suits you best, it is recommended to perform a test on the treadmill to find your sweet spots and also to determine your own unique pole-efficiency index.

If a large part of your training consists of using the ski ergometer, it is recommended that you regularly, perhaps once a month, perform a reference session on the treadmill. This way, you can easily verify that you are maintaining or increasing your pole-efficiency index and monitor your own reference values regarding ground contact time, frequency, and impulse and how these relate to standard values. Through your pole-efficiency index, you can also calculate how your Skierg power would hold up outdoors. You can improve on the ski trail both by increasing your pole-efficiency index and by enhancing your physical capacity. Skisens helps you understand where your greatest potential lies.

Now don’t forget to download the app and check how you stand!

Merry Christmas and Happy New Year!