Training and Racing with Ski Power: How to Use a Power Meter in Cross-Country Skiing

Training and racing with ski power

Dan Kuylenstierna, Johan Högstrand, Szhau Lai
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Any serious cyclist or coach is aware of Allen Hunter’s and “David Coggan’s book Training and Racing with a power meter [1]. The book summarizes and exemplifies how to best use power data during and after training and races. The two major elements in this work are Intensity control and capacity screening which are facilitated by power-based training zones and power profiling, respectively.

At the infancy of ski power meters, it is a huge advantage to be able to start from the framework already established by Hunter and Coggan. Key concepts such as functional threshold power (FTP), Maximum mean power output, and power-duration curve (also known as power profile) are valid also in skiing. Despite similarities there are also differences though. Obviously the numbers are different, while a world class cyclist may sustain 7 watts/kg for 20 min, a world class skier reaching beyond 5 watts/kg for the same duration is beyond our awareness. The main reason for this difference is different gross efficiency (GE). In Cycling GE can be as high as 25% while in skiing it is more like 17-19%. In the anaerobic exercise domain, the difference between ski power and cycling power is further accentuated, which means that the power profile in skiing normally is flatter than the counterpart in cycling. A hypothesis is that the relatively lower anaerobic ski power may be explained by lack of mechanical gears preventing the skier from working against optimal resistance.

Regardless of the difference in numbers, the training intensity in skiing can be prescribed from the same principles as in cycling with intensity zones based on the functional threshold power (FTP). Hunter and Coggan recommends 7 training zones related to power or heart rate according to Table 1. To be able to use the training zones as defined in Table 1, the first step is to determine the FTP, which fundamentally is the highest power an athlete can sustain for one hour of continuous work.

Table 1: Training zones based on FTP according to Hunter and Coggan.

Practically, it is challenging to motivate an athlete to go all out for one hour. A more practical approach is to run a 20 minute test and assess the FTP as 95% of the mean power output over those 20 minutes. In reality, only very well trained athletes can really maintain 95% of the 20 minutes maximum mean power output for a complete hour. Nevertheless, the standard is so well developed that this has become the practical definition of FTP. Laboratory methods for assessing the lactate threshold may instead be based on an incremental exercise protocol where lactate is measured during incrementally increased exercise intensity. Once FTP is determined, intensity control in training is significantly simplified by the numbers in Table 1. An open question is whether it may be reasonable to adjust these numbers for skiing. For now, we leave them the same.

Table 2: Ski power profile chart

A huge benefit of training with a power meter is that it helps you to identify your strength and weaknesses by comparing to average performance of other athletes at your competition level. This is where the performance screening and skill profiling comes in. As outlined in Hunter and Coggan, profiling can be based on maximum mean power over durations of 5s, 1m, 5m and 20m, reflecting neuromuscular ability, anaerobic ability, VO2 max, and FTP ability, respectively.

In skiing even more than cycling, it is very challenging to access a data set with a large group of riders being profiled over the full range of work durations. Instead our profiling is based on data from literature and a selected number of athletes of different performance levels. Then this data is scaled proportionally to what is expected for different performance levels. For a start, with the limited amount of data available to date, it is reasonable to use the same scaling as Hunter and Coggan have identified in cycling.

Hunter and Coggan categorize athletes at 9 different performance levels: World class, Except-ional, Excellent, Very good, Good, Moderate, Fair, Novice 1, Novice 2. Using their scaling rules and reference data from skiing, we end up with the Skiing profiling Chart in Table 2.

Everyone with a power meter at hand can now start collecting data and compare to numbers in Table 2. For those not having a power meter, it is possible to start with ergometer data, e.g, a Concept 2 skierg. Skierg FTP is rather well elaborated with documented world records and time trials over 5000 meter[2]. Outdoor validations can be based on race data [3] and records from Strava [4]. For those with access to a treadmill, it is possible to calculate power from inclination, friction and speed. Reference treadmill data can be taken from the Swedish Ski federation’s Requirement Profile [5] and conversions to watts was performed in a recent Skisens newsletter [6] .

In comparisons as discussed above, it is important to remember that power calculated from treadmill data by definition is identical to constructive propulsive work rate, i.e., external power, which is comparable to cycling where measured power goes directly to the drive line. Likewise on an ergometer, the power inevitably goes into the flywheel. However, due to variations in skiing technique it is far from obvious that the external power reached on the treadmill is the same as on the skierg. It takes a very good skiing technique to reach the same efficiency on the treadmill as on the Skierg. In this aspect a good skiing technique may be manifested by a high gross efficiency (GE), which is defined as the ratio between external power and internal power.

Talking about GE, we come to another difference between cycling and skiing. In cycling, GE is essentially invariant with speed, very much due to the fact that cyclists have mechanical gears allowing same cadence at high speed as for low speed. In skiing on the other hand, this is not the case, it is well known that the external power is lower at high speed compared to low speed. In a recent white paper [7], we discussed the difference between external power and Skisens Ski power. The ratio between the two is the poling efficiency index, i.e, the ratio of force in direction of motion to axial pole force. Laboratory testing on treadmill have validated that perceived intensity follows Skisens power rather than external power. For intensity control, it is therefore better to assume a constant pole-efficiency index, e.g, PE=0.6 which is found to be an average pole efficiency for trained skiers skiing at moderate to high intensity. It is also the number used in Skisens software. It is important though to remember that the assumption of constant PE has the consequence that Skisens SKI POWER is likely not identical to external power, which must be kept in mind while benchmarking performance versus Table 2.

To be able to benchmark performance, it is recommended to determine each athlete’s personal pole efficiency and its variation with speed or related parameter such as thrust duration which is described in [7]. This can be done by a dedicated test protocols on a treadmill. Once established, the PE model can be applied to outdoor skiing data in order to calculate true external power which can be benchmarked based on Table 2. The ambitious skier is recommended to repeat this test regularly as PE may change with progression in skiing technique. For skiers with mature technique the variations are small though, why this method works adequately.

Although it may appear confusing with two different power definitions, Skisens’ Ski power for intensity control and external power for capacity benchmark, it is not too different from the case in cycling where there are also different definitions such as normalized power and true average power. To fundamentally understand the difference between normalized power and true maximum mean power output, it is required to dig into the definition of intensity factor and its variability. However, after working with numbers for a while even the more mathematically minded persons tend to develop a feeling for the meaning of numbers rather than relying on direct calculations. The same applies as we are getting used to SKI POWER. It is not that difficult, and we must always develop a gut feeling for difference between circumstances, e.g., many cyclists have hard time reaching the same power outdoor as they do on the ergometer while for some it is vice versa. In skiing it is even more pronounced that some athletes are levels better on the skierg. If this is the case for you, it is like that you have a low poling efficiency. In skiing, there are also different biomechanical gears like diagonal stride, kick-double poling, and double poling in the the classical style and V1, V2, V2 alternate in the free style technique. This aspect, we will need to come back to later.

Training and Racing with Ski Power: How to Use a Power Meter in Cross-Country Skiing

Training and racing with ski power

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