SKI MECHANICS – How to Interpret Your SKISENS Data

Power, Energy, and Efficiency

Cross-country skiing, much like running, cycling, and swimming, is a sport where the human body performs mechanical work—moving a distance (s) against resisting forces (F). In competition settings, athletes aim to complete this work in the shortest possible time (t). A natural performance metric is therefore power, defined as work per unit of time and measured in watts (Nm/s).

Because power isolates the athlete’s physical capability from tactical choices, equipment differences, and environmental factors (weather, wind), it is often a more reliable comparison metric than speed. This is especially true in cycling, where power meters have been commercially available for many years, and—more recently—within running and skiing. Several types of gym equipment also display power output.

Before power is used too broadly to compare performance between sports, it is important to understand what the term actually means. In everyday speech, “power” is often used in a far less precise sense than the scientific definition. The term is sometimes used to describe an outcome—e.g., “the effect of the training made me faster at Vasaloppet.” This vague usage is reflected in some training-analysis platforms where “training effect” refers to training load based on heart rate or accelerometer data, not true mechanical power.

If one actually measures power directly, training load (energy expenditure) can be derived accurately. Energy consumption (e.g., kcal/hour) is mathematically identical to the integrated power output of a session. A power meter is therefore an excellent tool for estimating energy use—valuable both for recreational skiers who want weight control and for elite athletes optimizing energy balance.

However, to make use of this correctly, we must understand what type of work the power meter measures. Is it the body’s total mechanical work, or only the external usable power?

In cycling, strain-gauge–based power meters mounted in pedals or cranksets measure torque directly in the drivetrain, producing highly accurate measurements of external, usable power. Only about 20% of the body’s metabolic energy becomes usable power, with the rest lost as heat.

Newer running power meters (e.g., Stryd) use IMUs (inertial measurement units) to estimate mechanical power from movement—but without direct force measurement, there are uncertainties and it is unclear whether they estimate total body work or usable external power.

The SKISENS power meter measures pole power via a load cell consisting of four strain gauges that capture axial pole force, combined with an IMU that determines pole angle, allowing calculation of how much force is directed forward. Power is then computed by multiplying with the skier’s forward speed (GPS-derived). Thus, SKISENS measures usable external power through the poles. In double poling, this closely matches the skier’s total external propulsion. In techniques involving leg contribution (diagonal stride, kick double-poling, skating techniques), leg power is not captured.

To estimate whole-body energy expenditure, one can use typical skiing efficiency (≈15–17%) and research-based estimates of how much of the total propulsion is generated via the poles in different techniques. SKISENS software supports technique detection to enable this.

Force, Impulse, Frequency, and Force–Angle Curves

Cross-country skiing is a cyclical motion where propulsion is created by injecting energy forward in short, periodic pulses. To increase speed, the skier must generate forward-directed force that exceeds all resistive forces.

During double-poling, each pole stroke generates an impulse in the direction of travel. Impulse (Ns) depends both on force magnitude and the duration of force application. Scientifically, impulse is defined as the integral of the force-time curve over a pole cycle:

(1)  

where F(t) is instantaneous axial pole force, T is cycle time, and α(t) is pole angle relative to the ground.

To increase average forward force, the skier can either increase impulse per pole stroke or increase frequency f=1/T. Mean forward force is:

(2)  

This relationship places high demands on both physical capacity and technical skill. As frequency rises, maintaining high impulse becomes increasingly difficult—especially at high speeds, where the available time to generate force is shorter. Similar to running, explosive force application becomes more critical as speed increases.

Fig. 1 – Force-Angle Curve

Figure 1 shows a typical force–angle curve illustrating how axial pole force and pole angle vary throughout a pole stroke. The forward impulse depends not only on force magnitude but also on how well the skier maintains force late in the stroke when the pole is angled backward and more aligned with travel direction.

Figure 2 shows how forward-directed force compares to total axial force. Forward force peaks around a pole angle of ~45°, then drops to zero at pole release.

Fig. 2 – Force Components

By comparing force–angle curves at different speeds and gradients, one can evaluate a skier’s technical proficiency. Figure 3 compares a former Vasaloppet champion with a wave-2 recreational skier. The elite skier applies force more quickly, maintains it longer, and directs it more efficiently—producing much greater forward impulse.

Fig. 3 – Force–Angle Comparison: Vasaloppet Winner vs Wave-2 Skier