Nine female elite cross-country skiers (mean ± SD: age, 20.2 ± 1.0 years; height, 158.7 ± 5.0 cm; weight, 56.5 ± 4.3 kg) gave their informed consent for participation in this experiment. Their mean maximal oxygen consumption VO2max (56.8 ± 8.4 ml/kg/min) was measured during Nordic walking on a treadmill using an ergospirometry system.

All subjects used racing poles (Yoko Platinum Power Grip, Karhu Sporting Goods Oy, Finland). Specially made sensors were mounted on the right and left poles (Fig. 21.4). In the test, special roller skis were fabricated using square pipe aluminum (600 × 40 × 30 mm), bearing, and tire (610 mm, Marwe Oy, Hyvinkään Kumi Oy). To measure the ski reaction force on the roller ski, strain gages (N11-FA-5-1000-11, Showa Measuring Instruments Co. Ltd., Japan) were attached to the front and rear of the right and left roller ski frames (Fig. 21.4).

Testing was conducted on a flat, straight outdoor asphalt track. After accelerating over a 50 m section, the participant’s skating time over the next 20 m was measured using two photo sensors (Wireless Sprint System, Brower Timing Systems, USA). Each subject was instructed to ski at three different speeds (low: 3.5 m/s, medium: 5.0 m/s, and maximal effort).

In this study, one cycle was defined as running from the start of the ground contact of both poles to the start of their subsequent ground contact. The upper body work was divided into the poling and swing phases used by the pole reaction forces. The leg work was categorized into the glide, push-off, and swing phases used by the ski reaction forces (Fig. 21.5). The zero level of the pole and ski reaction forces was defined as the swing phase. The poling phase was the phase during which the pole reaction force was greater than zero. The glide and push-off phases were determined based on the point of lowest value among the bimodal ski reaction forces. The glide phase started with the placing of the roller ski on the ground, leading to a first peak in the force curve. It ended at the point of minimum value of the ski force, before it attained a second peak. The push-off phase started at the point of minimum force and ended when the ski reaction force decreased to zero. The duration of the poling, swing, glide, and push-off phases were calculated, and the bimodal peak values during the glide and push-off phases were defined as the peak force during each phase. The ratio of the peak force during the poling phase to the peak force during the push-off phase was calculated. The lowest force was determined as the minimum value between the glide and push-off peak forces.

The following data were obtained from each trial: velocity, cycle rate, cycle length, and peak and mean pole reaction forces during the poling phase; the lowest ski reaction force during the glide phase; the peak and mean ski reaction forces during the glide and push-off phases; and the poling, swing, glide, and push-off times. The variables were reported as the mean ± SD. They were compared across velocities using a one-way analysis of variance with repeated measures. When the F-values were significant, individual comparisons were made using Bonferroni post hoc analysis. Moreover, the correlation between the peak force during the push-off phase and the lowest force were analyzed using Pearson’s product–moment correlation analysis. Values of 0.05, 0.01, and 0.001 were accepted as the levels of statistical significance.

The cycle characteristics and kinetic parameters are listed in Table 21.1. The cycle rate gradually increased, from low to high velocities, up to 1.29 Hz (all P < 0.05), whereas the cycle length increased only from low to medium velocities (all P < 0.05).

The poling times decreased from low to medium velocities (P < 0.05), whereas the swing times gradually decreased across velocities, from low to high (all P < 0.05). The ratios of the poling and swing times, in each cycle, remained constant across velocities. The poling peak and mean forces increased from low to medium velocities (all P < 0.05).