Cross-country skiing is a complex endurance sport where the skier can choose between several sub-techniques over the course of a race. Essentially, the sub-techniques differ with regard to how propulsive forces are applied to the ground through the skis and/or the poles, where choice of sub-technique depends mostly on incline and speed. Double poling (DP) is the only subtechnique where the propulsive force is applied solely through the poles while the skis continuously glide forward parallel to the track. The upper-extremity muscles have therefore been considered the prime movers in DP. However, previous studies have shown that also the lower extremity plays an important role for optimal technique in high-performance DP, in part by generating mechanical energy through heightening of the body during the swing phase which subsequently can drive propulsion during the poling phase. Nevertheless, there is a lack of information concerning the specific sources of generation and destination of mechanical energy in DP (e.g., upper vs. lower extremity). Therefore, the main aim of this thesis was to examine the energetics and dynamics of DP in different conditions. One main question was how, and how much, lower-extremity power generation contributes to propulsion power through the poles in various DP conditions. This thesis is based on the three studies listed below which are referred to by their Roman numerals throughout the text. I Danielsen J, Sandbakk Ø, McGhie D, Ettema G (2018) The effect of exercise intensity on joint power and dynamics in ergometer double-poling performed by cross-country skiers. Hum Mov Sci 57: 83-93 II Danielsen J, Sandbakk Ø, McGhie D, Ettema G (2018) Mechanical energy and propulsion mechanics in double-poling roller skiing at increasing speeds. Pending submission to Sports Biomech III Danielsen J, Sandbakk Ø, McGhie D, Ettema G (2018) Energetics and dynamics of double-poling roller-skiing at different incline-speed combinations at equal work rates. Submitted to PLoS One A total of 21 male Norwegian elite skiers, of both national and international level, volunteered to participate. Study I examined the effect of intensity in ergometer DP, study II examined the effect of speed in roller-skiing DP on the level and study III examined the effect of incline-speed combinations in uphill roller-skiing DP. Motion capture analysis was used to derive body mechanical energy fluctuations and the rate of change. From kinematics and dynamics, linked segment modelling was used to compute joint moment and power, as well as upper-extremity (shoulder+elbow) and lower-extremity and trunk (trunk+hip+knee+ankle) power. The relative power contribution from the upper extremity and lower extremity and trunk towards total power output (external work rate; WR) was calculated. In ergometer DP (study I), the upper extremity contributed 51% of the total power output (i.e., pole propulsion power) during low-intensity DP (WR 116 W) and decreased (P<0.05) to 33% during maximal-intensity DP (3 min performance test; WR 306 W). In rollerskiing on the level (study II), this contribution amounted to 63% during low-speed DP (15km·h-1; WR 98 W) and increased (P<0.05) to 66% during high-speed DP (27 km·h-1; WR 176 W). In uphill DP (study III) on slight and steep inclines (5% and 12% incline, respectively), speed was set to give equal WR at both inclines. No effect of WR (142 W - 238 W) was found on upper-extremity contribution in either slight (9.3 - 15.5 km·h-1) or steep (4.8 - 7.9 km·h-1) uphill DP (P>0.05). However, upper-extremity contribution was 63% on slight incline, which was higher than the 54% contribution on steep incline (P<0.05). Based on these values, it can be concluded that the lower extremity contributes significantly to the total power output which is fully delivered externally through the poles. How this is made possible was similar between all conditions. Lower-extremity power generation occurs partly during the end of the poling phase, but mostly during the swing phase. The work done by the lower extremity during body heightening increases body mechanical (mainly potential) energy. As such, lower-extremity work is temporarily `stored` as body mechanical energy. During the following poling phase, the body is leaned forward and rapidly lowered and part of the body mechanical energy (potential and kinetic energy perpendicular to the surface) is transferred to pole (or rope) propulsion power as the body exerts force on the poles (or ropes). During the poling phase, the upper extremity (mainly shoulder) generates considerable power that instantaneously drives pole propulsion power. Some of the body mechanical energy generated during the swing phase is absorbed by the lower extremity during the following poling phase (that is, the part of the decreasing body energy not directly used for propulsion through the poles). This aspect was especially apparent in roller-skiing DP on the level (study II). Although this may seem energetically ineffective, some of this absorbed energy may be stored elastically and reutilised during the bouncing-like transition from body lowering to heightening. In uphill DP (study III), the amount of absorption by the lower extremity decreased at slight incline and was further reduced at steep incline. Increasing slope creates different boundary conditions, e.g., with the force of gravity acting at an angle to goal-directed movement (i.e., surface), not perpendicular as on the level. To maintain dynamic force balance, the skier adjusts body and pole positioning accordingly. In study III, a hypothesis related to incline was that the lower extremity could contribute less at steep incline because of this incline effect on the gravity-surface relation. This was not confirmed at the inclines and intensities studied here. The lower speed and gravity-surface relation on steeper incline leads to (much) longer poling times and shorter swing times. Altogether, on steep incline DP it seems as if body and pole positioning related to the boundary conditions become less advantageous for effective upper-extremity power generation. At the same time, more upper-extremity power was generated on slight incline at a lower perceived effort than at steep incline. From this it can be hypothesised that the upper-extremity muscles operate within a less advantageous range of the force-length-velocity relationship, which likely is a part of the interplay between several factors that make skiers not prefer DP on steep incline.
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