Biomechanical and physiological characterization of cross-country skiing techniques. Analysis of the selection between techniques

Cross country skiing can be seen as a form of locomotion performed with the use of passive locomotory tools, skis and poles, which allow to travel on snow with a cost of transport that is lower than that for running. Cross country skiing can be performed by using different techniques that are characterized by differences in movement patterns and the propulsive action of legs and arms in a proportion that changes between techniques. While several studies on cross country skiing have dealt with the description of movement patterns, of timing and force and/or the metabolic cost of some techniques, to our knowledge a systematic characterization of the biomechanical and the energetic aspects of all techniques over a wide range of speeds and grades has not been attempted so far. Cross country skiers can switch between the techniques accordingly to changes of track, slope and environmental conditions; the different techniques could be then considered in this perspective as a gear system. Despite the selection of the appropriate technique could then have important implications on efficiency of locomotion and on performance, the parameters that determine the selection of the techniques are still unknown. The aims of the thesis were: 1. Biomechanical and physiological characterization of the different cross-country skiing techniques in function of speeds and slopes 2. Determination of the preferred cross-country skiing technique for given combination of speeds and slopes 3. Assessment of the determinants of the selection between different techniques Similarly to what reported for walking and running, a comprehensive analysis of the transition among different techniques should consider metabolic, kinematic and kinetic parameters together. In order to answer to the aims of the project, the measurements have been divided into three different studies. The techniques considered in this study were the most used in classical skiing, diagonal stride technique (DS), double poling with kick (DK) and double poling (DP). All the studies have been performed by asking the athletes to ski with roller skis on a motorized big size treadmill that allow to impose to the skier precise and stable of slope and speed. The subjects recruited for the studies were 10 male cross country skiers. Study A- This study was planned to assess which techniques the athletes would prefer to use for skiing at different combinations of slope and speed (aim 2). Treadmill speed and grade were randomly selected, and sustained for at least 30 seconds. The subjects were asked to adopt the ski technique they feel most comfortable under that condition. Results are expressed as percentage of the subjects that selected that technique Study B – The aim of this study was to measure the energy cost of skiing, by accounting for both aerobic and anaerobic pathways and to consider general physiology. Oxygen consumption, blood lactate, rate of perceived exertion have been measured for skiing with all three techniques, at selected conditions of speed and slope (different speed from 6 km/h up to 14 km/h for DS and 16 km/h for DP and DK at a slope of 2°; different slopes from 0° to 4° for DP and DK and up to 6° for DS, at a speed of 10km/h ). Study C - In this study, cycle length, cycle rate, poling and leg thrust time, poling force external and internal mechanical work was obtained from the same group of skiers tested in Study B and Study C and in the same condition tested in Study B. Measurements of pole force and total body kinematics (18 reflective markers placed on well defined anatomical landmarks, skies and poles) will be synchronously sampled for 30s period to obtain 20 consecutive skiing cycles. Results of study B and study C are used to answer to aim 1 and aim 3. Discussion - aim 1 - We measured parameters investigated by other researchers and presented in the literature, however, to the best of our knowledge, none conducted a study by including both changes of speed and slopes on a wide range of increments and for all the three techniques. The changes of slope and speed were associated with several changes in many of the investigated parameters. An increment of slope is faced by increasing the cycle rate, by increasing the absolute and relative time of thrust action, by increasing the relative poling time and by augmenting poling force and force effectiveness. On the other hand, increments of speed do not imply increments of poling force nor induce increments in the cost of locomotion, at least in the range of the investigated speeds. The total mechanical work for locomotion was found to increase slightly with the speed, the increase being mainly attributable to increments of internal work, while external work increased with speed only for DS. A biomechanical adaptation to the increments of speed was observed in the increase of cycle rate and cycle length and in the decrease of both absolute and relative poling and pushing time. Energy curves of COM allowed us to characterize DS as a grounded running (in which aerial phases are substituted by gliding phases) and DP as a bouncing gait (as for running). The most evident differences between the three techniques are on the forces exerted through the poles: for DP these correspond to the total force required for propulsion while for DK and DS these are about 64% and 40% of total force, respectively. Even if the differences in the overall energy expenditure are rather small among techniques, large differences were observed in the anaerobic metabolism, with DP eliciting a higher lactate production in comparison with DS and DK. This result is in agreement with the observation that arm muscles are smaller in size and have a lower oxidative capacity in comparison to leg muscles. Discussion - aim 2 - It was found that DP is the preferred techniques by quite all the subjects for skiing on flat, at all the speed. When the slope is increased, skiers decided to abandon DP and declared to prefer DK for moderate uphill condition and they all indicated DS as the preferred for skiing at slope higher than 6-7°. When skiing at a slope of 2°, most of the skiers declared to prefer DP or DK at speed lower than 14 km/h while DP was the techniques preferred by all the skiers at higher speeds. Although preference for DS for skiing uphill and DP for fast skiing is well know, it was necessary to conduct this study in order to find the speed and the slope at which the technique transition is made for skiing in our experimental setup. Discussion - aim 3 - The variable that showed the greater variation among techniques is the force exerted through the pole The transition that occurs when increasing the slope can be tentatively explained with the hypothesis that there is a limit in the force a skier could chose not to exceed. A second parameter that can function as a trigger is the time of application of the force exerted by the legs. We have found that selection of DS and DK as preferred techniques occurs approximately in the ranges of speed an slopes in which leg’s thrust action has been hypothesize to be optimal. Taken together, the existence of a limit in the poling force and in the time for leg’s thrust can explain respectively why DP is abandoned in favour of DS at high slopes and why DS and DK are abandoned for DP at high speeds. Cost of locomotion is different between the techniques only on flat, and cannot be indicated as a good trigger for the transition. Even if it seems not possible to drawn an ultimate conclusion on the determinants of technique selection in cross country skiing since, we could hypothesize that transitions in cross country skiing are the results of a pool of determinants that can be ranked in hierarchical order and that transitions are determined by the activation of the most “sensible” trigger/variable in the pool.