The knowledge of the position of the centre of mass of a body and the calculation of its acceleration allow to evaluate the effect of the sum of the forces applied on it. In this study the hypothesis that the centre of mass (CM) motion influence pole force application during double poling was tested. Ten male members of the Italian cross country ski team participated in this study. They were requested to perform double poling using roller skis on a treadmill at 17km/h and 3 of slope. Total body kinematic was acquired by means of an optoeletronic system (Qualisys) placing 26 reflective markers on body landmarks. Two more markers for each pole were used to identify pole inclination. The pole force was recorded by means of a load cell mounted below the pole grip. Force and kinematic data were synchronized together and sampled at 100Hz. The beginning of the poling cycle, cycle and poling duration were calculated form force data. The pole inclination value was used to obtain vertical and horizontal force component. The CM motion on the sagittal plane was computed from kinematic of all segment. Results are reported as mean and SD over the group. The poling cycle duration was found to be 1.01±0.07s, the duration of the poling phase was 0.32±0.03s. The athletes start the poling action with a pole inclination of 76.4±3.4 and end with the pole at 25.3±1.4 . The mean force exerted on each pole was 147±15N, while the horizontal force component was 91±10N. The CM covered on the sagittal plane a ellipsoidal trajectory respect to the tips of the feet, length of 83.8±5.6cm. The maximum forward and backward displacement of the CM respect to the forefeet are respectively 9.1± 3.4cm and 18.2±1.8cm. Range of motion of CM in vertical direction was 28.6±2.6cm. Three of the athletes showed a pattern for CM different from the others, with the start of the poling phase occurring while the CM is moving in downbackward direction (strategy A) while the others showed a down-forward motion (strategy B). The athletes using strategy A are characterized by a higher CM forward displacement (12.2±2.6cm vs 7.9±3.0cm), lower poles inclination (72.1±1.2 vs 78.1±2.1 ), a higher ratio between horizontal and total poling force in the first part of the poling phase (45±2% vs 39±4%). The experimental setup and the data collected in this study allowed to precisely calculate the motion of the CM during a highly dynamic movement. These analyses showed that each athlete is characterized from a different shape of the CM trajectory. The synchronization between force and kinematic data allowed to point out, immediately before the poling phase, the use of two different strategies. It has been moreover seen that the two strategies lead to different force application in the first part of the poling cycle, with the strategies A that seems to be more efficient for forward progression.
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