This study aims to develop and validate an automated system for identifying skating-style cross-country subtechniques using inertial sensors. In the first experiment, the performance of a male cross-country skier was used to develop an automated identification system. In the second, eight male and seven female college cross-country skiers participated to validate the developed identification system. Each subject wore inertial sensors on both wrists and both roller skis, and a small video camera on a backpack. All subjects skied through a 3450 m roller ski course using a skating style at their maximum speed. The adopted subtechniques were identified by the automated method based on the data obtained from the sensors, as well as by visual observations from a video recording of the same ski run. The system correctly identified 6418 subtechniques from a total of 6768 cycles, which indicates an accuracy of 94.8%. The precisions of the automatic system for identifying the V1R, V1L, V2R, V2L, V2AR, and V2AL subtechniques were 87.6%, 87.0%, 97.5%, 97.8%, 92.1%, and 92.0%, respectively. Most incorrect identification cases occurred during a subtechnique identification that included a transition and turn event. Identification accuracy can be improved by separately identifying transition and turn events. This system could be used to evaluate each skier’s subtechniques in course conditions.
Two types of proˆles of ski reaction forces during V2 skating have been reported by previous investigations. One of the diŠerences between these two proˆles is in the existence of the``‰ight phase,'' i.e., the phase in between gliding and kicking oŠ, in which the skis ‰oat above the snow while skiing. It has been suggested that the diŠerence is caused by the skating velocity. The purpose of this study is to clarify whether or not there is a relationship between the occurrence of the ‰ight phase and the increase in velocity during V2 skating. Seven elite male cross-country skiers performed two types of trials at diŠerent velocities (high and medium speeds). The high and medium speeds correspond to the competitive pace for a sprint race and a 10-km race, respectively. The kinematics was measured for each trial using two video cameras and a panning direct linear transformation technique. The ‰ight phase was determined by the ski load data obtained from a sensor attached to the ski. No ‰ight phase was conˆrmed during medium-speed skating, but a ‰ight phase was conˆrmed during high-speed skating, indicating the existence of the ‰ight phase is related to an increase in skating velocity. However, the hip-and knee-joint angles and the vertical displacement of the center of mass were not changed by an increase in skating velocity. These results suggested that the ‰ight phase was a small change from the standpoint of kinematics, but it may cause changes in muscle activity since the leg muscle groups experience no ground reaction force.
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