| 1. |
- Jensby Nedergaard, Niels, et al.
(författare)
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The effect of light reflections from the snow on kinematic data collected using stereo-photogrammetry with passive markers
- 2014
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Ingår i: Sports Engineering. - : Springer Science and Business Media LLC. - 1369-7072 .- 1460-2687. ; 17:2, s. 97-102
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Tidskriftsartikel (refereegranskat)abstract
- This study aimed to compare kinematic data collected during ski-cross starts outdoors on snow in daylight (high albedo) to similar data collected indoors with infiltrating sunlight but without light reflections from the snow (low albedo) using a video-based motion capture system with the active filtering function enabled. A 12-camera 3D motion capture system (Qualisys AB, Sweden) was used to measure test objects and eight skiers performing a ski-cross start on a slope outdoors and on a wooden start ramp indoors. The average residuals and standard deviations of the length of the calibration wand calculated indoors and outdoors by the calibration software were compared using descriptive statistics. Static and moving fixed length measures and thigh length measures were compared using Bland-Altman plots. Calibration residuals were slightly increased outdoors (1.77 mm) compared to indoors (1.54 mm), while wand length varied by 3.63 and 1.51 mm, respectively. Fixed static lengths differed by -8.65 ± 4.94 mm (shorter indoors), whereas fixed moving lengths differed by 0.85 ± 1.05 mm (longer indoors). A randomly chosen marker pair on one segment (Thigh) showed a mean difference of 1.19 ± 22.05 mm (longer indoors). It is concluded that 3D motion capture outdoors on snow in daylight is feasible, provides kinematic data comparable to indoors, and could be used to research biomechanics in snow sports.
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| 2. |
- Nedergaard, Niels Jensby, et al.
(författare)
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Biomechanics of the ski cross start indoors on a customised training ramp and outdoors on snow
- 2015
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Ingår i: Sports Biomechanics. - : Informa UK Limited. - 1476-3141 .- 1752-6116. ; 14:3, s. 273-286
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Tidskriftsartikel (refereegranskat)abstract
- An effective start enhances an athlete's chances of success in ski cross competitions. Accordingly, this study was designed to investigate the biomechanics of start techniques used by elite athletes and assess the influence of different start environments. Seven elite ski cross athletes performed starts indoors on a custom-built ramp; six of these also performed starts on an outdoor slope. Horizontal and vertical forces were measured by force transducers located in the handles of the start gate and a 12-camera motion capture system allowed monitoring of the sagittal knee, hip, shoulder, and elbow kinematics. The starting movement involved Pre, Pull, and Push phases. Significant differences between body sides were observed for peak vertical and resultant forces, resultant impulse, and peak angular velocity of the shoulder joint. Significantly lower peak vertical forces (44N), higher resultant impulse (0.114Ns/kg), and knee joint range of motion (12 degrees) were observed indoors. Although movement in the ski cross start is generally symmetrical, asymmetric patterns of force were observed among the athletes. Two different movement strategies, i.e. pronounced hip extension or more accentuated elbow flexion, were utilised in the Pull phase. The patterns of force and movement during the indoor and outdoor starts were similar.
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