Oqus 400 cameras

Nine infrared Oqus 400 cameras (Qualisys AB, Gothenburg, Sweden) captured threedimensional position characteristics with a sampling frequency of 250 Hz. In total eight reflective markers (spherical, ⌀ 16 mm) were placed on the equipment: two markers on each ski (one 1 cm behind the front wheel and one 1 cm in front of the back wheel) and two markers on each pole (one ~5 cm below the bottom of the grip handle and one on the lateral side of the carbide tip). Before test start of each participant, the camera system was calibrated according to the manufacturer's specifications.

The volunteers were equipped with 27 retro-reflective markers placed on the participant's head (3 markers on a custom-build adjustable helmet), C7, manubrium, and placed bilaterally on the acromion, lateral epicondyle, dorsal side of the wrist joint, crista iliaca, trochanter major, thigh, lateral femoral condyles, tibial shaft, lateral malleoli, posterior on the calcaneum, and on the 1 st metatarsophalangeal joint. The positions of these markers were sampled at 300 Hz using a motion tracing system consisting of 10 Oqus 400 cameras (Qualisys, Gothenburg, Sweden).
The ground reaction forces were recorded at 1500 Hz using an AMTI Optima force plate (AMTI, Watertown, MA, USA). The cameras and the force plate were controlled by a computer running the software Qualisys Track Manager (Qualisys, Gothenburg, Sweden), which synchronized the data acquisition devices and calculated the 3D positions of the markers and the COP position.
All further data processing and analyses were conducted in Matlab (The MathWorks Inc., Natick, MA, USA). The data from one minute standing on the force plate, from second 20 to second 80, was selected and the COP data was down-sampled to 300 Hz.