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Perturbations. Kinematic features of the perturbations: North and South for both right and left feet; North-East, East and South-East for right foot; North-West, West and South-West for left foot. A perturbation consisted in: accelerating the belt up to the maximum speed, keeping the speed constant for a certain period, and stopping the belt. The N-W, N-E, S-W and S-E perturbations were obtained as a combination of the pure N, S, E and W ones.
