The 3-dimensional (3-D) movements of both horses and riders during walking and trotting on the ground were recorded by the accelerometers. Three riding-horses and 11 persons were examined. The accelerometers were fixed over the sternum of the horse and at the waist of the rider. The accelerations of horses and riders, that moved along the straight course at walk, sitting trot, and rising trot, were simultaneously recorded. Those of the person at walk (walker) and at run (runner) on the ground were also measured. The spatial positions of the movements were estimated by a double integration of the acceleration. To investigate the rhythm of the movements, the frequency and the amplitudes were estimated by Fast Fourier transform. The horse carrying the rider walked and trotted with higher frequency and smaller amplitude especially in vertical axis than that without rider. The frequency in the movement of the rider at walk was 1.84Hz, which was similar to that of the walker (1.94Hz), and was higher than at rising trot (1.50Hz, P<0.05) but lower than both the runner (2.81Hz, P<0.05) and at sitting trot (2.93Hz, P<0.05). The vertical amplitudes in the movement of the rider at rising trot and the runner were between 40 and 50mm, at sitting trot and the walker were between 30 and 40mm, and at walk was less than 10mm. The range of lateral amplitudes in every movements of the person was between 10 and 18mm, and the lateral amplitude of the rider at sitting trot was significantly smaller than that of the walker (P<0.05). The longitudinal amplitude in the movement of the person was the largest at rising trot (25.2mm) and the smallest at the runner (8.7mm). The lateral amplitude of the rider was significantly smaller than that of the horse at walk (P<0.01), while at the sitting trot and the rising trot, all three amplitudes of the rider were larger than those of the horse (P<0.01). The relationship between 3-D amplitudes in the movement of the waist at the walker was similar to that of the rider at sitting trot.

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