The objective of this research is to develop a wearable power assist device which helps a person to lift up a heavy object. The device is designed to support him by holding his upper body weight and reducing his inner force. It turns to the reduction of a compression force of his lower back discs, which is a major factor of a lower back injury. In a lifting-up motion, he twists and bends his body, which means three-dimensional motion analysis is necessary for the compression force analysis. However, most of related works on the compression force analysis have been carried out in a vertical plane. For understanding the characteristics of a lifting-up motion better, this paper presents three-dimensional analysis of it. Using a motion capture system, we measured a sequence of body positions with and without the power assist device in different object location and mass, as well as surface EMG (electro-myogram) signals. The compression force of the lower back discs is estimated by a three-dimensional biomechanical human body model utilizing the position data. The results show that the range of the twist angle of his body is about 30 degree in maximum and the compression force increases about 10% due to the twist angle. With respect to the effect of the power assist device, the magnitude of the surface EMG signals of spine muscles is decreased when wearing the power assist device. It means the proposed power assist device gives a good support to him.

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