In this paper, the development of a compact and lightweight power assist device for lower back flexion and extension, when carrying a heavy load, is presented. For investigating the effect of the device, we model a human body as a planar seven-link system, and we analyze a compression force in the lower back, particularly at the disc between the fifth lumbar vertebra and the first sacrum. We built two prototypes of the device, and we apply the model to real data acquired by the prototype. The analysis results show that the proposed device can reduce the compression force in the disc. A controller for the device is developed, which can follow a voluntary human motion using surface myosignals.