Recent progress in 3-D capture technology has made it possible to obtain much of realistic motion data of human subjects. Being captured in high frame rates, compression or extraction of key postures out of the motion data is useful for storage, transfer and browsing among them: this can serve as an important pre-processing for applications such as rehabilitation, ergonomics and sports physiology. This paper addresses these problems by treating the motion data as trajectory curves in a high-dimensional space and doing a novel application of a curve simplification algorithm, typically used for planar curves, to human motion data.