The objective of this study is to present a new method of physical activity monitoring, able to detect body postures (sitting, standing and lying) from only one kinematic sensor attached on the chest. Wavelet analysis in conjunction with a simple kinematics model is used to recognize different posture transitions during daily physical activity. The postures observed with this technique matched those obtained with a reference optical motion system. Accurate results were also obtained during real-life testing outside of the laboratory. This new method shows promises for efficient body posture classification