We present the design of sensor nodes for human posture tracking purposes. Each sensor node is equipped with a microcontroller, an RF transceiver chip and two sensors: a 3D accelerometer and a 3D magnetometer. Based on the signals of these sensors, the orientation of a sensor node can be estimated. Combining several of these nodes into a sensor network, allows for complete human posture tracking, as the human body can be approximated as a rigid body consisting of 15 links. The sensor nodes are designed on a single conductive layer and are assembled on a thin, flexible poly-imide substrate. The microcontroller code has been optimized for low power consumption, while keeping in mind the desired performance. Data is read from the sensors and transmitted wirelessly to a receiver at a rate of 100 samples per second, while only consuming an average current of less than 3 mA. The wireless communication protocol is based upon TDMA and allows for simultaneous use of 10 nodes per receiver.