A system is provided for capturing motion of a moving object via a plurality of motion sensor modules placed on various body segments. The sensor modules capture both 3D position and 3D orientation data relating to their respective body segments, thereby gathering motion data having six degrees of freedom with respect to a coordinate system not fixed to the body. Each body sensor collects 3D inertial sensor data and, optionally, magnetic field data. In embodiments, either DSP circuitry within the sensor modules or an external computing device, processes the sensor data to arrive at orientation and position estimates by using an estimation algorithm, such as a Kalman filter or a particle filter. The processing includes biomechanical model constraints that allow flexibility in the joints and provide characteristics for various joint types. To improve estimation accuracy, the system may be integrated with various types of aiding sensors.