Abstract: An alignment guide for determining a human or animal body part range of motion around a joint is disclosed. The alignment guide is adapted to be placed against the body part and to receive a sensor unit. By using the sensor unit, the range of motion of a human or an animal joint in a main plane of rotation can be estimated based on a relative angle between an orientation of the sensor unit in a first measurement position and an orientation of the sensor unit in a second measurement position. A system, as well as a method using the alignment guide and sensor unit is also disclosed.