The present invention relates to apparatus and methods for warning a user about undesirable motion of a first portion of the users body relative to a second portion of the users body. In one representative embodiment, an apparatus for warning a user about motion of a first portion of the users body relative to a second portion of the users body comprises at least one sensor configured to measure the angle of the first portion of the users body relative to a reference. The sensor is configured to be worn on the users body, and is in communication with a controller. A warning mechanism is also in communication with the controller, and the controller is operable to activate the warning mechanism upon movement of the first portion of the users body relative to the second portion of the users body beyond a predetermined angle relative to the reference.
