A person support apparatus supports a person in a laying-down or a seated position. One or more body-mounted sensors detect changes in the position of the person relative to a reference. A control system receives output from the body-mounted sensor or sensors through a remote coupling. The control system uses patient position information obtained from the body-mounted sensors to determine whether the person has experienced a change-in-position and/or a change-in-activity event. Alternatively or in addition, the position of the person situated on a person support apparatus is detected using one or more sensors that are not body-mounted, but rather are coupled to the person support apparatus or to another component of a patient support system. The patient position information is used to estimate the patient's torso angle. The estimated torso angle is used to assess the patient's condition.