A therapeutic device may be used in providing physical therapy for a patient's knee by moving the patient's leg through a plurality of cycles of motion in each of a number of treatment sessions. The device includes a “Comfort Zone” range of motion feature which allows an operator to temporarily increase the flexion angle (or decrease the extension angle) to alleviate discomfort the patient is experiencing as the upper leg support and lower leg support pivot or attempt to pivot through a portion of the operational range of motion. The preferred embodiment of the device will automatically decrease the flexion angle (or automatically increase the extension angle) at a predetermined rate over a period of treatment time, so that the device may return to operation between the preset operational limits of the range of motion with a lower chance that the patient will experience the same pain or discomfort which necessitated the establishment of a Comfort Zone limit. In a preferred embodiment of the invention, the carriage holding the patient's leg is decelerated, at a controlled rate over a controlled distance, from the operational speed to zero, as the carriage approaches the extension or flexion limit, and the carriage is accelerated in the same fashion as the carriage moves away from the extension or flexion limit.