Knee motion has been routinely analyzed using radiographic techniques in a static setting. Over the past decade, techniques of in vivo dynamic knee motion analysis have emerged, which have shed light on normal and pathologic knee motion. Most of these methods are either invasive or restricted to small indoor laboratories. This paper describes a new device that records in vivo dynamic knee motion without the restrictions of current techniques and shows results when this device is used with a patient with an anterior cruciate impairment. We believe that dynamic knee motion studies are critical to a full assessment of the effect of an injury and to subsequent rehabilitation and recovery and that this new device can be a useful diagnostic tool.