X-ray motion analysis is a technique used to track the movement of objects using X-rays. This is done by placing the subject to be imaged in the center of the X-ray beam and recording the motion using an image intensifier and a high-speed camera, allowing for high quality videos sampled many times per second. Depending on the settings of the X-rays, this technique can visualize specific structures in an object, such as bones or cartilage. X-ray motion analysis can be used to perform gait analysis, analyze joint movement, or record the motion of bones obscured by soft tissue. The ability to measure skeletal motions is a key aspect to one's understanding of vertebrate biomechanics, energetics, and motor control. Imaging Methods Planar Many X-ray studies are performed with a single X-ray emitter and camera. This type of imaging allows for tracking movements in the two-dimensional plane of the X-ray. Movements are performed parallel to the camera's imaging plane in order for the motion to be accurately tracked. In gait analysis, planar X-ray studies are done in the sagittal plane to allow for highly accurate tracking of large movements. Methods have been developed to allow for estimating all six degrees of freedom of movement from a planar X-ray and a model of the tracked object. Biplanar Few movements are truly planar; planar X-ray imaging can capture the majority of movement, but not all of it. Accurately capturing and quantifying all three dimensions of movement requires a biplanar imaging system. Biplanar imaging is difficult to perform because many facilities have access to only one X-ray emitter. With the addition of a second X-ray and camera system, the 2-D plane of imaging expands to a 3-D volume of imaging at the intersection of the X-ray beams. Because the volume of imaging is at the intersection of two X-ray beams, the overall size of it is limited by the area of the X-ray emitters. Tracking Techniques Markered Motion capture techniques often use reflective markers for the image capturing. In X-ray imaging, markers that appear opaque in the X-ray images are utilized. This frequently involves using radio-opaque spheres attached to the subject. Markers can be implanted in the subject's bones, which would then appear visible in the X-ray images. This method requires surgical procedures for implanting and a healing period before the subject can undergo a motion analysis. For accurate 3-D tracking, at least three markers