A high-speed biplane radiography system for in-vivo assessment of joint function is provided. The system can acquire stereo-pair radiographic images at rates from 30-4000 frames per second of nearly any motion or joint. The radiographic equipment can be mounted in a gantry system that provides sufficient positioning flexibility for imaging different joints of a subject's body, along with an imaging area large enough for a variety of dynamic activities (e.g., walking, running, jumping, throwing, etc.). Three-dimensional (3D) bone positions can be determined using software for matching the bones in each X-ray image with 3D models developed from subject-specific CT (computed tomography) scans. This system can provide accurate (e.g., ±0.1 mm) assessment and direct 3D visualization of dynamic joint function, and can overcome limitations of conventional gate or motion analysis.