Rigid Body Modeling, a 6 degree of freedom (DOF) method, provides state of the art human movement analysis, but with one critical limitation; it assumes segment rigidity. A non-rigid 12 DOF method, Pliant Surface Modeling (PSM) was developed to model the simultaneous pliant characteristics (scaling and shearing) of the human body's soft tissues. For validation, bone pins were surgically inserted into the tibia and femur of three volunteers. Infrared markers (44) were placed upon the thigh, leg and bone pin surfaces. Two synchronized OPTOTRAKI3O2OTM cameras (Northern Digital Inc., Waterloo, ON) were used to record 120 seconds of treadmill gait per subject. In comparison to the "gold standard" bone pin rotational results, PSM located the tibia, femur and tibiofemoral joint with root mean square (RMS) errors of 2.4°, 4.0° and 4.6°, respectively. These performances met or exceeded (P<.0l) the current state of the art for surface data, Rigid Surface Modeling. The thigh's measured surface experienced uniform repeatable changes in scale: 40% mediolateral, 5% anterioposterior, 5% superioinferior, and planar shears of: 25° transverse, 1 5° sagittal, 5°f rontal. With the brief exception of push-off, the lower leg demonstrated much greater rigidity: <5% scaling and <5° shearing. Thus, PSM offers superior "rigid" estimates of knee motion with the ability to quantify "pliant" surface changes.