Modification of the Grood and Suntay Joint Coordinate System equations for knee joint flexion

•Joint Coordinate System equations require corrections to ensure correct angles are calculated.•Flexion angle equations can be corrected by using a new tan function.•This will ensure the entire clinical range of motion has been considered.

Since its introduction, the Grood and Suntay Joint Coordinate System (JCS) has been embraced by the International Society of Biomechanics (ISB) and been widely used for biomechanical reporting. There is, however, a limitation in its ability to provide correct flexion values over a wide range of clinically relevant flexion angles. This technical note addresses the limitation of the JCS equations and introduces a new set of equations to overcome this problem.