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Measurement of the ground reaction force vector is important in clinical gait analysis and biomechanics research, for example to enable inverse dynamic calculations. Instrumented insoles allow biomechanical data to be collected outside of the motion analysis laboratory in many environments. However, current insole-based approaches typically measure only the vertical component of the reaction force and the plantar center of pressure. This work describes the development and evaluation of a silicone insole capable of measuring the complete three dimensional reaction force vector. The insole is optically based and low-cost with no complex manufacturing requirements. Accuracy over five nominal gait trails is shown to be on the order of 10% of the force range, with mean errors of 10.7 N in the shear directions and 68.1 N in normal. The insole can provide a simple mobile platform that allows kinetic gait data to be collected in many environments while minimally affecting the wearer's gait.