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Insects run with far greater speed and agility for their size than even the most advanced legged robots produced to date. The single-leg ground reaction forces of running insects such as the cockroach B. discoidalis provide valuable insight into the biomechanical basis for this rapid robust locomotion. To better study the running kinematics and biomechanics of these insects, a multiaxis silicon micromachined force sensor has been fabricated. The sensor consists of a 5.3-mm square plate that is supported at its corners by thin springlike beam elements. Each flexure beam is instrumented with two piezoresistive strain gauges, allowing the determination of both normal and in- plane bending force components. Typical unamplifled normal and in-plane flexure force sensitivities of 55 and 12 V/N, respectively, have been demonstrated for a sensor with 18-mum-thick flexures and a mechanical bandwidth of 1.3 kHz. Nominal normal force resolution is 2.2 nN/Hz1/2 at 1 kHz. This paper details the design, fabrication, calibration, performance, and analytical modeling of the first-generation micromachined ground reaction force sensor. Preliminary data obtained from running cockroaches show that this sensor represents a marked improvement in performance over the techniques previously available for studying small-animal biomechanics.