Design of a modular small dimension force platform for gait analysis of children and neurologic patients

Main topics: Analysis of gait and motor disorders; Technical developments in movement science Introduction and aim: Gait analysis of subjects with short and non-uniform gait, typical children, children with cerebral palsy, elderly and stroke patients, can result critical in terms of ground reaction force (GRF) quantification with traditional force platforms. The standard de facto of commercial force platforms dimension (0.60 m × 0.40 m) can determine problems resulting from the double contact of the foot on the same platform. The aim of this work is the design of a small modular, six components, force platform (0.40 m × 0.40 m) to solve the problem of double contact, maintaining the performance of commercial larger ones. The simplicity of the design and materials used allows the use of multiple force platforms at low cost. Electronics dedicated to the acquisition of the output signal is not required because a commercial evaluation board can be exploited, that is able to independently manage the output of the strain gauges. This solution simplifies the realization of instrumented corridors. Patients/materials and methods: The design of the proposed small modular force platform [1] was performed to obtain a performance comparable to that of commercial force platforms. The total dimension of the force platform is: 0.40 m × 0.40 m × 0.105 m. The minimum number of mono-axial strain gauges are used for deformation sensing: six for each post, longitudinally arranged. The output of each strain gauge is directly provided as input to a commercial evaluation board. This removes the need for ad-hoc electronics for signal acquisition. Static analysis was performed to estimate the platform calibration matrix. 576 simulations were implemented considering applied forces varying between −200 N and 200 N for Fx and Fy and between −2000 N and -200 N for Fz, in 9 different points of application. Which were used in order to cover most of the surface of the platform. Eighty-four additional simulations were performed to test the calibration matrix and quantify the error that the system commits in the estimation of applied loads and moments, which were not exploited in the estimation of the calibration matrix. Results: The median error made in the determination of the forces is between −0.0378% and 0.0041%. The maximum error made in the determination of the forces ranges from a minimum of 0.0325% to a maximum of 0.8075%. The median error made in the determination of the moments is between 0.0028% and 0.0372%. The maximum error made in the determination of the moments is in Mx 0.1112% at −45 Nm, in My 0.2401% at 15 Nm and in Mz 3.435% at 10 Nm. The median and the maximum error in the determination of the forces and moments, based on the calculated calibration matrix, resulting from the 84 additional simulations are illustrated in Tables 1 and 2.