Periodicalin-situ re-calibration of force platforms: A new method for the robust estimation of the calibration matrix

In-situ force plate calibration: 12 years' experience with an approach for correcting the point of force application

Force plates are common assessment tools used in biomechanics to measure ground reaction forces during motion capture or strength exercises. While the accuracy of the resulting kinetic data is critical for accurate gait analysis, factors such as plate mounting are thought to influence the measured point of force application (PFA) of the ground reaction forces. The aim of this study was therefore to present an accurate in-situ PFA calibration method. In addition, the approach was evaluated by examining the required PFA corrections over a period of ten years, which included plate remounting and a change of foundation. The in-situ PFA calibration was performed on six plates by assessing the locations of up to 98 points of force application per plate using an instrumented pole. Application of the in-situ PFA calibration approach reduced the root mean square errors by up to approximately 60% compared to the manufacturers calculation. Correction coefficients were strongly dependent on the individual platform as well as the location of the applied force on the plate. Remounting of the plates altered the plate coefficient corrections considerably, while changes over time were notable but not as extensive. We therefore recommend that plates should be recalibrated after remounting, but also at least every 5 years, in order to ensure the preservation of an accurate PFA.

Avaliação da simetria e descarga de peso entre os membros inferiores de atletas de futebol da categoria de base

INTRODUÇÃO: A avaliação instrumentada dos saltos verticais informa sobre as capacidades funcionais e variáveis neuromusculares, auxiliando no atendimento dos atletas. Essencial dar atenção maior para as categorias de base, pois contém fases importantes do desenvolvimento dos atletas que serão o futuro do esporte. Objetivo: Avaliar as características funcionais e neuromusculares de atletas de futebol da categoria de base, no desempenho em saltos verticais. METODOLOGIA: Foram avaliados atletas de futebol com idade entre 15 e 17 anos. Foram realizados em duas plataformas de força os seguintes saltos verticais: salto em agachamento, salto em contramovimento, salto pliométrico de 40 cm de altura. Para análise das variáveis das forças concêntrica, excêntrica, impacto e altura dos saltos utilizou-se a prova estatística de Shapiro-Wilk, descrição das variáveis em média e desvio padrão, considerando quando p < 0,05. RESULTADOS: Dos 30 atletas avaliados, obtivemos: três goleiros (10%), cinco zagueiros (16,7%), um lateral (3,3%), três volantes (10%), 12 meio-campistas (40%) e seis atacantes (20%). A categoria sub-17 apresentou maiores resultados comparada a categoria sub-15. Maior altura de salto do contramovimento em relação ao salto em agachamento (p < 0,05) e ao pliométrico (p > 0,05). Sem diferença na força concêntrica entre as categorias e os tipos de saltos. Maior força de impacto no contramovimento. Maior força em membro inferior direito em todos os saltos e variáveis de força. CONCLUSÕES: Encontramos assimetria entre os membros inferiores, podendo correlacionar com um déficit de potência muscular, má desaceleração do movimento e erros biomecânicos. Fatores predisponentes a lesões musculoesqueléticas, os quais podem ser prevenidos. A avaliação instrumentada da força através desses testes pode subsidiar os profissionais do clube como uma ferramenta de trabalho, capaz de aprimorar o treinamento, preparação e reabilitação. Entretanto, são necessárias mais pesquisas, de forma que momentos distintos da idade e da temporada do atleta sejam avaliados.

Design and evaluation of a new mechatronic platform for assessment and prevention of fall risks

Background

Studying the responses in human behaviour to external perturbations during daily motor tasks is of key importance for understanding mechanisms of balance control and for investigating the functional response of targeted subjects. Experimental platforms as far developed entail a low number of perturbations and, only in few cases, have been designed to measure variables used at run time to trigger events during a certain motor task.

Methods

This work introduces a new mechatronic device, named SENLY, that provides balance perturbations while subjects carry out daily motor tasks (e.g., walking, upright stance). SENLY mainly consists of two independently-controlled treadmills that destabilize balance by suddenly perturbing belts movements in the horizontal plane. It is also provided with force sensors, which can be used at run time to estimate the ground reaction forces and identify events along the gait cycle in order to trigger the platform perturbation. The paper also describes the customized procedures adopted to calibrate the platform and the first testing trials aimed at evaluating its performance.

Results

SENLY allows to measure both vertical ground reaction forces and their related location more precisely and more accurately than other platforms of the same size. Moreover, the platform kinematic and kinetic performance meets all required specifications, with a negligible influence of the instrumental noise.

Conclusion

A new perturbing platform able to reproduce different slipping paradigms while measuring GRFs at run time in order to enable the asynchronous triggering during the gait cycle was designed and developed. Calibration procedures and pilot tests show that SENLY allows to suitably estimate dynamical features of the load and to standardize experimental sessions, improving the efficacy of functional analysis.

Non-linear re-calibration of force platforms

Force platforms (FPs) are used in human movement analysis to measure the ground reaction force and the center of pressure (COP), and calculate derived kinetic and energetic quantities. We propose a re-calibration method that compensates for the FP non-linearity induced by top plate bending under loading. The method develops a previous solution that was proposed for a linear re-calibration and proved suitable for both local and global error compensation (Cedraro et al., 2008). The new method was experimentally tested on 4 commercial FPs by estimating the non-linear re-calibration matrix in a first training trial and by using it to assess the three force components and the COP in a validation trial, comparing the new method to the previously proposed solution for global, linear re-calibration. The average COP accuracy (mm) in the training trial was (mean±std): 2.3±1.4, 2.6±1.5, 11.8±4.3, 14.0±2.5 for the 4 FPs before re-calibration, and 0.7±0.4, 0.6±0.2, 0.5±0.2, 2.3±1.3 after non-linear re-calibration. In the validation trial, for one of the 4 tested FPs, mean errors for the three force components (N) and COP (mm) were: 3.6±2.3 (F(X)), 3.0±0.7 (F(Y)), 5.0±2.5 (F(Z)), 1.2±0.68 (COP) after linear re-calibration, and 2.5±0.7 (F(X)), 2.6±0.5 (F(Y)), 3.9±1.2 (F(Z)), 0.6±0.3 (COP) after non-linear re-calibration. The proposed global, non-linear method performed equally well as the local, linear re-calibration method, proving well-suited to compensate for the mild non-linear behavior of FP with the advantage of estimating a single re-calibration matrix.

A portable system for in-situ re-calibration of force platforms: experimental validation

A system for the in-situ re-calibration of six-component force platforms is presented. The system consists of a device, a data-acquisition procedure and an algorithm. The device, simple and lightweight, is composed of a high-precision, 3-D load cell, loaded through a triangular stage, and precisely positioned on the force platform under re-calibration by means of a template. The data-acquisition procedure lasts about 1h and requires up to 13 measurements consisting of manual positioning the load cell on the force platform, and then having the operator exerting loads on both load cell and force platform by his/her body movement. As a result, the procedure makes use of loads in the same range of posture and gait tests. The algorithm estimates the local or global six-by-six re-calibration matrix of the force platform through a least-squares optimization, and is presented in detail in a separate paper [Cedraro A, Cappello A, Chiari L. A portable system for in-situ re-calibration of force platforms: Theoretical validation. Gait Posture 2008;28:488-94]. The system was validated on four commercial force platforms (Amti OR6, Bertec 4060-08, Bertec 4080-10, and Kistler 9286A). The average accuracy in the measurement of the center of pressure were 2.3+/-1.4mm, 2.6+/-1.5mm, 11.8+/-4.3mm, 14.0+/-2.5mm before re-calibration, 1.1+/-0.6mm, 1.8+/-1.1mm, 1.0+/-0.6mm, 3.2+/-1.1mm after global re-calibration, and 0.7+/-0.4mm, 0.8+/-0.5mm, 0.5+/-0.3mm, 2.0+/-1.2mm after local re-calibration (results presented in random order). The force platform re-calibration influenced the value, sign, and timing of net joint moments, estimated during a gait task through an inverse dynamics approach.

A portable system for in-situ re-calibration of force platforms: theoretical validation

The periodic calibration of measurement devices, such as force platforms (FPs), guarantees and optimizes the quality of the acquired data. In this study, the theoretical validation of a portable system for the in-situ calibration of six-component FPs is presented. A least-squares algorithm, that estimates the FP re-calibration matrix, was designed and tested using a simulation approach. The algorithm is the core of a portable system, described in a separate paper, and represents a refinement of an algorithm previously presented by Cappello et al. [Cappello A, Lenzi D, Chiari L. Periodical in-situ re-calibration of force platforms: a new method for the robust estimation of the calibration matrix. Med Biol Eng Comput 2004;42:350-5]. The new algorithm assumes that the calibration inputs are known, 3-D, time-varying loads, applied to the FP at known coordinates, and not constrained in their direction of application. Simulation results confirmed the a priori identifiability of the re-calibration matrix and some of the algorithm features were optimized in the perspective of an actual building of the system. With the aid of simple sinusoidal loads, applied in at least five different points, we proved that the algorithm can ensure errors less than 0.2N and 0.4 Nm when calculating force and moment components of an applied load.