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

Validating a low-cost, consumer force-measuring platform as an accessible alternative for measuring postural sway

The Zibrio SmartScale is a low-cost, portable force platform designed to perform an objective assessment of postural stability. The purpose of the present study was to validate the center of pressure (COP) measurements in the Zibrio SmartScale. Simultaneous COP data was collected by a Zibrio SmartScale and a laboratory-grade force platform (LFP) under the dynamic motion of an inverted pendulum device intended to mimic the sway of a standing human. The inverted pendulum was placed on the Zibrio SmartScale which was placed on the LFP. The pendulum was then displaced to angles of 3° and 5° in both the anterior-posterior (AP) and medial-lateral (ML) directions. The findings of this study show low mean average error (MAE) among the measures taken simultaneously upon the LFP and Zibrio SmartScale with no appreciable difference in error in either AP or ML COP directions. Averaged over repeated trials, the MAE did not surpass 0.5 mm. This represented 0.4% of the total range (±50 to 60 mm in 5° displacement trials) of simulated COP. The results of this study strongly indicate that the Zibrio SmartScale can perform adequately as a light-weight and low-cost alternative method of COP measurement in comparison to a traditional LFP.

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.

Anatomical calibration for wearable motion capture systems: Video calibrated anatomical system technique

Inertial sensors are becoming widely used for the assessment of human movement in both clinical and research applications, thanks to their usability out of the laboratory. This work aims to propose a method for calibrating anatomical landmark position in the wearable sensor reference frame with an ease to use, portable and low cost device. An off-the-shelf camera, a stick and a pattern, attached to the inertial sensor, compose the device. The proposed technique is referred to as video Calibrated Anatomical System Technique (vCAST). The absolute orientation of a synthetic femur was tracked both using the vCAST together with an inertial sensor and using stereo-photogrammetry as reference. Anatomical landmark calibration showed mean absolute error of 0.6±0.5 mm: these errors are smaller than those affecting the in-vivo identification of anatomical landmarks. The roll, pitch and yaw anatomical frame orientations showed root mean square errors close to the accuracy limit of the wearable sensor used (1°), highlighting the reliability of the proposed technique. In conclusion, the present paper proposes and preliminarily verifies the performance of a method (vCAST) for calibrating anatomical landmark position in the wearable sensor reference frame: the technique is low time consuming, highly portable, easy to implement and usable outside laboratory.

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.

Test system for clinical force platforms

Our team has built a system for testing and calibration of clinical Force Platforms, able to generate a circular path Center of Pressure (CoP) that can be accurately applied in the horizontal plane at a given travel speed, selectable radius and vertical force. The CoP generated, which is applied directly onto the platform, is used as a precise reference for both static and dynamic measurements, to check the operation of unknown Force Platforms performance characteristics or to verify specifications indicated by the Force Platform's manufacturer. To confirm the feasibility and effectiveness of the proposed test apparatus, it was applied to a commercial clinical usage Force Platform manufactured by artOficio LTDA. The main advantages of the proposed Test System are the accuracy in fixing or setting a direct reference and the ability to maintain continuous monitoring through the generation of flexible and stable circular patterns during the tests.

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.