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Accelerometer and rate gyroscope measurement of kinematics: an inexpensive alternative to optical motion analysis systems

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Abstract

A general-purpose system to obtain the kinematics of gait in the sagittal plane based on body-mounted sensors was developed. It consisted of four uniaxial seismic accelerometers and one rate gyroscope per body segment. Tests were done with 10 young healthy volunteers, walking at five different speeds on a treadmill. In order to study the system's accuracy, measurements were made with an optic, passive-marker system and the body-mounted system, simultaneously. In all the comparison cases, the curves obtained from the two systems were very close, showing root mean square errors representing <7% full range in 75% of the cases (overall mean 6.64%, standard deviation 4.13%) and high coefficients of multiple correlation in 100% of cases (overall mean 0.9812, standard deviation 0.02). Calibration of the body-mounted system is done against gravity. The body-mounted sensors do not hinder natural movement. The calculation algorithms are computationally demanding and only are applicable off-line. The body-mounted sensors are accurate, inexpensive and portable and allow long-term recordings in clinical, sport and ergonomics settings.

Introduction

Optical motion analysis systems are often used in the study of human movement. However, these systems are expensive, only allow measurements in a restricted volume, and the markers are easily obscured from vision resulting in incomplete data. More recently, body-mounted sensors have also been used to obtain kinematic values (Bussmann et al., 1995; Bogert van den et al., 1996; Dai et al., 1996; Luinge et al., 1999; Tong and Granat, 1999; Veltink et al., 1996; Veltink, 1999). A fully portable system can be obtained if a portable data logger is used to gather the data from body-mounted sensors.

The system presented in this paper is general-purpose, combining accelerometers and rate gyroscopes, able to deliver the following kinematic parameters in the sagittal plane: shank angle, thigh angle, knee angle, shank angular velocity, thigh angular velocity, knee linear acceleration, shank angular acceleration and thigh angular acceleration. To verify the accuracy of the body-mounted sensor system, optical data were gathered simultaneously using a Vicon® System. Only the parameters of shank angle, shank angular velocity, knee linear acceleration and shank angular acceleration are presented in this paper as representative examples.

Section snippets

Model description

In the two-dimensional (sagittal plane) model, the shank and foot were considered as a single segment referred to as shank. Shank and thigh were represented by two rigid segments k and k+1 connected by a simple hinge knee joint (Fig. 1).

Calculation of linear acceleration

The accelerometer signal sk,i at any point i of a rigid body k can be expressed as a vector x=(x1,x2,x3)T on the basis of three independent signal vectors sk1,sk2,sk3 at positions rk1,rk2,rk3 (Veltink and Boom, 1996)ski=j=13xjskj.The coordinates

Measurement system verification

The number of cases (out of 20) in each comparison is indicated in Table 2, together with the RMS and CMC results. Missing cases are due to data lost to file archiving problems or to one or more of the markers being lost from vision. Fig. 3 shows examples of graphs of the four comparisons.

In three of the four comparisons, the RMS errors are <7% of the full range of the measurements and the found CMC are above 0.98. However, for the linear acceleration of the knee (comparison number 3), the

Accuracy of kinematic measurements

The body-mounted sensors give results that are very close to those of Vicon® presenting small RMS and large CMC values. Errors do increase at the highest speed for the accelerometer data, which for several subjects suffered from deformed peaks, probably due to the sensors being hit or vibrated during heel strike. This should be taken into account when designing future applications involving accelerometers. The rate gyroscopes were not affected in this way.

Contributions of this study and future work

In this study, a treadmill was used

Acknowledgements

We are grateful to the STW, Dutch Technology Foundation, for the funding for this project; to Andreas Heyn and Eric Camiro for their valuable assistance.

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