Development of a Device to Measure Human Response to Dorsiflexural Perturbation

 

 

J. Botha^I; R.T. Dobson^I; J.P. Driver-Jowitt^II

^IMSAIMechE, Department of Mechanical and Mechatronic Engineering, University of Stellenbosch, South Africa. E-mail: rtd@sun.ac.za
^IIOrthopaedic surgeon. E-mail: driver-jowitt@kingsley.co.za

 

 

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ABSTRACT

The ability to balance is of fundamental importance to the safety of human beings. Damage to the brain and spinal cord through illness, injury or old age can result in balance problems, leading to the tendency to fall A device named the dorsiflexometer has been designed for the purpose of recording and analysing a patient's response to a perturbation leading to dorsiflexion, in order to aid in the diagnosis of balance impairment, particularly impairment caused by the conditions already described. The device consists of two tilting force platforms which measure the position and magnitude of the component of the resultant force normal to the platforms before, during and after dorsiflexural perturbation. The dorsiflexometer was constructed, calibrated and a number of balance tests were conducted using young healthy adults between the ages of 20 and 25. Three parameters which may be of use in diagnosis of balance impairments were identified: the radius of a circle enclosing all the recorded positions of the normal component of the resultant force applied to each force platform, a sway index, defined in the article, and the Lyapunov exponent, which is an indicator of whether or not a process is chaotic in nature, as calculated for the movement of the point of application of the resultant force. A computer program was written to analyse the raw data and plot the change in position of the normal component of the resultant force applied to the force platform. The Lyapunov exponents indicated that a human's postural control processes are chaotic in nature. This implies that standard statistical methods may not be an appropriate tool to analyse such results.

Additional Keywords: Force platform, radius parameter, sway index, Lyapunov exponent, postural control, balancing ability

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Nomenclature

Roman

R Radius (mm)

SI Sway Index (m/s)

Greek

λLyapunov exponent

Subscripts

AD After Dorsiflexion

BD Before Dorsiflexion

DD During Dorsiflexion

TM Total Movement

Abbreviations

AD After dorsiflexion

BD Before dorsiflexion

COP Centre of pressure

DD During dorsiflexion

PC Personal computer

TM Total movement

LE Lyapunov exponent

l Length (mm)

∆t Time step (s)

 

1. Introduction

1.1 Background

The ability to evaluate a person's ability to balance is important in diagnosing and treating balance disorders. Balance disorders may be a result of certain diseases, such as cerebral palsy, multiple sclerosis and Parkinson's disease, or an aftermath of a stroke. It is also important to be able to measure the balancing ability of the elderly, to predict whether or not an individual has a high risk of falling. Falls create social problems for the elderly, and also cause large expenses for public health services throughout the world¹. Balance ability and methods to assess it are also of use in the field of sport and sports science.

A device has been constructed for the purpose of measuring an individual's ability to balance. This is done by recording the subject's response during dorsiflexural perturbation with the aim to detect balance impairment. The name Dorsiflexometer has been chosen for this device. The parameters used to interpret the results are defined such that an objective assessment of the patient's balance ability may be attained.

This article describes the design of the dorsiflexometer, the experimental set-up and test procedure and the balancing experiments that were conducted.

1.2 Existing technology

Various commercially available methods and procedures exist to assess a patient's ability to balance. Two companies, Neurocom and Micromedial Technologies, are involved in constructing and marketing devices for this purpose. Both companies appear to make two types of equipment using force-sensitive plates.

The first type of device consists of stationary force plates, on which the patient is required to stand while performing movements such as leaning in various directions so as to determine the limit of stability for the patient. The Balance Master device produced by Neurocom and the Balance Check device produced by Mircromedical Technologies are of this type. The most important difference between this type of device and the dorsiflexometer is that in the former, the subject voluntarily performs movements on a stationary platform, while with the latter the movements of the subject are involuntary, as a result of movement of the force-platforms.

The second type of device is more similar to the dorsiflexometer. The type consists of force platforms similar to those described above, but mounted on a moving base, with some sort of visual stimulus such as a moving screen in front of the patient. The Balance Quest device offered by Micromedical Technologies, and the EquiTest System offered by Neurocom are examples of this type of device. In the Balance Quest device, the platform is free to move in any direction, according to the movements of the patient, while the platform of Equitest can be caused to rotate about one axis, tilting the patient forwards or backwards and forcing the patient to compensate for the disturbance. This last device is the most similar in design to the dorsiflexometer of all those reviewed, although the dorsiflexometer is capable of tilting through á larger range of angles, as this is necessary in order to encompass the normal limits of dorsiflexion. The Equitest device also makes use of the movable screen to produce various types of visual stimulus, a capability which the dorsiflexometer does not possess or require at present. The Balance Quest device makes use of Virtual Reality glasses for the same purpose.

Several patents on such devices have also been filed, of which one by Mechling² resembles the device discussed in this article. This patent describes a device consisting of a force-sensitive platform which pivots freely in the dorsiflex-plantarflex direction.

1.3 The dorsiflexometer

The dorsiflexometer consists of two force-sensitive platforms which can be caused to tilt in a controlled manner to induce dorsiflexion in a subject, as shown in figure 1. It is thus more similar to the devices with a moving force-platform described in the previous sub-section than those with a stationary one.

 

 

The principal difference between the dorsiflexometer and all the other devices discussed lies in the type of balance impairment which the device is designed to detect. One of the possible effects of upper motor neuron deficiencies, or damage to the brain and spinal cord, is an increase in the tone, or tension at rest, of the extensor muscles controlling dorsiflection and plantarflexion of the foot³. This in turn limits the range of dorsiflexion achievable. This can result in a tendency for the sufferer to fall over backwards. When a disturbance in the sufferer's balance causes the dorsiflexion limit to be reached, various other strategies may be used in order to attempt to maintain balance, for instance bending forwards at the hips, if the problem is bilateral. The dorsiflexometer was designed specifically for the analysis of the dorsiflexion response and determination the limits of dorsiflexion in a patient. It could, however, also potentially be used for more general balance-disorder investigations. All the other systems discussed previously are designed for more general balance-disorder investigations, and do not lend themselves to the determination of the dorsiflexion limit.

 

2. Design of the Dorsiflexometer

The dorsiflexometer consists of two tilting platforms equipped with a force platform for each foot. The platforms tilt the test subject in the sagittal plane inducing dorsiflexion or plantar flexion of the ankle. A photograph of the Dorsiflexometer prototype⁴ and a simplified side-view illustration showing the movement of the dorsiflexometer are shown in figure 1. Tilting of each platform is achieved by means of a stepping motor connected to a worm gear. The platforms can tilt to a maximum of 11° in the dorsiflex direction and 20° in the plantar flex direction. A schematic diagram of this tilting mechanism is shown in figure 2. Each force platform consists of three footplates mounted on a load cell or force sensor, as shown in figure 3. From these force sensors, the position of the resultant force applied to the force platform in the plane of the platform, can be determined and expressed as a two-dimensional Cartesian point, with X- and y-coordi-nates relative to axes shown in figure 3. A railing was added to ensure the subject's safety and to provide reassurance during testing.

 

 

 

 

Other equipment with the constructed device include a personal computer (PC), bridge amplifiers with associated software and a National Instruments data acquisition and control card and software. The PC is used for controlling the stepping motors, using the Lab View card, and for storing the raw data acquired by the bridge amplifier during testing. The PC is also used for analysing the acquired data. A program was written for analysing the raw data and presenting the resulting parameters obtained in a useable form. A block-diagram of the experimental set-up is shown in figure 4.

 

 

3. Definition of Slip Factor

Three parameters were used for comparing different test subjects and quantifying their balance abilities. These parameters were calculated for data from three different time periods during the test - before dorsiflexion has started (BD), during dorsiflexion (DD), after dorsiflexion stopped (AD), as well as for the total movement (TM). All three parameters are based on the two-dimensional points of application of the resultant force component applied normal to each of the force-plates, on the system of axes illustrated in figure 3.

3.1 The Radius parameter (R)

The physical significance of this parameter is illustrated in figure 5. It may be thought of as the radius of the smallest circle that encloses all the recorded positions for the resultant force for each foot respectively, with its centre at the average x- and y-coordinates and for the n recorded positions for the resultant force for each platform. Naming the point most distant from the centre, (x_d, y_d), the radius is calculated as shown in equation 1.

 

 

3.2 The Sway index (SI)

The SI serves as an indication of the sway movement during the balance-ability test. It is the average rate of change of position of the component of the resultant force normal to the pressure plates, as shown in equation 2. This is calculated by summing the distances between consecutive data points ∆l_i divided by the time interval between the recording of these points, ∆t, and dividing by the number of such distances, or the number of intervals between the recorded points.

3.3 The Lyapunov exponent (LE, symbol λ)

The LE is calculated to establish whether the data behaviour is chaotic or not. A negative LE indicates that a process is chaotic in nature, and the more negative the number, the more chaotic the process is⁵. The LE is calculated by means of equation 3.

For this case, the function f'(x_i) was the derivative or gradient of the y-coordinate of the path along which the normal component of the resultant force moved in the two-dimensional plane of the force-plates, with respect to the x-coordinate; or mathematically This was evaluated for each time step using a finite difference approximation

 

4. Experimental Testing

4.1 Calibration

The force-plates were calibrated by recording the average readings from each of the six force-plates when not loaded, and when five known masses were placed on the force-plates. A linear curve-fit was then applied to the data obtained from each force-plate. The lowest correlation coefficient (R²) value for any of the six curve-fits was 0.9957.

4.2 Dynamic response testing

The dynamic response time of each force-plate was also determined by dropping a 0.125 kg mass on each platform from a height of 50 mm and recording the response. The response time for all force-plates was determined as being of the order of 10ms.

4.3 Test procedure

The test subject stands with each foot on the appropriate platform, barefoot, and arms relaxed and hanging from the shoulders. Data are captured for a total period of six seconds at a rate of 1200 samples/second. For the first two seconds, the platform is stationary and horizontal. For the following two seconds both platforms simultaneously tilt in the dorsiflex direction, until an angle of 9° is reached. This corresponds to a constant rate of tilting or rotation of 4.57s, or 0.0785 rad/s. After the platforms have reached the maximum tilt, and stopped, a further two seconds' raw data is captured with the platform in this position. The test is then completed.

The raw data are then analysed using a computer program. The position of the component of the resultant force exerted normal to each of the platforms, in the two-dimensional plane of the platforms is calculated for each time-step. For the purposes of this article, the position of this resultant force is expressed as a point in the two dimensional Cartesian plane of the force plates, with the x-axis in the direction in which the subject faces when standing on the force plates. The orientation of these axes is shown graphically in figure 3.

4.4 Test subjects

Five subjects were tested, all with no known balance disorder and between the ages of 20 and 25. Subjects participated in two tests; one with open eyes and the other with closed eyes. The latter test served as a simulation of balance impairment, as it has been shown that for a given individual, sway is more elaborate when standing with closed eyes than with open eyes⁶.

 

5. Test Results and Discussion

Figure 5 displays the calculated point of application of the component of the resultant force acting normal to the plane of the platform for each time step. Points captured during the BD period are indicated in black, DD in magenta, and AD in red.

From this figure, it is evident that the subject's resultant force on the platforms' normal component was slightly off-centre (in the second quadrant of the plane) BD. During the dorsiflexural perturbation, it moved rapidly towards the fourth quadrant where it stayed after perturbation stopped. The ability to gather this type of information could provide insight into the movements and mechanisms associated with balancing, and may be of use in diagnosing balance disorders.

Figures 7, 8 and 9 show the data from each of the three stages of testing shown in figure 5. It is clear that of the three stages, sway is much more elaborate during dorsiflexion (figure 8). This holds true for all the tests performed to date, as can be seen in table 1 and figure 6. After dorsiflexion (figure 9), the subject finds a new equilibrium position.

 

 

 

 

 

 

 

 

 

 

Bilateral comparison is also possible using the device. An illustration of how this could be conducted follows, based on figure 10. In this figure the compensation with the right foot appears to be more elaborate than with the left. From this, it appears that the subject is right-side dominant, which is plausible, as it was noted that this particular subject is right-handed.

 

 

From table 1 and figure 6, it is evident that definite differences exist between the open and closed eye tests for all subjects. However, standard statistical methods may not be appropriate to analyse the results obtained, as the LE values presented in table 2 are negative and therefore indicate chaotic behaviour. Even so, figure 6 suggests that the Sway Index has potential for quantifying and comparing a subject's balance capabilities.

 

 

6. Recommendations

In order to validate the use of the dorsiflexometer in the diagnosis of balance disorders, further testing should be conducted, initially with a larger number of normal, healthy subjects and later with patients with known balance disorders. These tests would then either verify the SI-, LN- and Radius parameters as useful indicators of human balance capabilities, or show that they should be discarded. Other parameters may be solicited from the raw data with the help of medical practitioners with knowledge and experience in the field of posturography. The design of the dorsiflexometer should be improved in two respects. The first is that more appropriate data capture and control equipment should be used in conjunction with the dorsiflexometer. Such equipment might be designed specifically for this purpose, and form integral components of the dorsiflexometer. The second is that the maximum dorsiflexural perturbation angle currently attainable with the device may be insufficient to determine the dorsiflexion limit in some individuals, and the range of perturbation should consequentially be extended.

 

Acknowledgements

The authors wish to acknowledge the contribution of Acorn Technologies for partial funding, and A. Gill, who assisted in correcting and updating the manuscript.

 

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