Analysis of the sit-stand-sit movement cycle: development of a measurement system

Analysis of the sit-stand-sit movement cycle: development of a measurement system K M Kerr BA MCSP’, J A White R A B Mollan MD FRCS~

PhD2f3,

D A Barr Ptm4, and

‘School of Physiotherapy, 3chool of Public Health Medicine, University of Nottingham; 3Nottingham Health Authority; 4Department of Orthopaedic Surgery, The Queen’s University of Belfast

Summary A novel approach to three-dimensional analysis of the sit-stand-sit movement cycle was developed using a measurement system, comprising vector stereography, electrogoniometry, and triaxial accelerometry, which could provide temporal and spatial information on the activity. Linearity, repeatability, and validity of individual elements of the measurement system were established and it was demonstrated that the attachment of the measurement system did not exert a significant influence on the sit-stand-sit activity. The application of the measurement system in repeated trials within a single subject showed coefficients of variation of &% on a number of spatial and temporal parameters extracted from the data. Output from the measurement system was recorded in analogue and digital form, from which data can be extracted on displacement and acceleration of the trunk in three dimensions, and angular displacement of the knee at any point within the movement cycle. It was concluded that the measurement system can provide accurate, consistent, and reliable information on patterns of acceleration and displacement, and definitive data on temporal and spatial parameters and the relationship between them. Key words: Movement accelerometry Gait & Posture

analysis,

sit-stand-sit,

1994: Vol. 2: 173-181,

vector

stereography,

September

Introduction Single measurements of movement are made regularly by clinicians to determine if it falls within normal ranges and to provide comparative information by which change can be monitored. However, total body and body segmental movement involves many joints, and complete definition of motion requires three-dimensional measurement *, The activity of rising from and descending to the Received: 19 January 1994 Accepted: 13 May 1994

Correspondence and reprint requests to: Kathleen M Kerr, The University of Nottingham School of Physiotherapy, Hucknall Road. Nottingham NGS 1PG. UK 0 1994 Butterworth-Heinnemann 09666362/94/030 173109

Ltd

electrogoniometry,

seated position is a fundamental characteristic of everyday living in normal individuals. Furthermore, Kralj et a1.2noted the functions of standing up and sitting down are physiological functions in man, and prerequisites for gait. Riley et a1.3 suggested that in some respects the activity of rising from a chair is ‘the most mechanically demanding functional task undertaken during daily activities’, citing Hodge et a1.4 and Berger et al.s who demonstrated higher forces in the hip and knee when subjects rose from the seated position than in other activities including gait, stair ascent, and other exercises. The fundamental aim of this study was to develop a measurement system that could provide accurate and reliable measures of the sit-stand-sit movement cycle, and to enable definition of the characteristics of the movement cycle.

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Review of the literature A review of research literature has revealed an increasing interest during the last 15-20 years into the activity of rising from a chair. Methods employed to obtain data have been varied, and have included electrogoniometry, electromyography, optoelectric devices, force plates, questionnaires, and cinematography, observation of functional parameters. Early studies made use of data obtained from force electrogoniometrylkL2, and optoelectric plates2+9, systems4J3J4 to calculate angular displacement and moments and forces at joints and muscles of the lower limbs during the activity of rising from a chair. These studies provided objective data which have been specific to certain aspects of the sit-stand-sit activity (for example, forces at the hip and knee), but apart from distinguishing between the forward lean and upward thrust elements of the activity of rising, they have not been concerned with defining the basic characteristics of the movement cycle, or with identifying the relationships between them. More recent evidence has suggested a slightly different approach to the analysis of the activity, with studies investigating the influence of momentum*5, the identification of time phases within the activity of standing up and sitting down on a chair5, and the potential importance of the relationship between forward displacement of the trunk and the initiation of lower extremity extension16. These studies have placed greater emphasis on the nature of and relationships between the spatial and temporal parameters within the activity, than absolute values of force and angular displacement which formed the focus of earlier studies. Consequently there are indications that the total and phasic time scales, and sequential nature of upper body and lower limb displacement may be powerful descriptors and discriminators of the activity of rising from and sitting down on a chair. Method Current methods of measuring movement can be divided into three categories, namely electrogoniometric linkage systems, stereometric devices, and accelerometry17. There appears to be no evidence to date of attempts to provide a direct and comprehensive measurement of displacement in the three measurement areas of the sitto-stand and stand-to-sit movements. It has been shown that the vector stereograph is capable of giving a direct and accurate measurement of linear displacement in the three major planes of movement1J8J9. This approach to obtain objective data would seem ideally suited to measurement of the sit-stand-sit movement cycle, as the measurement space is relatively small in comparison to other activities such as walking, where the system has obvious limitations. Similarly, although several studies have provided useful information on forces and moments at the joints

and muscles of the lower limb during rising from a chair2*4*69,few have addressed the issue of defining the temporal characteristics of the movement. Kralj et a1.2 provided a descriptive analysis of the time phases within the sit-stand-sit movements through identification of changes in torque and force values determined from photographic and force-plate data, but their results in terms of total time to rise and total time to sit are so much longer than those in any previously reported studies that their findings must be questionable. Furthermore there appears to be no existing evidence to suggest that a comprehensive analysis of the elements of the sit-stand-sit movement in terms of temporal/ displacement relationships has been carried out. The value of measuring acceleration as a means of analysing movement has been identified by several authors, who have demonstrated the discriminatory nature of acceleration patterns in distinguishing between different gait pattems20-24. The nature of the sit-stand-sit movement activity would appear to be subject to changes in acceleration throughout the cycle, and consequently, the measurement of patterns of acceleration should provide useful information on this activity.

Measurement system The measurement system consisted of three elements including a vector stereograph (developed ‘in house’ at The Queen’s University Department of Orthopaedic Surgery, and based on the system developed by Morris and Harris18), three uniaxial accelerometers aligned along mutually orthogonal planes (Vibrator Corp. Boston, MA USA), and an electrogoniometer (Penney and Giles Blackwood Ltd), with each element linked to an amplifier and a computer. The vector stereograph is a non-optical device capable of recording numerical information upon any selected point within its measurement space’. As such, it may be used to provide numerical data on both complex movement patterns, and on three-dimensional shape1J8J9. Any point in space may be defined by its scalar distance from three fixed points. The vector stereograph (Figures 1, and 3) consists of three potentiometers attached to constant spring-loaded spools or capstans (Figure 2), from which run three wires, which meet in space at a position designated the pointer of the system. The constant tension on the wires means that as the pointer moves, the spools wind or unwind according to the direction of the movement. The movement of the spools generates an electrical charge proportional to the length of wire paid out or gathered in. The position of the pointer can be determined by triangulation (Figure l), if the distances between the pointer and sites of origin of the wires are known, and also the co-ordinates of the sites of origin. The purpose of the vector stereograph was to provide a record of the pattern of linear displacement of its point of attachment on the subject in

Kerr et al.: Sit-stand-sit

each of three dimensions (traces 1,2 and 3 in Figure 7), and numerical values of the displacement. Accelerometers are vibration sensors which have been applied to several aspects of biomechanics; piezoelectric accelerometers contain a piezoelectric crystal made from an artificially polarized ferroelectric ceramic. A spring-loaded mass is attached to the crystal so that when the unit experiences an acceleration in tension, compression or shear, an electrical charge is generated between the two surfaces of the crystal, the size of which is proportional to the acceleration produced. The purpose of the accelerometers was to record the pattern of acceleration throughout the movement cycle, which could be linked to displacement in both temporal and spatial terms. Electrogoniometers are exoskeletal joint measurement systems in which angular displacement of a joint produces a voltage change in a potentiometer proportional to the displacement. More recent electrogoniometers are lightweight, and consist of a flexible element linking the fixation arms, which are aligned along the axes of the joint lever arms. The measured angles are independent of the shape of the bend along the length of the element, allowing the goniometer to measure angular movement of joint motion irrespective of linear movements due to skin stretch (Penney and Giles). In addition to providing numerical data concerning angular displacement, the electrogoniometer was also capable of indicating the initiation and completion of angular displacement in the temporal context. A moulded polypropylene trunk back slab with an attachment for placement of the vector stereograph

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175

Figure 2. Capstan of the vector stereograph.

Electrogoniometer Figure 3. Elements of the measurement

Figure 1. Diagrammatic representation of the vector stereograph. A, B and C represent the sites of origin of the wires, and the position of the capstans. X represents the position of the pointer of the system. Geometry of vector stereograph (x-(Y)2+(y-f3P)2+2-p2=o (x- A)2 + (y- 612 + 1- d = 0 (x-E)2+(y-X)2+22+72=0

system.

pointer and a housing unit to enable the accelerometers to be placed in mutually orthogonal planes (Figure 3), was strapped to each subject by means of canvas webbing straps (Figure 4). The pointer of the vector stereograph and the accelerometers were placed at approximately the level of the seventh cervical vertebra, a position which pilot studies had indicated gave consistent patterns of acceleration and displacement in the three planes, and which could clearly discriminate

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Gait & Posture 1994; 2: No 3

between the temporal and spatial elements of the sit-stand-sit movement cycle. The electrogoniometer was placed at the lateral aspect of the knee, with the alignment units placed in line with the longitudinal axes of the thigh and shank (Figures 3 and 4). Positioning the electrogoniometer at the knee permitted identification of the point of loss of contact with the seat (at initiation of knee extension during rising), and the point at which seat contact was regained (at the end of knee flexion during descending). The output from all three elements of the measurement system was in millivolts; in the vector stereograph the output from the three potentiometers was proportional to the linear displacement in the three planes, the output from the three accelerometers was proportional to the acceleration acting on them in the three planes, and in the electrogoniometer the output was proportional to angular displacement at the knee in the sagittal plane. Three leads from each of the vector stereograph potentiometers and the accelerometers, and the lead form the electrogoniometer were attached to an amplifier and a filter, then subsequently to a computer to give simultaneous recording of the seven measurement parameters in both digital and analogue form. For any system to provide valid information about movement the elements of the system must be shown to give accurate and reliable measurements, and for the system as a whole to demonstrate intrasubject reliability and interfere minimally with the activity being measured. Therefore a number of studies were carried out to determine: (a) the linearity and reliability of the vector stereograph, (b) the accuracy and reliability of the electrogoniometer, (c) the potential influence of the measurement system (specifically the tension in the vector stereograph) on the characteristics of the sit-stand-sit movement cycle, (d) the reliability of the

measurement system during repeated single subject.

Vector stereograph: linearity and reliability To establish the linearity of the vector stereograph it was necessary to move the node (pointer) along a welldefined locus, which in this case was a straight line that made an acute angle with each of the coordinate planes. In practice this was achieved by securely clamping the pointer of the vector stereograph to the ‘trolley’ of a monorail linear bearing, which in turn was secured in a fixed position in relation to the vector stereograph frame. Linearity of the vector stereograph measurement was then quantified by moving the trolley along the linear bearing to fixed incremental distances, recording each position, and calculating the coordinates of each point by means of the vector stereograph. Linear regression analysis was performed on the data from the three sets of orthogonal axes in turn. The results are shown in Table 1, and demonstrate a coefficient of determination R2 > 0.9999 (P < 0.0001) in each case, indicating that the vector stereograph is capable of measuring the position of any point in space in a consistent linear fashion. Repeatability of vector stereograph results was determined by moving the trolley cyclically from one end of the monorail to the other and back, for a total of five complete cycles,, or ten half-cycles. Linear regression analysis was performed on the three sets of data (for each orthogonal plane), recorded for each half-cycle, and resultant values for the slope and intercept of the locus were tabulated (Table 2). These results demonstrated that the locus derived for the vector stereograph node movement was offset from the actual locus by no Table 1. Linearity of vector stereograph Regression variables

Coefficient of Determination

xy

0.9999 0.9999 0.9999

X z y, z

Table 2. Repeatability ments R2 values analysis

Figure 4. Experimental

set-up.

trials within a

measurements Pvalue

R* co.ooo1 <0.0001 <0.0001

of vector stereograph measurederived from linear regression

Half-cycle movement

x,y

XZ

y,z

1 2 3 4 5 6 7 8 9 10

0.9996 0.9993 0.9997 0.9995 0.9996 0.9993 0.9996 0.9994 0.9996 0.9993

0.9996 0.9994 0.9997 0.9996 0.9996 0.9994 0.9996 0.9995 0.9996 0.9994

0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999

Kerr et al.: Sit-stand-sit

more than 2 mm, and that the derived slope of the locus was consistent to within 0.5%. These results confirm that the vector stereograph is capable of measuring the Cartesian coordinates of any point within its measurement space in a consistent and reliable fashion, and should therefore be capable of providing accurate and reliable measurement of the movement of any point on the human body during any activity within that space.

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The accuracy with which the electrogoniometer measures knee angular displacement was determined by comparing its output with that of an established reliable measurement system. A simultaneous recording of the angle measured by electrogoniometry and by a stereophotogrammetry system (ELITE gait analysis system, manufactured by BTS Milan) was conducted, and the waveforms were correlated to quantify any deviations of the goniometer output from the true value of the knee angle as measured by the ELITE system. The relationship of knee angle as measured by the electrogoniometer and actual knee angle as measured by the ELITE system is illustrated in Figure 5. Linear regression analysis confirmed that the electrogoniometer output was linearly related to actual knee angle, with a coefficient of determination of R2 > 0.985 (P < 0.0001).

opposes motion away from the frame, and assists motion towards the frame. To establish that this potential influence did not interfere with the normal sit-stand-sit movement pattern of the subject, acceleration patterns were recorded from 10 subjects during the sit-stand-sit movement cycle both with and without the vector stereograph attached. Three recordings were taken in each case, comprising a total of six complete sit-stand-sit movement cycles for each condition. Knee angular displacement was also recorded during these trials. Acceleration waveforms were subsequently aligned with respect to the start of knee angular displacement, and each of the two sets of six cycles were averaged to produce a characteristic waveform for each condition. Linear regression was performed on the pair of characteristic waveforms for each of the 10 subjects to quantify the correlation between the waveforms. A typical correlation between acceleration output between the two conditions is illustrated in Figure 6, and Table 3 lists the coefficients of determination for each of the ten subjects. The mean coefficient of determination was R2 = 0.89 (P < O.OOOl),and indicated that there was a high correlation between the two sets of waveforms. Furthermore, the x coefficient was very close to unity, leading to the conclusion that attaching the vector stereograph to the individual did not significantly influence the sit-stand-sit activity.

Evaluation of the influence of the measurement system

Repeated trials within a single subject

The cables of the displacement transducers of the vector stereograph must be continually be under tension, and consequently may exert a finite restraining force which

Having determined linearity, reliability and repeatability of the individual components of the measurement system, it was necessary also to establish the consistency

Accuracy and reliability of goniometric measurements

70 /

60

0

Direct

I

I

10

I

1

Angle

Figure 5. Goniometer

I

30

function

/

50

measuredhy “ELITE” system

output versus knee angle.

line

I

I

70

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80 60 40 20 0 -20 --40 -60 -80 -100 -120 -140 0 -160

0

q

0 0

0 0

#;ooo

-180

0

I

-300

-100

-200

,

0

100

Accele.donwithvcctmstereographaaacbed

Figure 6. Correlation

of accelerometer

Table 3. Correlation of acceleration waveforms without vector stereograph attached Subject

1 2 3 4 5 6 7 8 9 10

Coefficient of determination R2

Pvalue

0.927

<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

0.927 0.864 0.905 0.808 0.943 0.891 0.840 0.877 0.885

!

waveforms.

with and

of data obtained from the measurement system as a whole during repeated trials within one subject. A single healthy female subject aged 46 years, height 168.0 cm, weight 57.5 kg, performed the activity as described in the procedure section on two occasions during 1 day, and on three occasions on separate days, within 1 week. From the raw data obtained from this subject, six elements of the sit-stand-sit movement cycle which involved manipulation of both spatial and temporal data were selected to determine intrasubject variability during repeated trials. The six elements were selected to reflect what were considered to be potential determinants of the sit-stand-sit movement cycle, and included the total time to rise and total time to descend, the percentage

contribution of the forward lean phase to the total time in rising and in descending, and the velocity of forward lean in rising and descending. The coefficient of variation was calculated for all selected elements over the repeated trials, and was found to be less than 5% in all cases. Individual scores for each of the selected elements can be seen in Table 4. Output

from the measurement mystem

Ten normal healthy female subjects, mean age 25.2, mean height 161.2 cm, mean weight 59.5 kg, with no known history of neurological or recent musculoskeletal disorders participated in the study. All subjects were dressed in T-shirts and shorts, and were barefoot.

Table 4. Coefficient of variance for elements of the sit-stand-sit movement cycle during repeated trials with a single subject Element

Mean

SE

Coefficient of variation (%)

Time to rise Time to descend Forward lean velocity (rising) Forward lean velocity (descending) Forward lean time/ total time (rising) Forward lean time/ total time (descending)

178.2 181.6 6.2

7.6 4.9 0.2

4.2 2.7 3.8

2.5

0.1

4.0

0.52

0.035

3.5

0.38

0.006

1.5

Kerr et al.: Sit-stand-sit

Procedure Each subject sat on a wooden box topped with Styrofoam blocks wtandardize knee angle at between 95 and 100 degreesi3, and the angle between the shank and the vertical at 18 degrees2 for all subjects. The heels were positioned 10 cm apart2 and the arms were folded across the chest2,5. As temporal parameters were suspected to be powerful discriminators of the sit-stand-sit movement cycle, subjects in this study performed the activity at a selfselected speed. The subject and seat were positioned in front of the vector stereograph so that the strings from the pointer to the capstan potentiometers were under tension (Figure 4). The measurement system was attached to each subject as described previously. Subjects practised standing up and sitting down between 6 and 10 times to familiarize themselves with the equipment, after which knee and shank angles and the position of the feet were rechecked. Each experimental trial consisted of two sit-stand-sit movement cycles, and a total three trials was performed by each subject in each of two conditions, namely with the accelerometers and goniometer only attached. and with the total measurement system of accelerometers, electrogoniometer, and vector stereograph attached. The two conditions were necessitated by the observation during pilot studies that, as the strings of the vector stereograph must be under tension, it may exert a resistive influence during rising, and an assistive influence during descending; comparison of other measurement parameters between the two conditions would determine the influence of the vector stereograph element of the measurement system. Verbal instructions of ‘ready and stand’ and ‘ready and sit’ were given at 6-s intervals, with the subjects commencing the appropriate movement at their own self-selected speed on the words ‘stand’ and ‘sit’. The intervening standing and sitting positions were maintained as steadily as possible for approximately 34 s, the total time for each trial being 25 s. Data in analogue and digital forms was recorded for acceleration and linear displacement of the trunk in the three major planes of movement, and for angular displacement of the knee in the sagittal plane. Results Graphical representation of the data obtained from each trial, consisting of two complete sit-stand-sit movement cycles, was recorded for each of the seven measurement parameters, and is shown in Figure 7;

(4 Sagittal displacement of the trunk (b) vertical displacement of the trunk Cc) coronal/lateral displacement of the trunk angular displacement of the knee sagittal acceleration of the trunk vertical acceleration of the trunk (iit) coronal/lateral acceleration of the trunk.

(4

Ir:

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Each trace represented changes in each of the parameters measured, for example the sagittal displacement trace followed an initial upward direction (forward movement of the trunk), a peak (the point of maximal forward lean), and a downward orientation (backward movement of the trunk on attaining the standing position). The level section of the trace represented steady standing. During the descending phase the initial upward orientation of the trace represented forward lean of the trunk, the peak representing the maximal forward position during the descending phase, and the final downward slope of the trace represented the backward movement of the trunk as the subject regains the sitting position. Similarly the other traces represented changes in vertical and coronal displacement of the trunk (traces 2 and 3) angular displacement of the knee (trace 4) and acceleration in the coronal, vertical and sagittal planes (traces 5, 6 and 7). Sampling of the recorded data enabled numerical data to be extracted for linear displacement and acceleration of the trunk in three dimensions, and for angular displacement of the knee at any point of the movement cycle. Furthermore, as the measurements were made along a continuous time-scale, the temporal characteristics of the cycle could also be defined. Discussion To date, the analysis of the sit-to-stand movement has been somewhat piecemeal, with researchers investigating individual elements of the activity, rather than analysing the movement as a whole; the stand-to-sit activity has received scant attention. For example, forces and moments produced at the joints of the lower limb’*5-s,angular displacement of the joints of the trunk and lower limb13, and development of momentum in the trunk4.‘5 have been investigated. Because of the widely varying approaches to the analysis of the activity, direct comparison of findings from the investigations has not been possible, and consequently consensus on the characteristics and determinants of the activity has not yet been established. This is in obvious contrast to the development of research into the gait cycle, which has been facilitated by an accepted definition of the characteristics and determinants of the activityZ5.Z6. This paper describes a novel approach to the analysis of the sit-stand-sit movement cycle, using a relatively simple and inexpensive measurement system. Most previous research concerned with the sit-to-stand movement has involved the use of complex and expensive equipment such as force plates and optoelectric systems, which although capable of providing information that may further elucidate the sit-stand-sit activity. have obvious financial drawbacks and often require specialist laboratory settings for their use. Since many of these systems have been accepted in terms of validity and reliability, it has not been necessary to report on these aspects. However, the outcomes of these investigations have concentrated almost exclusively on the quantification of forces and moments at

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Sagittaldisplacement

Verticaldisplacement

Lateraldisplacement

Knee angulardisplacement

Sagittalacceleration

VefticaIacceleration

Lateralacceleration

10

0

20 Time [seconds]

Figure 7. Analogue

output from the measurement

the hip and knee, which while identifying the biomechanical demands of the activity have done little to elucidate the fundamental nature and pattern of the movement cycle. Similarly, kinesiological studies have quantified angular displacement at a variety of lower limb and trunk joints 13,but have not attempted to relate individual joint displacement to segmental motion in the temporal context. The most basic analysis of the sit-to-stand movement, in terms of forward displacement of the trunk and extension of the lower limbs, would tend to suggest that investigation into the temporal relationships of these elements was warranted. This concept was supported by the findings of Pai and Rogers15 and Berger et a1.5, who recognixed the importance of the development of forward momentum of the trunk, and by Car@, who has suggested that the timing of lower limb extension in relation to forward displacement of the trunk may be critical to the successful performance of rising from a chair. The measurement system described in this paper provided the opportunity to identify patterns of displacement and acceleration from the analogue

system.

output, and to extract numerical data at any specified point of the activity from all seven measurement parameters. The raw numerical data was manipulated to provide absolute measures of linear displacement of the trunk, and angular displacement of the knee. Absolute measures of acceleration were also possible with this system, but were not extracted in this study since it was believed that the patterns of acceleration and their relationship to other displacement parameters would be more meaningful in the analysis of the activity. Extraction of numerical data from this system can therefore be used to quantify the extent of linear and angular displacement during any phase or element of the activity, and because all parameters were recorded simultaneously along a continuous time-scale the duration of these selected phases and elements can also be calculated, and relationships between different elements can be determined within the total temporal framework. Consequently this system has the potential to provide information in both analogue and digital form, to enable identification of patterns of displacement and acceleration within the sit-stand-sit movement cycle,

Kerr et al.: Sit-stand-sit

and to investigate the nature and relationship between the different elements of the activity. In the development of a novel system of measurement designed to analyse any activity the ultimate success of the system must rely on the reliability and validity of the system, and on evidence that under normal conditions the performance of the activity is consistent. The system described in this paper comprised three elements, namely vector stereography, electrogoniometry, and accelerometry. Of these, perhaps only accelerometry has been used extensively in the analysis of movement, the accuracy and reliability of which has been accepted23.24. In the present study the linearity and reliability of the vector stereograph has been established, and the electrogoniometer has been shown to compare favourably with an established reliable system (ELITE). The final process in the development of a new system for the analysis of a specific activity is that having established the validity and reliability of the system, the actual performance of the activity by normal healthy individuals under normal, controlled conditions must be consistent. Otherwise it is impossible to define the characteristics of the activity within normal limits, and beyond this to analyse and explain abnormalities. The application of the measurement system to a series of repeated trials within a single subject has demonstrated consistency of performance when the elements of the activity, selected to reflect both temporal and spatial parameters were analysed with a coefficient of variation of less than 5%. The validity and reliability of the elements of this measurement system have been established and its application to the analysis of the sit-stand-sit movement cycle has demonstrated consistency in the performance of the activity, and the potential of the measurement system to provide a comprehensive analysis of the movement. Conclusion The aim of this study, which was to develop a measurement system capable of analysing the sit-stand-sit movement cycle, has been achieved. Tests of individual elements of the measurement system have demonstrated acceptable accuracy and reliability, and the application of the system as a whole in repeated trials within a single subject has produced consistent results. The output from the system in both analogue and digital form has been shown to be capable of providing information on the patterns of displacement and acceleration, and definitive data on temporal and spatial parameters, and the relationships between them. References 1 Grew ND, Harris JD. A method of measuring human body shape and movement - the vector stereograph. Eng Med 1979; 8(3): 115-18 2 Kralj A, Jaeger RJ, Munh M. Analysis of standing up

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