Assessing Reliability of Measurement of Gait Velocity

313 Key Words Reliability, gait velocity, walking speed, statistical analysis.

Assessing Reliability of Measurement of Gait Velocity Summary Background and Purpose Gait velocity reliability was evaluated using the GAITRite® system to measure healthy subjects at free walking speed. Methods A convenience sample of 60 subjects (mean age 22.3 years) was measured on two separate occasions (24 hours apart) with each subject traversing a 7.5 metre walkway. Instructions to each subject were standardised in an attempt to control for variability of gait. A strict protocol was used to ensure consistency. The mean velocity for the three repetitions on each occasion was recorded. Data were analysed to assess reliability by the Bland and Altman method to evaluate the clinical importance of the error inherent in the data. Results For the measurement of gait velocity, the limits of agreement (0.13 m/s to –0.09 m/s) demonstrated that two measurements taken 24 hours apart, and according to the protocol described, would be within 0.11 m/s of each other in 95% of cases. Conclusion The clinical implications from these results suggest that the relatively small error described by the limits of agreement reflects the accuracy of the measurements. The system would be useful in a busy clinical practice to assess gait parameters.

Batey, P, Rome, K, Finn, P and Hanchard, N (2003). ‘Assessing reliability of measurement of gait velocity’, Physiotherapy, 89, 5, 313-317.

Introduction Assessment of gait forms an important aspect of daily clinical practice in neurology, rheumatology, orthopaedic and rehabilitation medicine (Mulder et al, 1998). The majority of physiotherapists assess gait by observation owing to the cost of gait analysis equipment and the space needed to store it. If it is standardised and well documented, obser vational analysis may give a reasonable basis from which to judge improvement in a patient’s condition, but

by Paul Batey Keith Rome Paul Finn Nigel Hanchard

objective data for research cannot be obtained in this way (Riley et al, 1999). Gait velocity has been widely used as a measure of patient status and treatment efficacy in clinical care and research studies (Roth et al, 1997; McDonough et al, 2001; Selby-Silverstein and Besser, 1999; Cutlip et al, 2000). In clinical practice freely selected walking speed is one of the better indicators of how well people walk. Speed decreases with injury and pain and increases with recovery (Weir and Childress, 1997). Walking velocity has been found to be related to other temporal gait variables in postsurgical orthopaedic patients and normal control subjects (Roth et al, 1997). Velocity has also been shown to be sensitive and specific in screening older people for both balance and mobility impairments (Harada et al, 1995; Guimaraes and Isaacs, 1980; Judge et al, 1993). There have been many attempts to devise objective gait measurement systems that are simple and economical (Wall and Scarbrough, 1997; Smith and Rome, 1996). The GAITRite® is a relatively new system for clinical gait analysis, which measures spatial and temporal parameters of gait automatically. Unlike other walkway systems, the GAITRite® system is easy to use, portable, and can be laid over any flat surface. It requires minimum set-up and test time and no devices are placed on patients. A wealth of literature refers to gait speed reliability (Andriacchi et al, 1977; Smith and Rome, 1996; Bohannon, 1997; Fransen et al, 1997). The problem arises on the clinical interpretation of the results. Previous studies have employed Pearson’s r correlation coefficient (Hill et al, 1994; Wilkinson and Menz, 1997). However, other studies have reported Physiotherapy May 2003/vol 89/no 5

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Authors and Contribution Paul Batey BSc MCSP is currently a junior physiotherapist at James Cook University Hospital, South Tees AHA Trust, and was a student at the University of Teesside, School of Health and Social Care, Physiotherapy. This paper was part of his final dissertation. Keith Rome PhD MSc BSc is a reader in musculoskeletal rehabilitation and head of the Rehabilitation Research Unit, Postgraduate Institute of Health and Social Care, University of Teesside. He was Mr Batey’s supervisor. Paul Finn MSc is a medical statistician, Postgraduate Institute of Health and Social Care, University of Teesside. He gave statistical advice. Nigel Hanchard MSc MCSP is a senior lecturer in physiotherapy at the University of Teesside and was involved in the statistical analysis. This article was received on July 22, 2002, and accepted on December 10, 2002.

that Pearson’s r correlation coefficient is inappropriate because this measures the strength of linear relationship whereas the information required is the strength of agreement (Bland and Altman, 1986; Rankin and Stokes, 1998; Bruton et al, 2000). Furthermore, Keating and Matyas (1998) suggested that correlation coefficients alone do not provide insight into the possible usefulness of an assessment procedure for identifying the differences of interest. They warned readers to be sceptical if a study claims reliability on the strength of correlation alone. Therefore correlation coefficients can offer only limited information to clinicians. Other studies have reported the use of intraclass coefficients as a measure of reliability and limits of agreement either independently or together (Bland and Altman, 1986; Rankin and Stokes, 1998). The Bland and Altman method (limits of agreement) considers measurement error, whereas intraclass coefficients do not (Bland and Altman, 1986). The former is therefore essential to reveal clinical significance. The primary purpose of measuring systems is to provide clinicians with tools to help their subjective assessment of gait. Furthermore, with all new measuring systems key issues such as reliability and validity need to be addressed before being applied in a clinical setting. Indeed, if a measuring instrument is to be considered clinically useful, a physiotherapist should know that studies have been carried out to demonstrate that it yields results that can be reproduced (CSP, 1999). Therefore, the aim of the current study was to determine the intra-tester reliability of the GAITRite ® system in measuring gait velocity using the Bland and Altman method in young healthy subjects. Methodology A convenience sample of 60 subjects (35 female and 25 male) from the School of Health and Social Care, University of Teesside, were selected. They were aged between 18 and 43 years (see table). The inclusion criteria included pain-free movement of the lower limbs and lumbar spine joints; no previous history of trauma or surgery to the lower limbs or lumbar spine, no history of balance disorders, and no diagnosed musculoskeletal

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Descriptive statistics for sample population Minimum Maximum Mean Height (m)

1.20

1.86

1.70

SD 0.11

Weight (kg)

43.5

95.0

69.1

11.3

Age (years)

18

43

22.3

5.2

disorders which might affect gait. Ethical consent was granted from the University of Teesside School of Health and Social Care Ethics Committee. Subjects were informed that they had the right to withdraw from the study at any time throughout the period of testing. The 7.5 m GAITRite® electronic walkway contains six sensor pads encapsulated in a roll-up carpet that produces an active area 61 cm wide and 366 cm long (fig 1). As the subject moves across the walkway, the system captures the relative arrangement, the geometry and the applied pressure of each footfall as a function of time. The application software controls the function of the walkway, processes the raw data into footfall patterns and computes temporal and spatial parameters. Gait velocity (metres per second) readings were measured for each subject on two separate occasions, 24 hours apart. Written instructions to each subject were standardised in an attempt to control for extraneous variables. Each subject was asked to walk along the length of the electronic walkway and the velocity for each walk was recorded. The mean of three repetitions was recorded for each subject. A taped line was placed 2 m before the start of the walkway and an additional taped line was placed 2 m after the walkway to ensure an accelerative and decelerative period of walking. Miller et al (1996) demonstrated that the end of three full steps accomplished a steady state in gait. Subjects were instructed to walk across the mat at their own preferred comfortable walking speed (Smith and Rome, 1996). Results Descriptive information of the sample is shown in the table. The results demonstrated a mean (standard deviation) of 1.49 (1.43) m/sec for occasion 1 and a mean (standard deviation) of 1.48 (1.51) m/sec for occasion 2. The range of values found in this study is consistent with previous work (Whittle, 2002).

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Walkway connector Power connector

Power cord Power supply

Network controller

Interface cable

Pad controller

Serial port connector

GAITrite Walkway

Fig 1: Walkway connections

Figure 2 demonstrates the relationship between means of the two measurements of gait velocity taken on two occasions against the difference between the two measurements. The upper and lower bold lines represent the limits of agreement as defined by Bland and Altman (1986). Figure 2 also suggests that the mean velocity increased by approximately 0.02 m/s over the two replications. This

indicates that the subjects tended to walk slightly faster on the second occasion. The magnitude of this difference is unlikely to be of great clinical significance. The limits of agreement demonstrated that for the measurement of gait velocity the reading obtained on the second occasion would fall within 0.13 m/s and –0.09 m/s of that obtained on the first occasion in 95% of cases.

0.15

Difference between two measurements (m/s)

0.12 0.09 0.06 0.03 0.00 0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

-0.03 -0.06 -0.09

-0.12 -0.15

Mean of two measurements (m/s)

Fig 2: Limits of agreement for the measurement of gait velocity

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Address for Correspondence Dr Keith Rome, Rehabilitation Research Unit, Postgraduate Institute of Health and Social Care, School of Health and Social Care, Physiotherapy, University of Teesside, Middlesbrough TS1 3BA. e-mail [email protected]

Discussion The results of the present study, using limits of agreement, indicate that the GAITRite® system is a reliable means of measuring gait velocity. Clinically this suggests that provided two measurements for the same subject show a difference greater than 0.11 m/s then on 95% of occasions a real difference between performance exists. Or to put it another way, that the difference noted is greater than would be expected due to the error of the measuring system. Previous studies suggest that fast walking speeds are more reliable than slow or free gait velocities (Fransen et al, 1997; Friedman et al, 1988). The current study was concerned only with ‘normal’ walking speed. Fast walking speed might have been more appropriate. However Smith and Rome (1996) state that although they found that the biggest difference is at free speed, when gait velocity is considered in terms of standard deviation, the least variation is apparent at this speed. Although the current study reviewed free-walking speed, a complete range of walking speeds should be recorded on different occasions to obtain a reliable measure of gait velocity. This should be considered for future research. The current study encountered problems with the GAITRite® in recognising subjects’ footfalls. This was a systematic error, which displayed itself when subjects shuffled their feet. Controversy currently surrounds the ability of the GAITRite ® system to interpret correctly the foot pressure pattern, and as a result it may be unable to calculate the specific temporal and spatial parameters of gait (Kirtley, 2000). However the system does have the facility to edit the footfalls manually. The authors of the current study found this to be accurate, although more timeconsuming. It is highly likely that this would cause problems clinically as many neurological patients shuffle, for example those suffering from Parkinsonism or hemiplegia. The results of the current study are relevant for clinical practice. The GAITRite® system provides clinicians with

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a simple tool for gait analysis. In these changing times in the National Health Ser vice there is a need to evaluate critically the role of physiotherapists (Wakefield, 2000). Evidence-based practice now embraces the interventions of all clinicians delivering healthcare ser vices (Barnard and Wiles, 2001). Therapists need to demonstrate that current methods of practice are reliable, cost-efficient and appropriate, while questioning traditional theories and treatment methods (Wakefield, 2000). Standard 1 of the guidelines for administering tests and taking measurements, produced by the Chartered Society of Physiotherapy (1999), states that a physiotherapist should select an instrument which has been shown to be useful for the purpose intended, and ensure that the test is based on theoretical principles and is valid, accurate and reliable. Standard 3 suggests that physiotherapists should select a measuring instrument which provides an objective measure and that by using the same instrument in a standardised way they will facilitate audit and skill development of staff. The GAITRite® system would be an acceptable measurement tool to conform to these standards. Conclusion The current study found the GAITRite® system to be useful as a clinical tool for measuring gait velocity in healthy subjects, providing clinicians with an easy to use, efficient method of producing objective measurements in accordance with evidence-based practice and CSP standards. While the intra-rater reliability of the system indicates a relatively small degree of error, future research is required into the effect on the system of different walking speeds. Research is under way to evaluate the reliability of the system in different client groups, the impact of foot orthoses in seronegative arthropathy patients, and the synchronisation of other gait systems to enable a full gait analysis to be conducted.

Research report

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Key Messages ■ Physiotherapists need to provide objective documentation on the effectiveness of their treatment. ■ Walking velocity has been used as a measure of patient status and treatment efficacy. ■ Various statistical methods have been reported to evaluate walking velocity. ■ The Bland and Altman method of describing the error of measurement provides useful clinical guidance.

Selby-Silverstein, L and Besser, M (1999). ‘Accuracy of the GAITRite® system for measuring temporal-spatial parameters of gait’, Physical Therapy, 79, S59. Smith, W and Rome, K (1996). ‘Reliability of walking speed in podiatric patients’, Gait and Posture, 4, 130-135. Wakefield, A (2000). ‘Evidence-based physiotherapy: The case for pragmatic randomised controlled trials’, Physiotherapy, 86, 394-396. Wall, J C and Scarbrough, J (1997). ‘Use of a multimemory stopwatch to measure the temporal gait parameters’, Journal of Sports and Physical Therapy, 25, 277-281. Weir, R F and Childress, D S (1997). ‘A portable, real-time, clinical gait velocity analysis system’, IEEE Transactions on Rehabilitation Engineering, 5, 310-320. Wilkinson, M J and Menz, H B (1997). ‘Measurement of gait parameters from footprints: a reliability study’, Foot, 7, 19-23. Whittle, M (2002). Gait Analysis: An introduction. Butterworth-Heinemann, Oxford, 3rd edn.

Keating, J and Matyas, T (1998). ‘Unreliable inferences from reliable measurements’, Australian Journal of Physiotherapy, 44, 5-10.

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