An Accurate Inexpensive System for the Assessment of Walking Speed
S. B. Mitchell, MSE
J. E. Sanders, PhD
ABSTRACT
An easy-to-build and inexpensive device was developed to assess walking speed. The device measured to a best resolution of 0.001 second the time interval between the triggering of two dual beam photoelectric intrusion-detection systems positioned 12.0 m apart. A preliminary study on a population of three normal subjects demonstrated that the variance in the time to walk between the detectors was greater than the resolution of the measurement system, thus the resolution of the device was adequate for walking applications. Potentially, this device could be used to assess postoperative patient progress and functional outcome.
Keywords: Gait analysis, walking velocity, functional outcome
For the assessment of walking performance, velocity is the best single index of walking ability.1 At a self-selected walking velocity, an individual will naturally employ both the mannerisms and speed that will provide for maximum energy efficiency. A deviation from that velocity could indicate an abnormality. Such information can be useful to a prosthetist or orthotist during treatment and fitting. Furthermore, when asked to walk at a fast velocity, healthy subjects can increase their speeds by as much as 44%.2 However, pathological subjects will not be able to perform such large increases, indicating that an abnormality is present and should be addressed.
A number of systems exist to measure walking velocity. They typically operate on an infrared (IR) beam transmission and detection principle. The Speedlight Timing System (SWIFT Performance Equipment, Alstonville, Australia), at a 1999 cost of $3,390, calculates velocity and acceleration using IR timing gates, computer interface hardware, and custom software. Velocity and acceleration data are presented to the recorder on a computer screen. The Stride Analyzer (B & L Engineering, Tustin, CA), at a cost of $9,900, calculates velocity, cadence, stride length, the duration of single and double support for each limb, and the pattern of contact for each foot. The Powertimer (Newtest, Oulu, Finland), at a cost of $4,300, delivers reaction times, velocity, and acceleration for various activities including running and jumping. While these systems do indeed determine walking speed, they are overdesigns if the measurement of velocity is exclusively of interest. Their costs are prohibitory for exclusive velocity assessment. Thus, a simple, relatively inexpensive system is needed and of strong potential utility in a clinical prosthetics and orthotics practice (< $500) for the assessment of walking velocity. The system should have a timing resolution consistent with or better than the above-described systems, ie, approximately 0.01 second based on the manufacturers' product brochures, so as to ensure comparable or improved accuracy.
Methods
Device Design
Two sending and receiving pairs from an intrusion-detection system (model DS422, Detection Systems, Inc., Fairport, NY) are used. They are IR transmitters and photoelectric detectors in a dual beam configuration. Each sender and receiver is mounted to a plastic pole approximately 1.4 m (adjustable ±0.10 m) from the floor (Figure 1
). Hollow plastic bases filled with sand stabilize the poles. Shielded cables (approximately 8.0 m long) attach the photoelectric systems to a stand-alone timing circuit with a miniature display module timer (1.5 × 3.5 cm) (model MDMU, Red Lion Controls, York, PA). The circuit is designed so that a subject starts the timer by passing through either photoelectric detector and stops the timer by passing through the second detector. The elapsed time is displayed on the timer. The cost of the system in 1999 dollars is $260 for the IR detectors, $49 for the MDMU timer, $89 for the electronics, $50 for the stands, and $20 for the cables, a total of $468.
The timing circuit consists of two digital, integrated circuits (HC7400, HC7404), two on-momentary switches (a stop switch and a reset switch), a double-pole single-throw (DPST) toggle switch, and the MDMU timer (Figure 2
). The normally open switches on the IR detectors are connected to the HC7400 in such a manner that a flip-flop is formed. The flip-flop enables one IR detector to start the timer and the other to stop the timer. Each IR detector is configured such that both beams within each IR detector need to be interrupted for triggering to occur. The toggle switches and the HC7404 are used to control the trigger direction of the timer. This feature allows the subject to start from either end of the timing gates. One on-momentary switch is used as a stop button for accidental starting of the system, and the other on-momentary switch is used as a reset for the timer. All components with the exception of the IR detectors are placed in a shielded box with a power supply. Cables extend from the box to the IR detectors and an AC power source. The MDMU timer has a best resolution of 0.001 second. However, a 0.01-second resolution was considered adequate for this application and is consistent with similar timing devices.
Functional Testing
Tests were conducted to determine the smallest object size needed to trigger the timer. This evaluation helped to determine whether a measurement error would occur by triggering from swinging fingers or arms rather than translation of the torso. The height of a test object was decreased incrementally until the IR detector did not sense the object.
A preliminary study was conducted on three subjects to determine whether the resolution of the system was less than the subject gait variability from trial to trial. All subjects were men between the ages of 26 and 34, had no limb abnormalities, and had a self-selected walking cadence of approximately 60 strides/min. The photoelectric detection pairs were placed 12.0 m apart, with each sending unit 2.0 m from its pair-receiving unit. Subjects started walking approximately five paces before the first detector and stopped approximately five paces after the second detector. An audible metronome was used such that the subjects matched their cadence to the specified rate. Six trials at each of three rates (40, 60, and 80 strides/min) for a total of 18 trials were randomly ordered for each subject over an approximately 15-minute session. Half the tests were performed in one direction, and half were performed in the other direction. The distance between the detectors was divided by the elapsed time to calculate walking velocity. The variance in the time to walk between the detectors was determined for each subject at each cadence.
Results
Results from the evaluation tests demonstrated that the smallest object that could pass through the detector without triggering the timer was 7.62 cm in height. Hand breadth across the thumb is 10.5 cm for the 50th percentile man and 9.2 cm for the 50th percentile woman.3 It should be noted that the detectors have a 0.035-second response time, meaning that an object that passes through within 0.035 second will not be detected.
Velocities for the three subjects altogether ranged from 0.78 m/s to 2.46 m/s. The variance for the time to walk between the two detectors was at least 0.03 second for all cadences (Table 1
), greater than the 0.01-second resolution of the time measurement system.
Discussion
The threshold vertical dimension for triggering (7.62 cm) is well within the hand dimensions for the 50th percentile man (10.5 cm) and woman (9.2 cm). Thus if only object dimensions are considered, a swinging hand in front of the torso could trigger the timer, potentially causing an erroneous reading. However, the detectors are configured with a 0.035-second response time, meaning that objects that pass through the detectors within 0.035 second do not cause triggering. A swinging hand during walking will likely pass through in this time interval. However, to minimize the risk of error, the height of IR detectors should be adjusted so that the subject breaks the beams near to shoulder level. For the velocity data presented here, the IR beams were positioned at shoulder level for all subjects.
The time analysis demonstrated that, for the walking velocities achieved here, the variance in velocity from trial to trial (> 0.03 second) was greater than the time resolution of the instrument (0.01 second). Thus, the resolution of the system was adequate.
The system described here is relatively simple to build and is of relatively low cost ($468) compared with commercial systems that assess walking characteristics (> $3,390). Therefore, the potential for the device to be affordable and useable in a clinical prosthetics or orthotics treatment facility is strong. Analysis of self-selected or maximum walking velocity changes postoperatively could provide an assessment of a patient's progress and improvement after initial fitting. Assessment over the long term could help to indicate when a prosthesis is uncomfortable and needs to be replaced.
Conclusion
The device described here measures velocity to an accuracy sufficient for the assessment of walking speed on normal subjects. Because the system is easy to build with relatively inexpensive components, it represents a potentially useful functional outcome assessment device for prosthetic and orthotic fitting and treatment.
Acknowledgments
This research was supported by the National Center for Medical Rehabilitation Research at the National Institutes of Health (HD-31445).
Copyright ©2000 American Academy of Orthotists and Prosthetists.
S. B. Mitchell, MSE, is a PhD student in the Bioengineering Department at the University of Washington, Seattle, WA.
J. E. Sanders, PhD, is a professor in the Bioengineering Department at the University of Washington, Seattle, WA.Professor J. E. Sanders, Bioengineering, Box 357962, Harris 309, University of Washington, Seattle, WA 98195. Phone: (206) 221-5872; Fax: (206) 221-5874; E-mail: jsanders@u.washington.edu.
References:
- Ayyappa E. Normal human locomotion, Part 1: Basic concepts and terminology. J Prosthet Orthot. 1997;9:10-17.
- Finley FR, Cody KA, Finizie RV. Locomotion patterns in elderly women. Arch Phys Med Rehabil. 1969;50:140-146.
- Pheasant S. Bodyspace: Anthropometry, Ergonomics and the Design of Work, 2nd ed. Bristol, PA: Taylor and Francis, Inc.; 1996.
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