The PushTracker: An Activity Monitor for Manual Wheelchair Users

ABSTRACT

To assist manual wheelchair users in avoiding upper limb pain and pathology we created a device that can track their activity level throughout and across several months.  To do this we custom designed and built a printed circuit board with associated circuit components.  The processor uses our algorithm to detect individual pushes from onboard accelerometer data and distance measurements from the wheel mounted magnet system in real time.  The PushTracker then records this data to an onboard SD card, which can be accessed using a computer.  After each logging period the graphs on the screen update to include that session’s data on average cadence, push distance, and total number of pushes.

BACKGROUND

Wheelchair users are living longer, fuller lives as the result of innovative medical and technological advances. While the progress has been considerable, there are still areas of significant need in this population. Wheelchair users are over twice as likely to be obese as people in the general population.(1) Obesity leads to secondary conditions such as heart disease and upper extremity (UE) pain and injury. Cardiovascular disease is a leading cause of death among wheelchair users.⁽²⁾ UE pain and injury affects over half of wheelchair users and heavier users are more prone to injury.^(3,4) The solution to obesity in the wheelchair user population is the same as it is in the general population, eating healthy and being as active as possible.

In addition to being overweight, how a user pushes is believed to contribute to the development of UE injury, with those users who push less efficiently having a greater risk of developing UE injuries.^(3,4) The PVA Clinical Practice Guidelines for the Preservation of Upper Limb Health following Spinal Cord Injury recommends training users to push with less frequent, long, smooth strokes.⁽⁵⁾ However, there isn’t a product on the market with the ability to measure how users push in their natural environment, so users are unable to assess, monitor and track their propulsion technique.

PROBLEM STATEMENT

Manual wheelchair users currently have no way to track changes in their activity level.  Our device will give them that ability at an hourly, daily, and monthly level in a user friendly fashion.  This information will help the user better follow the PVA Clinical Practice Guidelines to decrease their probability of upper extremity pain and injury.

SOLUTIONS CONSIDERED

Before we created the PushTracker we attempted to get the same functionalities from bike cyclometers and pedometers.    The bike cyclometer met with failure because it was not capable of identifying individual pushes.  The cyclometer measures only speed and distance while acceleration is needed for push detection.  The pedometer failed because it assumes a gravity vector which was absent when it was turned sideways and attached to a wheelchair.  We purchased three pedometers with different step detection mechanisms and attempted to use them to count pushes. The three detection mechanisms were: 1) a mass-spring system, 2) a single-axis accelerometer system, and 3) a three-axis accelerometer system. None of them were able to register a single push.

METHOD

To create the Pushtracker we combined an accelerometer and a wheel mounted magnet system to detect pushes and measure distance.  To read and power the accelerometer we created a custom printed circuit board (PCB) with an onboard ARM7 microcontroller.  This microcontroller uses our algorithm to filter and interpret the accelerometer’s output in real time to detect pushes.  In addition, the PCB contains a battery recharge circuit, which allows the device to be recharged with a computer’s USB port.  The PCB also holds the onboard microSD card to store data and an OLED screen to display data.  An attached thumb knob allows users to cycle through a menu showing their cadence, distance, and number of pushes over time periods ranging from hours to months.  The microcontroller pins are set to either sense switch closures from the magnet during use or are switched to USB data transfer mode when downloading data to the computer for additional viewing.

Figure 1: The main portion of the PushTracker (left) and the wheel-mounted magnets and sensor (right).

A magnet and magnet sensor arrangement was developed to mimic a cyclometer. The system uses a small plastic ring with embedded magnets. The ring is placed on the outer face of the wheel bearing and held in place by its own magnetic force. The magnet ring spins around with the wheel as it rotates. A magnet sensor is then placed at the distal end of the axle receiver and secured in place. In our design, the magnet sensor was integrated into a threaded plastic enclosure that was threaded onto the end of the axle receiver. Because the magnet sensor is above the axle tube and within the wheelchair frame, it is fairly protected during transport. The magnet sensor was connected to the PushTracker via a USB cable, allowing it to be easily disconnected and reconnected for viewing.

RESULT

To evaluate our prototype we conducted a ten person evaluation study.  Those ten wheelchair users provided informed consent (WIRB, Olympia, WA) to participate in the evaluation of the PushTracker prototype. The PushTracker and two OptiPush instrumented wheels were mounted to the subject’s wheelchair.  The Optipush wheel is an instrumented wheel which measures torques and forces using a 6 degree of freedom load cell and was considered the gold standard in this study. (6)  Subjects were asked to propel their wheelchair on an overground test course. The course consisted of indoor and outdoor surfaces including asphalt, sidewalk concrete, low-pile carpet, and cement. After completing the course, subjects were given the opportunity to handle the PushTracker and to try out the different functions. Subjects were also asked to complete a brief questionnaire designed to gauge interest in the PushTracker and to help guide future developments.

PERFORMANCE

Distance, speed, push count and cadence measurements made by the PushTracker were compared the values obtained by the OptiPush. OptiPush values were averaged across the left and right sides. Percent differences were computed for each subject. Paired t-tests were used to check for significance across the variables with alpha set to 0.05. The results are shown in Table 1. For all variables, the PushTracker was accurate to within 2%. No significant differences were found between the PushTracker and the OptiPush results.

Table 1: Comparison of PushTracker and OptiPush measurements

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Survey results indicated positive user response to the PushTracker. Nine of the ten subjects said they found the PushTracker information valuable and would consider buying one. In addition, the majority of subjects said they would use the PushTracker to monitor exercise and activity level. Based on the results of this study, we believe the PushTracker will appeal to most wheelchair users.

COST

Table 2: Prototype Costs, production costs will be lower

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IMPLICATIONS

The PushTracker gives users the ability to monitor their pushing habits over time.  The capacity to monitor trends will empower users to take control of their own health and mobility.

ACKNOWLEDGEMENTS

We would like to thank Mark Richter, Liyun Guo, Josh Curtis, Ken Shafer, and Andrew Kwarciak for their help in the design and implementation of the PushTracker.

REFERENCES

1. Weil E, Wachterman M, McCarthy EP et al. Obesity among adults with disabling conditions. JAMA 2002;288:1265-8.

2. Bauman WA, Spungen AM, Raza M et al. Coronary artery disease: metabolic risk factors and latent disease in individuals with paraplegia. Mt Sinai J Med 1992;59:163-8.

3. Sie IH, Waters RL, Adkins RH, and Gellman H. Upper extremity pain in the postrehabilitation spinal cord injured patient. Arch Phys Med Rehab 1992;73:44-8.

4. Boninger ML, Cooper RA, Baldwin MA, Shimada SD, and Koontz AM. Wheelchair pushrim kinetics: body weight and median nerve function. Arch Phys Med Rehab 1999;80:910-5.

5. Consortium for Spinal Cord Medicine. Preservation of upper limb function following spinal cord injury: A clinical practice guideline for health-care professionals. Consortium for Spinal Cord Medicine: Clinical Practice Guidelines. 2005.

6. http://max-mobility.com/products/optipush