Just like shifting gears on a bicycle, multiple gears on a manual wheelchair can make it easier to complete many tasks of daily living. We developed an automatic gear-shifting system (AGS) for manually propelled wheelchairs. The AGS has three speeds ??? first gear: for ascending slopes or traversing compliant surfaces, second gear: traditional direct-drive for everyday situations, and third gear: for improved ergonomics during fast propulsion. The AGS is packaged as an add-on set of two wheelchair wheels (each with its own 3-speed transmission connecting the hand rim and tire) and onboard electronics for gear shifting. A previous gear shifting design for manual wheelchairs (Magic Wheels, Magic Wheels, Inc.; Seattle, Washington), required the user to stop completely and manually shift gears, whereas the AGS can shift on the fly automatically. The Magic Wheels device was shown to significantly reduce the level of shoulder pain when used for long periods of time, so it is likely that the AGS can provide the same benefit but with increased usability. A prototype of the AGS was constructed and tested over the course of a year. Although many mechanical and electrical components experienced a number of issues, each was analyzed and redesigned. At the very least, clear guidelines were provided for future development. Additionally, a study was done to evaluate the AGS???s usability by assessing user perception of the AGS ability to reduce the effort of propelling a wheelchair. This study also documented changes in wheelchair propulsion metrics including task completion time, number of pushes, and push frequency. Eleven fulltime manual wheelchair users (ages 34 ?? 14 years) were recruited to compare the AGS to a traditional direct-drive configuration and rate their level of perceived exertion while completing 10 tasks of daily living. Nine participants perceived the AGS as easier for at least one task of daily living, and four participants rated the AGS as easier to use for more than half of the tasks. Subjects perceived a significant advantage when traversing up a steep slope and crossing an uneven doorway threshold. Interestingly, subjects with less trunk control (higher spinal cord injuries) consistently rated the AGS configuration more favorable than subjects with more trunk control. Although the low gear increased the time and number of pushes to complete a task, the AGS did not increase the push frequency of wheelchair propulsion which suggests that the design encourages optimal propulsion strategies. Gear-shifting may be an effective option for making wheelchair propulsion easier, especially for individuals with reduced trunk function.
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