Push for Power

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March 2004

By Rory A. Cooper, PhD; Rosemarie Cooper, MPT, ATP; Mark Schmeler, MS, OTR/L, ATP; and Michael L. Boninger, MD

Rory A. Cooper, PhD

Until recently, people who required a wheelchair had three basic types of products available to them: a manual wheelchair, an electric powered wheelchair, or a scooter. All three are helpful to people and provide valuable mobility. However, a number of compromises are required with each type of mobility device, and there was a need for a new class of device that was lightweight, maneuverable, and simple to transport. From this need arose the pushrim activated power assisted wheelchair (PAPAW).

A pushrim activated power assisted wheelchair uses motors and batteries to augment the power applied by the user to one or both pushrims during propulsion or braking. Applying a torque to the pushrim activates the wheelchair. The torque applied to the pushrim is amplified by the motors and gear-train. A microcontroller operates the rear wheels. Software simulates inertia (ie, allows the wheels to coast between strokes), compensates for discrepancies between the two wheels (eg, differences in friction), and provides an automatic braking system activated when applying a reverse torque to the pushrims. The PAPAW represents an entirely new class of wheelchair.

Why a PAPAW?
Some people cannot functionally propel a manual wheelchair, while at the same time they may have barriers preventing them from using an electric powered wheelchair effectively. Until recently, these clients needed to make a difficult decision between manual or powered mobility. However, with the introduction of pushrim activated power assisted wheelchairs, there is another option that may be most appropriate for some clients. The PAPAW novel control technology amplifies the force applied to the pushrim to propel or brake the wheelchair. This has the net effect of lowering the force that needs to be generated by the user’s upper extremities and hence reducing the strain on the arm joints. Because the user and the PAPAW work in tandem, the motors and batteries can be smaller and lighter weight. The reduction in weight, as compared to an electric powered wheelchair, permits the use of an ultralight wheelchair (K0005) frame. Therefore, the overall PAPAW system may weigh as little as 45 pounds, which is considerably lighter than an electric powered wheelchair.

PAPAWs operate much like manual wheelchairs, although they are heavier and more difficult to transport. For people who have been using a manual wheelchair and who have developed or are at risk for upper extremity joint degeneration, a PAPAW may be the most appropriate wheelchair. It is well established that about 30% to 70% of long-term manual wheelchair users eventually develop a degenerative condition of one of the upper extremity joints that often leads to pain limiting their mobility. A PAPAW can provide mobility similar to that of a manual wheelchair with less stress on the arm joints, while at the same time offering some of the aerobic exercise benefits of manual wheelchair propulsion. PAPAWs have been shown to reduce the amount of force required for propulsion, to lower the stroke frequency, and to reduce the range of motion required for efficient propulsion.

There are also clients who desire a manual wheelchair, but who have difficulty negotiating ramps, side slopes, soft surfaces (eg, carpet), and other mobility barriers. These individuals may have impaired upper extremities, or their limitation could be due to disease- or injury-related reduction in cardiorespiratory capacity. A PAPAW may provide the assistance needed to negotiate obstacles independently and to propel distances necessary to perform functions of everyday living. While it is difficult for the wheelchair user to load a PAPAW into a car or van that is not equipped with a lift or ramp, it is not difficult for an assistant. This makes a PAPAW attractive for individuals who use transportation with an assistant such as a family member or friend.

Studies have shown that manual wheelchair propulsion efficiency is between 5% and 18% depending on the style of the wheelchair and the fit to the user. The low efficiency of manual wheelchairs can make them ineffective for some individuals to use during activities of daily living. Manual wheelchair users also experience upper extremity pain and joint degeneration. Between 25% and 80% of long-term manual wheelchair users are reported to have injuries to the wrist, elbow, or shoulder. The risk of injury tends to increase with age, while cardiovascular fitness tends to decrease. These issues provide compelling reasons to consider a PAPAW for a client. Besides electric powered wheelchairs and scooters, several other alternatives have been developed with varying levels of success, including lever-drive units, crank drives, and geared hubs. However, none of these approaches have been found to be practical or acceptable to consumers.

The right PAPAW client
People who have severe difficulty walking or who have the inability to walk should be evaluated for a PAPAW. In order to use a PAPAW effectively, the individual must have the coordination and strength to propel a manual wheelchair over a smooth hard floor with at least one arm. The PAPAW is beneficial for reducing stress on the upper extremities, increasing propulsion efficiency, and lowering the cardiopulmonary demand of wheelchair propulsion. Arguably, nearly every manual wheelchair user is a potential candidate for a PAPAW due to the high risk for joint and cardiovascular degeneration. Certainly, individuals who experience pain or fatigue negotiating ramps, carpets, or outdoor surfaces are ideal candidates for a PAPAW. Individuals with spinal cord injury or dysfunction, people with cerebral palsy, and elderly people may benefit from a PAPAW.

There are basically two types of PAPAWs: those that retrofit to an ultralight manual wheelchair and allow conversion from a manual wheelchair to power assist, and those that are integrated into the complete wheelchair package. A retrofit PAPAW is typically assembled by fitting an ultralight manual wheelchair with PAPAW wheels using customized hardware. Most PAPAW wheels use quick release axles (ie, axles that allow the wheels to be removed without tools). The retrofit PAPAWs will accommodate standard wheelchair wheels in order to serve as a manual wheelchair as well. PAPAWs have features that are either selectable (ie, available from the manufacturer) or tunable (ie, may be altered by the supplier or a clinic). When evaluating a person for a PAPAW, all of the considerations given to fitting a manual wheelchair must be reviewed.

However, a PAPAW has several additional features that are much like those of an electric powered wheelchair. The amount of assistance must be determined. Too much assistance may make the PAPAW difficult to control, whereas too little assistance may not allow the user to perform critical activities. PAPAWs also assist with braking. Braking assist is critical, as users need to be able to safely descend any ramp that they can climb. In some PAPAWs, the amount of assistance that is provided to either side can be adjusted. This is helpful for compensating for asymmetrical strength or range of motion. Some PAPAWs allow for communication between the two wheels. This has the advantage of allowing them to be used to create a one-arm-drive wheelchair with assistance from the factory. It also has the added safety feature of being able to compensate for the loss of assistance from one wheel. When the two wheels communicate with one another, compensation can be used to help keep the wheelchair going straight or to turn smoothly.

There are some special considerations when selecting a PAPAW for an individual. Because of the power added by the motors, it is much more difficult to perform a “wheelie” in a PAPAW than a manual wheelchair. This requires applying a different technique when climbing curbs or similar obstacles. With a typical manual wheelchair, the user pops a “wheelie” and approaches the curb with some momentum. With a PAPAW, the individual approaches the curb and then turns the power off—essentially converting the PAPAW to a manual wheelchair—to pop a “wheelie” and set the front casters on the top of the curb. Once the wheels are on the curb, the person activates the PAPAW and applies torque to the pushrim while leaning forward. The clinician and client must work together to determine the best means of performing activities with a PAPAW.

One client’s experience
Carl is a 40-year-old man who was diagnosed with multiple sclerosis (MS) about 10 years ago. Carl’s condition has been stable for several years. He has occasional exacerbations of his condition. Carl fatigues readily and can propel his ultralight manual wheelchair for short distances on smooth hard surfaces without requiring any assistance. He is incapable of climbing ramps, curb cuts, or hills without assistance. He is unable to hold a “wheelie.” He rents an apartment and lives with his wife and two children. His wife and friends help him with driving and load the wheelchair in their car for him. Carl can walk for a few steps, and he has good sitting balance. He is very intelligent and trained as an engineer. Carl is currently employed, and his job requires moderate travel primarily by airline. He needs a wheelchair that he can take with him when he travels. He typically travels with his wife or a colleague.

Carl has come to an assistive technology clinic in order to replace his manual wheelchair, which is 5 years old. The clinician must consider Carl’s current mobility skills, his living arrangements, and the availability of assistance when helping Carl to select his new wheelchair. Carl is an excellent candidate for a PAPAW. He requires a wheelchair that will help him climb and cut curbs, ascend and descend hills, and push around the office and community. His inability to hold a “wheelie” is further indication of his marginal manual wheelchair skills. Carl needs a wheelchair that his wife or friends can load into their cars. The clinician must also consider that Carl is likely to develop upper extremity pain if he continues to use a manual wheelchair. Furthermore, his MS may progress to where he will be unable to function in a manual wheelchair before he is prepared to transition to an electric powered wheelchair. Since Carl rents an apartment, it is likely that he would need to move if he were to begin using an electric powered wheelchair or scooter.

Carl and the clinician decide upon a PAPAW based on an ultralight manual wheelchair. The PAPAW will help Carl to remain independent, to continue to be gainfully employed, and to participate in family and community activities.

At the University of Pittsburgh Medical Center, Rory A. Cooper, PhD, is chairman, Department of Rehabilitation Science and Technology; Rosemarie Cooper, MPT, ATP, is an evaluation specialist for the Center for Assistive Technology and an instructor for the Department of Rehabilitation Science and Technology; Mark R. Schmeler, MS, OTR/L, ATP, is director of the Center for Assistive Technology; and Michael L. Boninger, MD, is medical director of the Human Engineering Research Laboratories, and associate professor and research director, Department of Physical Medicine and Rehabilitation.

Arva J, Fitzgerald SG, Cooper RA, Boninger ML. Mechanical efficiency and user power requirement with a pushrim activated power assisted wheelchair. Med Eng Phys. 2001;23(10):699-705.
Boninger ML, Dicianno BE, Cooper RA, Towers JD, Koontz AM, Souza AL. Shoulder magnetic resonance imaging abnormalties, wheelchair propulsion, and gender. Arch Phys Med Rehabil. 2003;84(11):1615-1620.
Boninger ML, Baldwin M, Cooper RA, Koontz A, Chan L. Manual wheelchair pushrim biomechanics and axle position. Arch Phys Med Rehabil. 2000;81(5):608-613.
Boninger ML, Cooper RA, Baldwin MA, Shimada SD, Koontz A. Wheelchair pushrim kinetics: weight and median nerve function. Arch Phys Med Rehabil. 1999;80(8):910-915.
Cooper RA, Corfman TA, Fitzgerald SG, et al. Performance assessment of a pushrim activated power assisted wheelchair. IEEE Transactions of Control Systems Technology. 2002;10(1):121-126.
Cooper RA, Fitzgerald SG, Boninger ML, et al. Evaluation of a pushrim-activated, power-assisted wheelchair. Arch Phys Med Rehabil. 2001;82(5):702-708.
Cooper RA. Wheelchairs: A Guide to Selection and Configuration. New York: Demos Medical Publishers; 1998.
Cooper RA, Quatrano LA, Axelson PW, et al. Research on physical activity and health among people with disabilities: a consensus statement. J Rehabil Res Dev. 1999;36(2):142-154.
Corfman TA, Cooper RA, Boninger ML, Koontz AM, Fitzgerald SG. Range of motion and stroke frequency differences between manual wheelchair propulsion and pushrim-activated power-assisted wheelchair propulsion. J Spinal Cord Med. 2003;26(2):135-140.

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