A power add-on unit is a device that converts a manual wheelchair to a powered wheelchair. There are 3 different types of power assist devices: ones that attach to the front of the manual wheelchair to assist by "pulling" the wheelchair, ones that attach at the rear of the manual wheelchair to "push" the wheelchair, and ones that attach to the hub of the rear wheels to assist by multiplying the human force that is applied to the handrim. This last type generically is known as a handrim-activated power assist drive system and is the subject of this month's Clinical Corner.
Manual power assist wheelchairs, or handrim-activated power assist wheelchairs (HAPAW), are manual wheelchairs – either adjustable folding frame or rigid frame wheelchairs – to which motorized wheels have been added to provide power to the wheelchair. [Note: HAPAW previously were known as PAPAW when handrims were referred to as pushrims.] A manual power assist wheelchair may be considered a hybrid between a manual wheelchair and a power wheelchair1. A person using a HAPAW continues to hand propel the wheelchair using the handrims; however, motors in the hubs of the wheels are activated by the force on the handrims of the wheels. The motors in the hubs of the wheels engage only when there is force directed through the handrims. In essence, the motors allow for greater distance to be travelled with one push stroke compared to the use of a regular manual wheelchair.
Power assist wheels can be prescribed either at the time of the original manual wheelchair prescription or as an addition to an existing manual wheelchair. The quick release axles on the power assist wheels allow for the easy exchange between the power assist wheels and regular wheels on the manual wheelchair. Thus, a HAPAW can convert easily back to a manual wheelchair, if required.
The method by which the power assistance is delivered in a power assist wheel varies by manufacturer. Nevertheless, HAPAWs have been studied extensively by researchers and reported on in the literature. There have been new advances in the technology of the power assist wheels since many of these articles were written, and as such, perhaps even greater clinical benefits may be seen with the use of HAPAWs.
Manual wheelchair propulsion with the upper extremities puts people at risk of repetitive strain injury (RSI) due to the repetitive motions under force. A person moves not only their own body weight, but also the weight of the wheelchair and seating system. HAPAWs provide mechanical efficiency that reduces the risk of RSI. One study found that propelling a HAPAW required one-third less power to propel when compared to subjects' own manual wheelchairs at the same speed and resistance. The study also found that there was an increased mechanical efficiency in propulsion by an average of 80% when using the HAPAW.2 Another study found that the use of a HAPAW decreased the upper extremity joint range of motion in shoulder flexion and wrist extension and decreased the frequency of strokes taken when compared to propelling a subject's own manual wheelchair.3 By decreasing the upper extremity range of motion and decreasing the number of strokes required to propel a manual wheelchair, the risk of RSI is reduced.
Studies have also found that the metabolic demands associated with manual propulsion of a wheelchair are decreased when using a HAPAW.3,4,5 The average heart rate and oxygen consumption was lower for people using a HAPAW than compared to a regular manual wheelchair. For people who fatigue easily, the use of a HAPAW may assist to conserve energy to enable improved function in other areas, such as transfers or leisure activities.
When tested in the community, one study found that the average speed travelled was greater for individuals using a HAPAW versus a person's own manual wheelchair.6 This allowed individuals to propel the HAPAW at a "normal walking speed required to safely cross the streets."6 (p. 827).
In addition, the use of a HAPAW has been found to improve a person's ability to climb obstacles compared to manual wheelchair use.7 The use of a HAPAW enables an individual to propel over carpets, up steep inclines, and over uneven surfaces, such as grass and gravel, with less effort due to the mechanical advantage provided by the power assist wheels. When travelling along a flat surface, less assistance may be required by an individual. More assistance is provided to climb a slight incline and even greater assistance is provided to climb a steep incline. The HAPAW also assists to control the speed going downhill to ensure safety.
While there are many clinical benefits of using a HAPAW, there are some considerations that must be taken into account. One factor is that the overall width of the wheelchair is increased by the addition of power assist wheels as the small motors are in the hub of the wheels, adding to the width of the manual wheelchair. For some people, this added width may cause accessibility issues in confined spaces when compared to a regular manual wheelchair.
Another consideration is transportability. An advantage of a HAPAW over a power wheelchair is the ability to transport the mobility device without requiring any vehicle modifications. The power assist wheels can be removed from the manual wheelchair and the wheelchair and wheels can be lifted into the vehicle. It should be noted, however, that due to the weight of the wheels (approximately 17 pounds per wheel for one model of HAPAW), some individuals may not be able to lift the power assist wheels into a vehicle independently.6
There are many factors to consider when deciding if a HAPAW is an appropriate choice for an individual. Ding et al. (2008) provide a summary: "When choosing between a PAPAW and a manual wheelchair or a power wheelchair, it is important to consider a user's preference, life style, physical conditions such as the injury level and upper limb strength, and environmental factors such as accessibility conditions at home including steps at entrance, stairs, doorways, and maneuvering spaces, and transportation issues with motor vehicles" (p. 828).
While the above authors were studying people with tetraplegia, it is important to note that the use of a HAPAW can benefit not only people with tetraplegia, but also people who have an "arm injury, insufficient arm strength or low cardiopulmonary reserves. Also, subjects who have difficulty propelling a wheelchair in a challenging environment can benefit from power-assisted wheelchair use."1 (p. 1.)
Handrim-activated power assist wheelchairs have many clinical benefits for manual wheelchair users, from decreased risk of repetitive strain injury to improved ability to climb obstacles with fewer metabolic demands.
Please send your comments, questions, and suggestions for Clinical Corner topics to Sheilagh Sherman, OT Reg. (Ont.) at Sheilagh.Sherman@sunmed.com. Thank you!
- Kloosterman, M.G., Snoek, G., Van der Woude, L.H., Buurke, J.H., & Rietman, J.S. (2012). A systematic review on the pros and cons of using a pushrim-activated power-assisted wheelchair. Clinical Rehabilitation, 0(0),1-15.
- Arva, J., Fitzgerald, S.G., Cooper, R.A., & Boninger, M.L. (2001). Mechanical efficiency and user power requirement with a pushrim activated power assisted wheelchair. Medical Engineering and Physics, 23, 699-705.
- Algood, S.D., Cooper, R.A., Fitzgerald, S.G., Cooper, R., Boninger, M.L. (2004). Impact of a pushrim-activated power assisted wheelchair on the metabolic demands, stroke frequency, and range of motion among subjects with tetraplegia. Archives of Physical Medicine and Rehabilitation, 85, 1865-71.
- Haubert, L.L., Requejo, P.S., Newsam, C.J., & Mulroy, S.J. (2005). Comparison of energy expenditure and propulsion characteristics in a standard and three pushrim-activated power-assisted wheelchairs. Topics in Spinal Cord Injury Rehabilitation, 11, 64-73.
- Levy, C.E., Chow, J.W., Tillman, M.D., Hanson, C., Donohue, T., & Mann, W.C., (2004). Variable-ratio pushrim-activated power assist wheelchair eases wheeling over a variety of terrain for elders. Archives of Physical Medicine and Rehabilitation, 85, 104-112.
- Souza, D.D., Cooper, R.A., Fitzgerald, S.G., Kelleher, A., & Boninger, M.L. (2008). A preliminary study on the impact of pushrim-activated power-assist wheelchairs among individuals with tetraplegia. American Journal of Physical Medicine & Rehabilitation, 87, 821-829.
- Algood, S.D., Cooper, R.A., Fitzgerald, S.G., Cooper, R., & Boninger, M.L. (2005). Effect of a pushrim-activated power-assist wheelchair on the functional capabilities of persons with tetraplegia. Archives of Physical Medicine and Rehabilitation, 86, 380-386.
Note: The content of this article is not meant to be prescriptive; rather, it is meant as a general resource for clinicians to then use clinical reasoning skills to determine optimal solutions for individual clients. Sheilagh is unable to answer questions from members of the general public. Members of the general public are directed to their own therapists or other health care professionals to ask questions regarding needs.
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