Wading Through the Possibilities

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April 2002

By David Kreutz, PT, and Chris Maurer, MPT

Today, the number of options to consider when choosing power mobility can be overwhelming, especially with advances in technology. Power mobility options fall into several different classifications: scooters, power assist, power add-on, conventional power wheelchairs, and power bases.

Power Mobility Options

Scooters typically have long, narrow bases with a tiller for negotiation and a swivel seat mounted on a single pole. Most scooters are designed to allow the seat and batteries to be removed and the tiller to be folded down for transport. Positioning options on scooter seats are limited for people with significant deformities or decreased motor control. Thumb or finger control is required to maneuver the scooter.1

The pushrim activated power assist units allow manual wheelchair users the option of powered mobility when necessary. The power assist unit is composed of two wheels with motors and batteries enclosed in the hubs, which attach to the manual wheelchair frame. The user propels the wheels using the pushrim, but is provided with a power assist to decrease the effort required to traverse the same distance as with the traditional method of propelling a manual wheelchair. The degree of sensitivity and power of the units can typically be adjusted to the user. The units can also be adjusted to compensate for discrepancies in left and right upper extremity strength. A study by Arva et al2 determined that using the pushrim activated assist wheels reduces the user’s metabolic demand, requires less power generation, and increases the user’s mechanical efficiency compared to manual propulsion. These wheels are quick-release to allow easy transfer from the traditional manual wheels to the power assist wheels. Each wheel with the battery pack weighs approximately 20 pounds more than the standard wheel, and may present management problems when loading the wheelchair. Not all manual wheelchair frames can accept the power assist units, so care must be taken when choosing a final product.

Power add-on units have recently resurfaced. They are also an alternative means of providing power mobility to manual wheelchair users. These units are made up of in-wheel motors with an external battery pack and joystick controller that are added onto a manual wheelchair frame.3 When choosing the power add-on unit, the manual wheelchair frame may have to be reinforced to handle the extra weight. This may alter the characteristics of propulsion in the manual wheelchair mode. Another type of power add-on unit is a power trike, which is a third motorized wheel with throttle controls that can be added to a manual wheelchair.

A conventional power wheelchair consists of an integrated frame and seat, batteries, an integrated controller, and an input device. Off-chair seating components have been designed to provide custom seating options using the standard power wheelchair seat.3 Conventional power chairs typically have a cross frame and some can be disassembled and folded for transport. Most conventional power wheelchairs are rear-wheel drive. A consideration with conventional power wheelchairs is that the overall width of the chair increases with the seat width.

Rigid frame power base wheelchairs have gradually replaced conventional power wheelchair frames. Power bases are simply a frame with the motors, controller, batteries, and wheels on which a seating system is placed. Power bases tend to offer higher performance than conventional power wheelchairs.3

Performance Guidelines

The advent of the microprocessor (controller), the invention of new switches, and third-party funding are the chief reasons that power mobility is becoming a viable option for more and more individuals with impaired motor control. It was not so long ago that a joystick or a tiller represented the only option for operating a power wheelchair or scooter. Today, people with only one or two movements can potentially drive a power wheelchair if they have the desire and can develop the necessary skills.4

Power wheelchair performance is based on the basic design of the wheelchair and how the wheelchair is programmed to respond to user commands. There are three basic design categories for power wheelchairs: front wheel, mid-wheel, and rear wheel drive. Each of these designs has its advantages and disadvantages.

Performance of the power wheelchair will also depend on how the wheelchair control module has been adjusted. Programming or fine-tuning the wheelchair to meet the needs of the individual will improve control, safety, and general satisfaction with the wheelchair. Fast and simple adjustments can be made to set forward, reverse, and turn speeds, acceleration and deceleration, and sensitivity of the joystick. These basic adjustments affect how the wheelchair will respond to different commands and are made to improve the patient’s control and safety. Each manufacturer may use different terminology, but the basic adjustments are present in nearly every power wheelchair.

Maximum speed varies significantly between different manufacturers and models. Some power wheelchairs are capable of going two to three times faster than other models. These wheelchairs are targeted toward outdoor use. Even the fastest wheelchairs offer four or more different drive modes so that the wheelchair can be programmed for slow, safe operation. A slow mode of operation is needed for maneuvering indoors or onto a van lift. Braking or deceleration is another critical adjustment. A too hard setting can cause patients to be thrown forward when they release the joystick. A too soft setting can cause the wheelchair to coast and not stop in time to avoid an obstacle. Sudden braking by pulling back on the joystick or accidentally turning the chair off can cause a patient to be thrown from the chair. In addition to properly programming the wheelchair, use of a properly fitted seat belt and properly adjusted footrests is essential to prevent falls or injury.5

Additional controller functions offer the patient improved control and improved efficiency of operation. For example, adjusting the joystick throw for patients with limited arm movement allows them to have access to full power and better control with smaller motion of their arms. Calibrating the intensity of hard and soft commands for a patient using a sip and puff input device is another controller function that can readily improve patient control and efficiency. Latch mode is yet another programming function that can be used to improve efficiency. Latch mode allows the patient to give a single command and experience a sustained response from the wheelchair. For example, by latching forward drive, a single forward command with the input device causes the chair to drive forward. The wheelchair continues to move in a forward direction until the patient either gives a stop command or activates a safety (mode/reset) switch. The advantage of latch mode is that it allows a patient with poor endurance more efficiency during operation of the wheelchair.

Today a controller can be quickly programmed to recognize a variety of input devices and alter the response for each device. Numerous preprogrammed options allow the therapist to quickly set up a wheelchair based on patient experience. Therapists also have the option of trying different input devices during the evaluation because of this plug and play technology. This technology allows the input device to be changed as the patient’s condition changes without having to replace the electronics or wheelchair. In addition to improving control and safety, these recent developments are also helping to reduce costs.

Input Devices

A safe, consistent, and reliable method of accessing power mobility can sometimes be the most difficult step of the evaluation. Input devices are not only used for controlling the wheelchair, but also to access environmental control systems and computers. Input devices are categorized as either digitally or proportionally controlled.

Proportional controls allow the control output sent to the wheelchair to be proportional to the deflection of the input device from a neutral position. Examples of proportional input devices include the standard joystick, chin control, head control, and drive control. Joystick placement is usually located at the hand, but can also be placed at the foot, chin, or head. The head control responds to forward and lateral flexion of the head. The drive control sensor can be placed anywhere on the head, hand, finger, or foot. The sensor needs to be positioned where it can move along an X-Y axis to allow forward/reverse, left/right movements of the wheelchair. The sensor can be set to work in an off-center position when the body part where it is located is unable to be positioned in an upright manner. Some type of single switch is required to activate the sensor.

Pneumatic switches (sip and puff), single switches, head array, voice command, and scanners are types of digitally controlled input devices. The user provides a command to activate the input device by a breath, tapping a switch or moving near a proximity switch, or using their voice to move the wheelchair in the desired direction. The chair will move at a predetermined speed until a reverse command is given (as in a latched mode) or the input to the switch is removed.

Success in controlling an input device is largely dependent on stable positioning within the seating system. Locations for input devices can include the head, headrest, chin, shoulder, elbow, arm, hand, knee, foot, tongue, and mouth, as well as utilizing voice or breath control. The placement site of the input device is crucial in regard to accuracy and consistency of using the device for safe, functional power wheelchair negotiation.

Safety Issues

Safe operation of a power wheelchair should include: the ability to turn the chair on and off independently; the ability to stop the chair and/or consistently and independently access a mode or reset switch when using latch mode to drive the wheelchair; user stability while driving the wheelchair; and knowing how the wheelchair will respond on different terrains or under different environmental conditions.

All power wheelchairs have a warning label regarding radio-frequency interference. The purpose of these labels is to warn patients about the possibility of the wheelchair moving on its own when the power is on, but the input device is not activated. It is important that the patient be able to turn the chair off immediately if the chair begins to move. Use of a reset/mode switch is mandatory when operating a wheelchair in latch mode, should the patient ever lose access to the switch being used to drive the wheelchair. The reset switch acts as an emergency stop switch and must be located so that the patient can activate it quickly and consistently.

Operator stability is key to preventing injury. Postural supports may be needed for stability while driving the wheelchair. Use of properly adjusted foot supports and a seat belt is strongly recommended to prevent falls from the wheelchair. Education regarding abrupt stops or the use of a switch guard would help to prevent this type of injury.4

The options available with power mobility today appear endless. Recommendation should not be done hastily. The positive aspect of the advances in power wheelchair technology is the opportunity offered to meet the needs of people with all types of disabilities.

References

1 Boyd B. Unlocking the facts about scooters. Paraplegia News. 1995;49(1):30-38.
2 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;23:699-705.
3. Cooper RA. Wheelchair Selection and Configuration. New York: Demos Medical Publishing Inc; 1998:227-251.
4. Cook AM, Hussey SM. Assistive Technologies: Principles and Practice. St Louis: Mosby; 1995:525-571.
5. Cooper RA, Dvorznak MJ, O’Connor TJ, Boninger ML, Jones DK. Braking electric-powered wheelchairs: effect of braking method, seatbelt, and legrests. Arch Phys Med Rehabil. 1998;79:1244-1249.

David Kreutz, PT, is the Seating and Mobility Clinic Coordinator, and Chris Maurer, MPT, is a senior physical therapist in the seating clinic at Shepherd Center, Atlanta.

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April 2002