The Rear Wheel Big Deal
Kendra Betz, MSPT
VA Puget Sound / University of Washington / Independent Consultant
The rear wheel deserves critical consideration for manual wheelchair configuration, performance, and skills training. A comprehensive client evaluation will reveal key information for determining appropriate choices in rear wheel orientation, dimensions, components and education needs. Published research findings surrounding rear wheel interaction with the chair, propulsion mechanics, and injuries associated with wheelchair use, carry significant implications for best practice surrounding manual wheelchair prescription. The PVA supported Clinical Practice Guideline(CPG): Preservation of Upper Limb Function Following Spinal Cord Injury” (1) provides a thorough review of pertinent research and will be referenced relative to rear wheel topics. Understanding the many options available for rear wheels and making appropriate selections for each individual will greatly improve functional outcomes when prescribing and fitting manual wheelchairs.
Manual wheelchairs vary in the availability of rear wheel adjustments and configuration options. The ultralight manual wheelchairs (HCPCS K0005) are the only chairs that allow horizontal adjustment of the rear wheel forward and a wide variety of rear wheel options. Within the class of ultralights, rear wheel position options vary from highly adjustable to fixed. Dependent on chair design, some chairs allow rear wheel vertical adjustment via moving the wheel while others utilize adjustment of the seat height relative to the wheels to change vertical orientation. Many ultralights offer partial adjustability of the rear wheel where the wheel can be adjusted in the horizontal and lateral dimensions for a fine tuned fit while the vertical dimension remains fixed. Critical consideration of rear wheel position in all dimensions is necessary to properly prescribe and fit manual wheelchairs.
The position of the rear wheel in the horizontal plane has specific implications for push mechanics. Per CPG recommendation #8, the rear wheel should be adjusted as far forward as possible without compromising rearward stability. With a forward wheel position, rolling resistance is decreased (2), the hand contact angle with the handrim is increased (3) and propulsion requires less muscle effort with a smoother joint patterns and lower stroke frequencies (4). Boninger et al. (5) demonstrated that a forward wheel position results in lower peak forces, less rapid loading of the pushrim, fewer strokes and greater contact angles. Mulroy et al. recently demonstrated lower subacromial shoulder forces with the seat behind the wheel. (6). Adjustment of the wheel forward creates inherent rearward instability. Observation while pushing in varied environments (i.e. levels, inclines, unevens) and with usual chair conditions (i.e. backpack loaded on backrest) is necessary to ensure safety. The orientation of the seat and back carry significant implication for chair stability and must therefore be addressed prior to wheel adjustments. Wheelchair propulsion evaluation and training allows the individual to maintain chair stability with the rear wheel forward. One way to quickly test ideal position of the rear wheel horizontally is a straight push test. If the wheelchair front end pops up with level pushing, the chair is likely to be rearward unstable. The amputee axle plate must be discussed relative to horizontal wheel position. The amputee axle plate was designed to prevent rearward instability. However, a rearward position of the wheel necessitates propulsion with the shoulder in extension, abduction and internal rotation with the wrist in excessive extension which is not a desired push position. Clinical experience reveals that a well configured wheelchair frame and seating system provides rearward stability without the need for an amputee axle plate even for individuals with bilateral above knee amputations.
Rear wheel horizontal position must also be considered relative to mobility skills. The rear wheel in the forward position causes increased rearward tippiness which is desirable for performing wheelie skills. Attempting a wheelie with the wheel in a rearward, stable position creates an unnecessary challenge; therefore the wheel should be adjusted forward before attempting to teach or learn wheelie skills. Client transfers are also impacted by the rear wheel position. The wheel in the recommended forward position reduces the amount of forward frame available for transfers. In most cases, transfer training by a skilled clinician empowers the wheelchair user to implement safe transfer techniques despite the forward wheel position.
Vertical orientation of the rear wheel must also be critically evaluated. CPG recommendation #9 indicates that the rear axle be positioned vertically so that when the hand is placed at top dead center of the pushrim, the angle between the upper arm and forearm is between 100 and 120 degrees. This is supported by two studies (5,7). Clinical experience indicates a strong correlation with that recommended angle and the center of the middle finger at the center of the axle. A 2001 study supports the concept of minimizing the distance between the shoulder and the center of the wheel hub (8). The vertical position of the rear wheel is directly linked with rear seat to floor height. As the axle is raised, the seat is lowered and conversely when the axle is lowered, the seat is raised. While lower seat heights give greater access for more efficient and joint protective propulsion, a seat that is too low increases the risk of upper extremity injury (1). Seat height must be considered in addition to elbow angle when determining an ideal vertical placement for the rear wheel.
Lateral orientation of the rear wheel includes selection of camber settings and orientation of the wheel next to the frame. Determination of ideal camber is individual for each person and is based on a balance between chair maneuverability, architectural limitations and personal preference. A moderate amount of camber allows improved ease of turning. With increased camber, lateral chair stability is improved while rearward stability is impaired (9). Greater amounts of camber further increase ease of turning, but may impair straight line pushing and greatly increase the overall width of the chair. Two studies found that increased camber results in alteration of push kinetic parameters (10,11). Clinically, camber settings between 3 and 6 degrees are preferred by most full time users. Some chairs offer dual camber or multi-position camber adjustments that may be beneficial as long as the increased weight is justified. For optimal push mechanics, the superior aspect of the wheel should be as close to the person’s body as possible. Narrowing of the rear wheel lateral footprint may require moving axle receivers toward midline and removal of armrests when not required for client stability.
The selection of rear wheel size is based on client evaluation, environmental and architectural profile, interface with the wheelchair frame and client preferences. Typically, chairs designed for adults can accommodate a range of wheel sizes from 22” to 27” with 24” being the most commonly prescribed. Height of the rear seat must be considered in conjunction with the roll resistance differences for varying tire sizes. Smaller wheels position the seat lower to the ground, however require more force and a greater push frequency to propel over a given distance. Larger wheels position the rear seat higher and require less force to propel given the longer lever arm between the handrim and the center of the axle. In a chair with the rear axle position fixed in the vertical dimension, the rear wheel size cannot be changed as any alteration in rear wheel diameter shifts the front caster housing from perpendicular. Wheel size relative to camber must be considered as alterations in camber have the same impact as changing the wheel size. Increased camber lowers the rear wheel height. A strategy to maintain rear wheel height with greater camber is to add a larger profile tire to accommodate for the vertical height loss.
There are a variety of options available in rear wheel materials and design. The most basic is the composite or “mag” rear wheel which requires little maintenance, yet is heavy and offers little shock absorption. Steel spoked wheels are lighter weight and provide shock absorption but require consistent maintenance to preserve the tension and alignment. There are currently many lightweight wheels available which are advertised as being more durable and require less maintenance while providing a smooth ride and easy push. While there are many apparent benefits of these attractive wheels, a key consideration is cost with many current wheels approaching $1000/pair (US). A recent study compared Spinergy wheels with a standard spoke wheel relative to efficiency with straight line wheeling and comfort. Results indicated no significant difference in wheeling efficiency, however ride comfort was significantly better with the Spinergy (12). Lighter weight wheels are easier to stow and reduce the overall weight of the mobility system. With the many options that exist in rear wheel materials and design, attention must be directed toward the wheelchair user’s needs, preferences and available funding.
Tires have many implications for overall chair maneuverability, efficiency of propulsion and chair configuration. There are a multitude of tire options available for manual wheelchairs which vary in size, material, inflation and tread. Selection of tires will be based on the user’s needs and preferences with a balance between efficiency and maintenance. By intuition, high pressure treadless tires (i.e.“primos) create less roll resistance than a high tread mountain tire. However, significantly less traction is available in a treadles tire compared to other choices. There are a variety of solid tires available that vary in width and tread. A 2004 study (13) found that two different solid tires demonstrate more roll resistance than three pneumatic tires with significant air loss. Attention to tire pressure for pneumatics is critical as tire pressures below 50% of recommended inflation result in an additional 25% increase in energy expenditure during wheelchair propulsion (14). Relative to chair configuration, tire tread must be considered. Changing to a tire with greater tread increases the overall diameter of the wheel which consequently will impact rear seat to floor height and front caster housing alignment. When changing from a low tread to high tread tire, a smaller wheel may be required or increased camber added to maintain chair alignment.
Rear wheel alignment in three planes is mandatory for allowing efficient propulsion and preventing pull of the chair to one side. The two wheels must be symmetric in all dimensions. The center of the right and left rear axles should be the same distance forward from the rear of the frame in the horizontal dimension. The left and right vertical axle positions must be level. The lateral position of the wheels must also be symmetric which requires that two specific adjustments be addressed. First, the axle tube must be centered under the frame. Second, the axle receivers mounted to an axle tube or axle plate must show equal number of exposed threads. A specific alignment issue relative to camber is toe-in/toe-out. The distance between the left and right center vertical height of the front of the tires should equal to the distance between the left and right center vertical height of the rear of the tire. Any asymmetry in rear wheel orientation impairs the wheelchair user’s ability to propel. When the rear wheel is moved, subsequent secondary adjustments are necessary to preserve chair configuration.
In attempt to improve mobility and decrease pain and injury for individuals who propel manual chairs, add-on rear wheel options that provide assistance during propulsion are available. One option is the Pushrim Activated Power Assist Wheel (PAPAW). These battery powered wheels provide supplemental power output when the handrim is engaged. Studies have shown that use of PAPAW’s provides significant benefit to wheelchair users (15,16). Another add-on option is a recently developed geared 2-speed wheel that allows the individual to switch between standard pushing and a low gear with a 2:1 torque ratio that assists propulsion. Preliminary findings of a recent investigation indicate that shoulder pain is reduced with the use of this geared wheel (17). Considerations for add-on wheels include additional weight, ease of operation, maintenance, and need for additional wheelchair skills training.
Wheelchair skills training and joint protection education are critical for maximizing independent wheelchair use with safe mobility techniques. With prescription, issue, and adjustment of a manual chairs, client education for push techniques is needed. Research indicates that the client should use long, smooth strokes that limit high impact on the push rim and a semi-circular push pattern (18). Additionally appropriate skills should be addressed for chair negotiation in varied environments and terrains, and wheelie skills should be maximized for appropriate candidates. Wheelchair users require an appropriate flexibility and strengthening program with stretching of the anterior shoulder musculature and strengthening of posterior musculature which is supported by CPG recommendations 17 and 18.
The handrim is an integral component of the rear wheel that has significant implications for chair propulsion and upper extremity injuries. Handrim selection must be combined with propulsion and skills training to decrease the risks of injury associated with the repetitive task of pushing a chair. Handrims vary in materials, size and tube diameter with several options for friction coating to allow improved grip. Custom designed and manufactured handrims are available for individuals with unique needs. More recently, several ergonomically designed handrims are commercially available. One is the Natural Fit Handrim (NFH) distributed by Three Rivers Holdings, LLC . The NFH provides an oval shape anodized rim with a contoured trough next to the wheel which allows a neutral position of the hand during propulsion. Research studies support the use of this ergonomic handrim for addressing upper extremity injury risk and treatment in manual wheelchair users (19,20). Appropriate handrim selection combined with propulsion training is supported by CPG recommendations #4 and #5.
Objective measurement of wheelchair propulsion parameters is available for clinical settings. The Clinical SmartWheel (Three Rivers) mounts to most manual wheelchairs and interfaces with a computer via Wi-Fi technology to collect and report propulsion data. Parameters measured include time, distance, average speed, highest speed, number of pushes, peak forces for forward and backward propulsion, speed, off-rim acceleration, speed/push frequency ratio, push length, push frequency, peak/average force ratio, average push force, push mechanical efficiency, peak forces for individual pushes. A standardized protocol has been developed for use of the SW in clinical setting. Appropriate clinical applications include client education for proper push mechanics, objective data comparisons for wheelchair selection and configuration and power mobility justification for those with marginal ability to push, all of which are supported by the CPG (21).