Ask a Clinician: How Design Impacts Ultralightweight Propulsion
Editor’s Note: Many users of ultralightweight manual wheelchairs self-propel for years… even for decades. That makes efficient self-propulsion a critical factor that can affect a wheelchair user’s independence and health.
Christie Hamstra, PT, MSPT, DPT, ATP, is a Clinical Education Specialist with Motion Composites, and has worked both as a physical therapist in a seating clinic and as an ATP supplier who specialized in seating and wheeled mobility. Mobility Management asked Hamstra these questions about efficient propulsion and ultralightweight wheelchairs.
Q: Why is it important to learn efficient propulsion mechanics from the very start? What’s an example of an efficient propulsion practice?
Christie Hamstra: Shoulder preservation, coincidently, falls on the shoulders of wheelchair users. The user must incorporate strengthening; optimal equipment and setup; and proper propulsion techniques from the start. The evidence shows 40 to 100 percent of individuals who utilize a manual wheelchair as their primary means of mobility will develop shoulder problems after use over a prolonged period of time. The research explains the degree of pain/derangement/injury depends on length of use of the manual wheelchair. For long-time wheelchair users, it isn’t a matter of “if” the individual will develop pain, but when.
Prevention through proper wheelchair prescription is extremely important, as shoulders were not made to be weight-bearing joints. Wheelchair users can prevent or delay onset and/or reduce severity of shoulder pain by utilizing an exercise program, optimally configured wheelchair, and proper propulsion techniques. Common injuries that full-time users report include rotator cuff impairment, impingement, and carpal tunnel, according to Lighthall-Haubert et al 2009.*
Taking into consideration what is at stake, it is extremely important for users to maximize propulsion and be as efficient as possible from the start. The average number of propulsion strokes for a full-time manual wheelchair user is 2,000 to 3,000 per day. This, in addition to all of the overhead and upper-extremity tasks for activities of daily living (ADLs), makes efficiency even more important. However, it is impossible to separate efficiency of propulsion and stroke patterns from proper setup and fit of the wheelchair. But assuming appropriate dimensions of the chair and optimum configuration, there are more efficient strokes in certain situations than others.
The gold standard of propulsion is the “semi-circular” pattern. This preferred hand stroke consists of grabbing the wheel at 10 o’clock, releasing at 2 o’clock, and dropping the hand down under rim for a recovery phase, like a locomotion wheel, or “choo-choo” for pediatrics. Hardly an inherent movement, this skill must be learned, practiced, and reassessed. Often users may default to “arc,” which covers the least amount of stroke length, especially if the chair is not set up in an optimal position. This usually starts at about 12 o’clock, releases at 2 o’clock, and recovers by tracing the hand along the push-rim back to start. This inefficient default stroke subjects the shoulder to unnecessary forces and posturing over long distances, as well as drastically increases the total stroke count throughout the day. However, this is the ideal stroke for traveling across inclines because of the sustained contact with the rim.
Regardless of the stroke selected, shoulders should remain in a neutral position during strokes. There should not be excessive shoulder elevation throughout propulsion. If this occurs, there will need to be adjustments to the vertical height of the rear axle, or a smaller rear wheel, depending on individual needs and goals.
Proper wheelchair selection, setup, and propulsion patterns do make a difference in maintaining shoulder health and can significantly delay onset of shoulder issues. The importance of this multi-faceted approach with professionals and end users cannot be underemphasized.
Q: How does wheelchair design impact the wheelchair’s ease of propulsion? For example, is a rigid chair always easier and more efficient to propel than a folding wheelchair? Does the distribution of the user’s body weight and the center of gravity/mass make a difference to efficient propulsion?
Hamstra: A rigid wheelchair is inherently more efficient than a folding wheelchair. They are generally lighter in weight, have fewer moving parts, and roll with less resistance. Therefore, less forward momentum can be lost into the frame itself. Movement in the frame and crossbrace can create torsion; therefore, a folding frame by definition is going to have torsion, which leads to inefficiency.
Balance and proper weight distribution is important. Engineering of the frame, both rigid and folding, can help to keep proper weight distribution. Rigidizing bars on the rigid frame, and a symmetrical cross brace on a folding frame do assist with proper weight distribution.
Proper setup of the center of gravity is extremely important, and research shows it can be one of the most important measurements to take in ensuring the chair is set up efficiently. The center of gravity should be as far forward as possible without compromising the safety of the user. A farther-forward center of gravity will allow the chair to appear lighter and roll easier. However, if the chair is popping the casters off the ground with each stroke, the center of gravity is too aggressive.
When the user’s center of gravity is well balanced over the rear wheels, the rolling resistance reduces, and the load is easier to move, like a wheelbarrow. By configuring the center of gravity on the chair in the forward direction, the chair can feel up to 10 lbs. (more than 4.5 kg) lighter to the wheelchair user. Yet this setup and measurement of center of gravity is often overlooked and left to the manufacturer’s default setting. There are people propelling themselves in the lightest state-of-the-art chairs on the market, but they aren’t feeling the benefits because the center of gravity (COG) isn’t optimized.
Q: Why is it important for ultralightweight wheelchair users to have a wheelchair that’s fit especially to their measurements and needs?
Hamstra: It is extremely important for users to have a chair that is fit to their specific measurements, and to the functional needs of the client. Many physical and occupational therapists are taught in school to “measure the hips and add an inch to either side.” This could end up leaving the chair a full 2 inches (5+ cm) too wide, putting that user at an even higher risk of injury and decreased efficiency. For every inch of additional width that is not necessary in a wheelchair, propulsion efficiency is decreased by 10 percent. The argument is “They’re going to grow, they’re going to wear a winter coat.” It is important to fit the chair to what the user will be need 90 percent of the time, not the 10 percent when they may need the winter coat.
Seat depth also needs to be determined due to its effect on form and function. Consideration for foot propulsion, seat slope, and backrest selection are imperative. If the seat depth is too long, the user will inevitably slide forward, causing a shift in the center of gravity. This shift may affect pelvic posture, wound risk, and shoulder health and function during propulsion.
Camber is another way to get the wheels in toward the body, and to allow a more natural push pattern with the upper extremities. Camber can decrease strain on the shoulders and upper extremities by allowing the wheels to be in better alignment with the body.
Proper fit in combination with appropriate handrim selection can enhance push for a user with tetraplegia.
It cannot be said enough that any user who will utilize a manual wheelchair for an extended period of time (greater than 6 months) per the RESNA wheelchair provision guide should be fitted by a therapist and an ATP who specialize in seating and mobility. This will allow them to provide a custom chair that will be optimally configured to be as lightweight, fitted, and efficient as possible.
The Consortium of Spinal Cord Medicine (2005): “Preservation of Upper Limb Function Following Spinal Cord Injury: A Clinical Practice Guideline for Healthcare Professionals.”
RESNA Position Paper (2012): The Application of Ultralight Manual Wheelchairs.
Sawatzky, B. et. al. “The Need for Updated Clinical Practice Guidelines for Preservation of Upper Extremities in Manual Wheelchair Users: A Position Paper.” American Journal of Physical Medicine and Rehabilitation, 2014.
Medola, FO. et. al. Aspects of Manual Wheelchair Configuration Affecting Mobility: A Review. Journal of Physical Therapy Science. 26: 313-318, 2014.
Freixes O, Fernandez SA Gatti MA. Crespo, MJ, Olmos LE & Rubel IF (2010). Wheelchair axle position effect on start-up propulsion performance of persons with tetraplegia. Journal of Rehabilitation Research & Development, 47(7): 661-668.
About the Author: Christie Hamstra is a Clinical Education Specialist with Motion Composites. She received her Master’s of Science in Physical Therapy from Andrews University, and a transitional Doctorate of Physical Therapy from Oakland University. Hamstra has provided education throughout the United States, and at the European Seating Symposium and the Canadian Seating & Mobility Conference. This story is sponsored by Motion Composites.