It might seem like nothing: a rough patch of asphalt, a crack in the sidewalk, a pathway made of brick or cobblestone.
But to a wheelchair user, those bumps on the rolling surface cause jolts that add up quickly. “Bad vibes” can have an enormous impact on wheelchair ride and the long-term health of wheelchair users in ways that researchers are still working to understand.
The Science of Vibrations
There is a lot of science behind the vibrations that wheelchair users experience.
Josh Anderson is VP of product & brand development for TiLite and, as an ultralightweight wheelchair user, knows about wheelchair vibrations first hand. He makes an important distinction between vibrations and impacts.
“When we talk about vibration, we’re really talking about highfrequency vibration,” he notes. “That’s like uneven surfaces: asphalt, concrete or cobblestone. Cobblestone can also cause impact if it’s 200-year-old cobblestone and you’re in Europe.”
Anderson describes vibration as occurring on uneven surfaces and resulting in “non-stop vibrating of the chair or product. Impact, on the other hand, is a threshold between two sidewalk slabs. It doesn’t have to be a big impact, like bouncing down a curb — which is also an impact. But it is more of a jolt to the chair.”
So why are vibrations and impacts a problem for wheelchair users?
Jonathan Pearlman, Ph.D., is associate director of engineering in the Human Engineering Research Labs at VA Pittsburgh Healthcare System, and also an assistant professor at the University of Pittsburgh’s department of rehab sciences & technology.
“The body responds (to vibrations) based on frequency,” Pearlman explains. “The organs in the body respond and have a resonant frequency, meaning that they shake in an amplified way. So if you go over a certain type of rough terrain that has a certain roughness to it, your internal organs will vibrate at a high frequency.”
Not surprisingly, then, the more often the body is shaken, the more potentially hazardous.
“There are a couple of ranges that are really dangerous,” Pearlman says. “The ones that we focus on are between four and six hertz. That’s a terrain that makes you shake between four and six times per second. When that occurs, the organs in the body can resonate, and it causes discomfort.”
How Vibrations Are Transmitted
One of the challenges of vibrations is how commonly they’re encountered by wheelchair users. The possibility of experiencing vibrations seems to be everywhere.
Steve Boucher, OTR/L, clinical education specialist at Sunrise Medical, points out, “Both the wheelchair’s components and the user’s propulsion environment may play a part.”
He notes “the crossbrace, swing-away legrests, footplates, back posts, and armrests are among wheelchair-derived contributors. These items tend to loosen over time and can cause vibrations experienced by users.”
And while rough sidewalks and streets are easy to identify as vibration causing, Boucher adds that other common outdoor environments, such as gravel, grass and dirt, can be culprits.
“Another source of vibration exposure would be when you’re in a vehicle,” Pearlman says. “You’re not maneuvering, so to speak, over ground. You’re just riding in a bus, and the bus is hitting bumps, and you’re experiencing it more or less like any other rider.”
Wherever it comes from, once it is produced the vibration transmits “right up through the casters, through the frame, through the cushion, to the user,” Anderson says.
Boucher believes there are differences in the vibration experiences of power chair users vs. manual chair users.
“For example,” he says, “a power wheelchair user in a higher-end chair may drive over rougher terrains at faster speeds than a manual wheelchair user. Depending on the terrain and speed, the quantity and intensity of vibrations can increase.
“For those using manual wheelchairs, counter forces may be a greater contributor than speed. Take the example of a user pushing a wheelchair through terrain. As they lean and push forward, the obstacles they encounter push against the wheels, creating a counter force.”
How the Human Body Reacts
Much of the knowledge we have about how our bodies respond to being frequently jolted is rooted in studies of occupational hazards, Pearlman says.
He points to research that examined the experiences of long-haul truck drivers, bus drivers and people who operate heavy equipment. People with these occupations work long hours while seated and frequently navigate over rough terrains. They also were found to experience “pain and spinal problems,” Pearlman says.
“The thing with vibrations is they slowly wear you out over time,” Anderson says. “A lot of times, it’s fatigue over the entire day. It’s fatigue on the user, and fatigue on the chair itself.”
He notes that both steady, lower-amplitude vibrations and more occasional but stronger impacts “can trigger spasticity, for example. If it’s a pretty rough asphalt surface, oftentimes that’s enough to trigger spasticity for me and for a lot of other people.”
Boucher adds, “Research shows that over time, vibrations can lead to lower-back, neck, shoulder, elbow and wrist pain and abdominal issues and strains. Vibrations can negatively impact ride quality in most situations. Vibrations can lead to increased workload not only in propelling or driving the chair, but also in the efforts to maintain functional positioning in the chair throughout the day.”
As Anderson points out, the impact of vibrations on the wheelchair user is a cumulative effect.
Boucher adds, “Research suggests that as the length of time in a chair increases, so does the quantity of vibrations and level of fatigue, which may lead to greater susceptibility to injuries.”
Additionally, all those big and little jolts to the body can literally bounce wheelchair users out of their optimal positioning.
“Power chair vibrations can cause challenges with access to drive controls, and can make it difficult for therapists and suppliers to find solutions for optimal driving techniques,” Boucher says. “Manual and power chair users who increase their energy output to maintain body position due to vibrations report that the time they are able to stay in their mobility devices is often reduced — sometimes severely reduced. This impacts clients not only physically, but mentally as well. Negative experiences can lead to discouragement and a decline in motivation to use these mobility devices. Eventually this affects all areas of work, rest and play.”
There is some good news, Pearlman says, about how the human body responds to vibrations.
“If your body does get a break — so if you’re only on rough terrain for an hour or so — your body can recuperate,” he explains. “You can get back to a state that’s pretty healthy.”
But there’s a caveat.
“The concern we have is we haven’t done research over a long period of time to monitor whether or not we can change the prevalence of back pain, for instance,” Pearlman notes. “What we’re concerned with is if you do go to work and you have to drive your power chair or propel your manual chair for an hour or so on rough terrain, then over time the damage accumulates, and you do increase the severity of your back pain. It’s similar to the types of trends that happen with carpal tunnel syndrome and other repetitive pain injuries of the upper extremities.
“We think this is sort of a parallel process. More or less, all manual wheelchair users we know have some repetitive strain injury from transferring and from propelling their wheelchairs, and that builds up over time. We believe the same consequences occur related to vibrations.”
Able-bodied truck and bus drivers have an advantage over wheelchair users, Pearlman says: “The driver can get up and stretch and get into other postures. The type of pain they experience can be ameliorated at some level by doing other activities.”
In comparison, “A wheelchair user is more or less seated. Other than being in bed and maybe intentionally getting out of their chair to change postures, they’re always in a vulnerable position, and they’re already at a higher risk for things like back and neck pain because of that seated position. So it is a more compromised situation compared to somebody who can stand up and readjust their posture and do stretches.”
In addition, the injuries or conditions that cause consumers to use wheelchairs in the first place may also predispose them to additional difficulties upon encountering vibrations.
“It’s an unfortunate combination of situations,” Pearlman says, “where in a lot of cases, you have a lack of control and muscle tone, which then compromises the stability of the spine under circumstances in which you have high accelerations. Even if they’re low-amplitude vibrations, they’re over a long period of time because you might be pushed a couple of hours over a sidewalk surface that’s pretty rough.”
Getting Vibes Under Control
While eliminating all vibrations isn’t a reasonable expectation, it is possible to lessen their impact. On the environmental side, Pearlman notes that he’s part of a team working to develop accessibility standards for sidewalks that would include surface roughness guidelines.
As for improving the ability of wheelchairs themselves to combat the effects of vibrations, Pearlman again points to gains made in the occupational safety realm for long-haul truck drivers.
“The air seats in those trucks were designed around the fact that you need to be able to move the body at a different frequency than the body is receiving from the road,” he explains. “So you’ll see the truck driver move up and down 6″ — (the seat) shifts the frequency of what’s coming into the truck and what’s coming into the body, and shift s it to a frequency that’s less harmful to the driver.”
But he says, “The designs of wheelchairs and suspensions of wheelchairs have not reached the level of where it’s really shifted the frequency or made it much safer to travel over the terrains that wheelchair users have to travel over on a day-today basis.”
Pearlman says that while they’re not perfect solutions, adding suspensions to wheelchairs has been “helpful for absorbing that low-amplitude, high-frequency vibration that occurs when you’re driving over rough sidewalks. We haven’t seen a great solution that helps with the bigger-amplitude (vibrations) while going down a curb. We’ve seen helpful designs, but they’ve been equally problematic for wheelchair users to propel and feel like the chair is very responsive.”
For ultralightweight chairs, the vibration-dampening challenge is magnified, he points out, by the need to maintain a light overall weight: “Adding a big suspension element to a chair is heavy. It really absorbs a lot of the propulsion energy, also. What would be a pretty nice holy grail would be to find a way for a suspension system to make the chair feel rigid until you jumped off a curb or over a threshold. It’s a tricky design problem, but there are people working on it.”
As for reducing the impact of vibrations on power chair users, Pearlman says, “For power chairs there’s some technology that I would frankly say should be applied to power chairs that has not been applied yet in terms of smart suspension systems that could really sense what an issue is. This type of technology already exists in automobiles, but hasn’t been applied to wheelchairs: Suspensions systems that have sensors that know we’re in this type of terrain, let’s adjust the suspension.”
Bad Vibes: A Consumer’s Perspective
Josh Anderson isn’t just VP of product & brand development at TiLite; he’s also a wheelchair user who knows all about wheelchair vibrations.
“Besides that it’s fatiguing on the body and on the chair,” he says, “it can also trigger spasms, just another thing you have to deal with.” Minimizing those spasms by reducing the vibrations he feels, he says, is one of his major goals when building and fine-tuning his own chair.
“I’ve tried to use a combination of things to make my chair light and still very rigid and vibration dampening,” he says. “A lot of times, people think a chair is vibration dampening when it’s really not vibration dampening — it’s just not that rigid.”
When that’s the case, Anderson explains, the chair is indeed absorbing the vibrations…but it’s also absorbing productive energy, such as energy the consumer expends to push on the handrims.
“The big thing with bicycle technology is transmitting energy from the user to the pedals,” he says, “so you can propel the bike forward. The same holds true for a wheelchair: We want that rigidity because we want to have efficient transfer of energy when we’re propelling.”
So how can you reduce vibrations without too high a trade-off ?
Anderson touts the value of titanium as “a natural vibration-dampening material” that doesn’t become brittle over time.
“The frame material makes a difference,” he says. “And I think your front wheels make a difference. I love the ‘soft roll’ technology that’s available. They’ve looked for the best durometer of materials for the wheel itself so that they absorb vibration but at the same time, they have minimal rolling resistance.”
Past iterations of soft-roll types of casters, Anderson says, were “kind of mushy. When you went to turn, the tire would dig in, and you’d have this lag in performance. A lot of the newer front tires don’t have that loss of performance, but still absorb that vibration.”
Rear wheels are another opportunity to improve ride quality.
“High-end wheels — Topolino, Spinergy — all have a very good vibration-dampening property,” Anderson says.
And choosing the right tires can also make a big difference: “With pneumatic tires, not only do they offer less rolling resistance, but they roll so much smoother because they absorb that vibration,” he notes. “True, a flat-free insert is no maintenance. A solid tire is no maintenance. That’s about the absolute worst thing you can do to a high-end wheelchair, to put a flat-free insert on it, because it’s just a solid chunk of rubber. There’s no vibration dampening, there’s no impact absorption. You’re compressing that insert, so you have a lot of roll resistance.”
Anderson is especially wary of using flat-free inserts on pediatric chairs. “You’ve got a child that may weigh 50 lbs. and a chair that weighs 20 lbs. The relationship of the weight to the user is much greater. Sometimes by putting more weight in the chair, more force in the chair, you see a lot less of that vibration. But for kids, they’re just so light that they’re feeling all of that vibration.”
Good Vibes for the Future?
Understanding the full effects of vibrations on wheelchair users is still, in some ways, in its infancy.
Says Pearlman: “The science isn’t advanced enough related to wheelchair use to say if somebody is using a wheelchair with suspension versus not using a wheelchair with suspension, the severity of their back pain is not going to go up as fast. It’s sort of very intuitive: small injuries or small exposures to vibrations over time will lead to chronic conditions.
“We haven’t demonstrated that in wheelchair users, but we know sort of anecdotally that wheelchair users have a high prevalence of back and neck pain, much higher than the able-bodied population, and although we can’t single out vibration because there are these other compromising situations related to muscle tone and other postural issues being seated all the time, it certainly makes a lot of sense to us that (vibrations) would be a pretty big contributing factor.”
Meanwhile, the studies and research go on: Recent projects have looked at the transmissibility of vibrations through various types of seating systems and seat cushions, for example.
“The problem with vibrations doesn’t get nearly enough press,” Pearlman says. “We know from a research standpoint that — if it were their job to push along in a wheelchair every day, it were their occupation — their employer would be in trouble because it’s an occupational hazard.
“We know there are health consequences related to vibrations, and there needs to be a bigger effort related to the design of chair, improvements to the maintenance of sidewalks and appropriate standards, and more awareness of clinicians and users of how they can resolve the problems.”
Are Your Clients Getting Bad Vibes?
Clients may not know how much vibrations impact their health and quality of life. Steve Boucher, OTR/L, clinical education specialist at Sunrise Medical, suggests asking clients these questions to help pinpoint vibration- or riderelated problems:
- How do you like your current chair?
- How many hours do you use your chair? Why not longer?
- Have you had or do you have discomfort or pain when driving or propelling your chair? Is this situational? Do you ever off set your pain by adjusting your positioning?
- Have you changed your seating system or anything on the chair recently?
- Have you experienced any medical or physical changes since the chair was provided to you? (If yes, this is a good reason to reevaluate the current system.)
- Have you used a product in the past that did not create or induce discomfort, pain or improper positioning in your chair?
- Have your environment or activities of daily living changed?
As for ATPs, Boucher recommends keeping up on both power and manual wheelchair models and accessories. “New studies discuss the vibrationdampening qualities in both aluminum alloys and titanium alloys used in ultra lightweight manual chairs today,” he says as an example. “Ask your manufacturers’ reps about the suspension systems power chairs offer and how different drive-wheel positions could affect your client. Inquire about the electronics and options for programming.”