Getting Lost in Gravity
- By Elisha Bury
- Nov 01, 2014
Gravity isn’t something many of us think much about. We know the story of Isaac Newton and the apple, of course, and we might recall that gravity is responsible for how much we weigh. But we rarely consider the part gravity plays in our everyday lives.
Gravity, as it were, is pretty important for people who use manual and power wheelchairs because gravity can greatly impact a wheelchair setup. In fact, center of gravity (CoG) may be one of the most important adjustments a clinician can make to a wheelchair.
“It’s a foundation measurement that many other measurements flow from and are affected by,” explains Jeff Adams, president/CEO of Icon Wheelchairs. “Changes in cushion height, growth, weight gain, weight loss, skill development and others can all have profound effects on the CoG setting and vice versa.”
Yet as important as this measurement is, CoG is often one of the toughest measurements to get right, partly because CoG is not only about empirical data.
“Center of gravity is also one of the measurements that is set by feel,” Adams says.
So how can a clinician balance both client demand and proper positioning so that clients get the most out of their mobility systems?
Science Behind CoG
The first step to setting CoG is figuring out what it is exactly. By definition, CoG refers to the point at which the weight of the wheelchair is balanced.
“Objects are balanced when the weight is evenly distributed,” says Kay E. Koch, OTR/L, ATP, rehab clinical consultant for Invacare Corp. “The center of gravity is this balance point.”
That might be a hard concept to visualize. Instead, think of CoG as a balanced seesaw. If no one is on the seesaw, the CoG is in the middle of the seesaw. If people of different weights are on each end of the seesaw, then all of the different weights must be taken into consideration. The CoG will be where the seesaw is balanced, which will be closer to the end of the heavier person.
In this example, the seesaw represents the wheelchair, which has its own unique CoG. But when you add a person to the chair, just like with the seesaw, the CoG changes. A wheelchair has a CoG low to the ground, but when the chair is occupied, the CoG shifts higher. As you can imagine, this shifting of balance can distort stability. That’s one reason it’s so important to get CoG right.
To illustrate the point, Koch uses the example of moving the rear wheels on a manual chair.
“When the rear wheels on a manual wheelchair are shifted forward, the center of gravity is changed, making it easier to pop a wheelie. If the wheels are shifted backward, the center of gravity is changed, making the wheelchair more stable and harder to pop a wheelie,” Koch says. “By understanding how this relates to performance, stability can be changed to help the client navigate his or her environment.”
Of course, stability isn’t the only factor to consider with CoG. In fact, stability has a negative relationship with ease of propulsion, which might spell trouble for manual users, especially on an incline and decline. For example, if more effort is required to change direction, fatigue is increased, Adams says.
“The less energy we have to spend steering, the more is left over to generate speed or cover more distance,” he says, which can dramatically affect how easy it is for someone to navigate his or her environment. Of course, when the body moves in tilt or recline or when maneuvering obstacles in the environment, CoG will change again, Koch says.
Therefore, a clinician must consider more than simply the client’s weight when determining CoG.
Center of gravity (CoG) is the point in which the weight of an object is balanced. For example, if a seesaw is empty, the CoG is in the middle of the seesaw. If people with different weights occupy the seesaw, the CoG is closer to the heavier person. Likewise, a wheelchair has its own unique CoG. When a person occupies a chair, the CoG changes. A wheelchair has a CoG low to the ground, but when the chair is occupied, the CoG shifts higher.
Benefits of the Correct CoG
So what does this complicated measurement mean for clients, and why is it so important to get it right?
As you already know, CoG can have a big impact on a client’s everyday efficiency in terms of propelling a manual chair.
“These adjustments can provide better wheel access, which leads to more efficient propulsion even if only minimal adjustments are made,” Koch says.
Everyday efficiency becomes especially important as clients age or for clients with the most limitations, including those with weakness, reduced endurance and impaired balance, Koch says.
CoG can impact many other aspects of a client’s life, however, including shoulder/wrist/finger/joint health, fatigue, comfort, posture and confidence, Adams says.
For example, if a chair is too front heavy, a user might be in danger of incurring a shoulder injury because the chair is harder to turn, Adams says. Likewise, if a chair is too unstable, a user will have to worry about tipping over backward, resulting in posture issues or even loss of confidence.
“If feeling like tipping over backward is a constant concern, that can easily result in an enormous amount of anxiety,” Adams says.
Shoulder injuries can be particularly concerning for someone with lower-extremity amputation. For these clients, “the adjustment in the center of gravity is crucial,” Koch says, as it will reduce the stress on the upper extremities.
Setting It Right: Manual Chairs
Setting CoG for manual chairs requires a fine hand. What clinicians are attempting to do is set the CoG for the optimal weight distribution over the back wheels and front casters, Adams says. As easy as that sounds, there are many ways to go about it.
“Getting the weight distribution somewhere in the 80/20 rear/front range is a good starting point for most riders,” Adams says. “From there, small adjustments can make a big difference.”
Tina Roesler, PT, MS, ABDA, director of international sales at TiLite, says that ideally the rear wheel would be in line with the shoulder or slightly forward of the shoulder to maximize propulsion.
Another way to determine CoG, Roesler says, is to look at the height of the front caster when the person is in a balance position in the chair.
“When holding the wheelie, the front caster should really be no more than 3" or 4" off the ground for safety reasons,” she says.
Koch says that clinicians must take into account the user’s height, weight and body type, strength and balance, position in the wheelchair, and the wheelchair and seating system when determining CoG adjustments. She recommends evaluating the user’s skill level and then moving the rear wheel forward in 1/2" increments.
Of course, Adams says the battle between stability and maneuverability is a tough one. “Testing the setting in as many different environments as possible is helpful, and making sure that the testing happens in a safe environment and in small steps is critical.
“I always try to make gradual changes (1/2" at a time) and do repeated tests — steering around pylons, going up and down ramps and curb cuts, doing a wheelie off of a curb, and other things that are real-world tests. I’ll often install anti-tippers, but set at a height where they would only touch as a fail-safe,” he says.
CoG isn’t a set-it-and-forget-it adjustment. Roesler says adjustments to CoG should progress forward over time as a new user acquires skills.
She says that although the research shows that a forward CoG is better, it’s important to consider the stability of the user.
“It is important to remember that stability is relative to the user, and a new rear-wheel position may require some skills training,” Roesler says. “We need to give the client time to acclimate to the more forward wheel. We shouldn’t just move the wheel rearward because their wheels are popping off the ground when they first try to push. We need to give instructions on how much force to use and how to propel correctly.”
Finding the CoG in Power
Unlike manual chairs, power chairs do not require exact measurements for CoG. Instead, the drive-wheel configuration determines CoG for a power chair user, explains Magdalena Love, OTR, ATP, at Permobil.
“Ideally, you want the majority of the individual’s weight over the drivewheel tire,” she says.
To do so, the clinician will have to take into account the user’s weight and weight distribution. For example, in a rear-wheel-drive configuration, a user with a lot of weight anteriorly, such as a client with lower-extremity edema, will have that weight positioned over the front casters.
“Consequently, the drive wheel will not be getting as much traction as it needs for optimal performance. When using power seat functions such as tilt and recline, all the weight of the user and seating system shifts backward, potentially making the system have a tendency to rock backward. This sensation often is very alarming for individuals to experience — potentially leading to a decreased likelihood of completing pressure-relieving activities,” Love says.
On the other hand, that same user might have other issues in a frontwheel-drive chair.
“Depending on the individual’s weight distribution, the wheelchair may have a tendency to rock forward if the user does not tilt back,” Love says. “This is especially true when going downhill or if there is significant lower-extremity edema. However, the system would be well balanced during seat function use.”
The mid-wheel-drive chair might be the best option for this client, as it would provide the most stability with the least number of restrictions during driving and power seat function use, Love says.
Koch says that when the drive wheel is behind the user’s CoG, such as on a rear-wheel-drive system, the chair will be stable but have a larger turning radius. The front-wheel-drive configuration provides stability with a tighter turning radius, and “the drive wheel in the center or the mid-wheel drive has the user’s center of gravity over the wheelchair and has the tightest turning radius,” she says.
When determining the best drive system, clinicians must also consider the user’s driving skills and environment. That often means explaining the differences among rear-, front- and mid-wheel drive. Love uses the example of a person driving down a hallway and then turning right into a room. She says the rear-wheel drive requires a wide turn, but a wide turn with front-wheel drive would swing the rear casters into the wall. Love recommends a trial of the equipment in the home or community as well as specific training on driving techniques.
So what happens when the chair is configured with power tilt, power recline or both?
“The bases are designed to accommodate this change created by the seating system function, and the actual center of gravity on the base remains the same,” Koch says.
Even on manual chairs with a tilting seating system, the system is designed with fixed CoG changes that do not need adjusting, she says.
A Word on Adjustability
Adams tells the story of a client who is a professor at a local university and an experienced manual chair user. She ordered a fully welded chair. After the chair arrived, Adams did the final fitting and tweaks, and the client was happy. The professor then picked up her backpack and flipped over.
“We had done the eval and ordered the chair midsummer, and it didn’t come in until mid-September, after school had started,” Adams explains. “Her backpack was now filled with books and papers, which made the CoG that we ordered unmanageable. Her chair was just way too tippy with her backpack mounted. Her requirements from September to April were about 2" off of her requirement during summer break from May to August.”
Although Adams found a solution that required mounting some of the professor’s books to the front of the chair, this situation convinced him that adjustability is critical, even for experienced users.
“Adjustable center of gravity is one of the most critical adjustments that any chair should have,” she says. “The position of the wheel would dictate someone’s long-term function and their ability to function within their environment.”
Adams says that even a 1/4" can make a difference, especially because “if someone lives in Topeka or San Francisco, the practical application of the theoretical setting of the CoG can be wildly different.”
In fact, the ability to adjust CoG on the go would be ideal.
When determining center of gravity (CoG) for a power chair, consider the drive-wheel confi guration. “Ideally, you want the majority of the individual’s weight over the drive-wheel tire,” says Magdalena Love, OTR, ATP, at Permobil. Consequently, a rear-wheel configuration places the weight over the rear wheel, which provides stability. A front-wheel configuration places the weight over the front casters, which could result in less stability depending on the user’s weight distribution. The mid-wheel drive configuration places the weight right in the center of the chair, giving the user the benefit of stability as well as a tight turning radius, says Kay Koch, OTR/L, ATP, rehab clinical consultant for Invacare Corp.
“One of the things that often messes up the center of gravity of my personal chair is having to carry something — groceries or laundry. Or horsing around with my nephews on my lap causes the CoG of my chair to be very front heavy, and my chair becomes cumbersome to maneuver,” Adams says. “On the other hand, if I’m carrying something up a ramp and am prevented from leaning forward, my chair becomes way too unstable. Being able to make on-the-fly changes to the CoG while out in the real world could have dramatic benefits.”
As the client changes over time, CoG might also need to change. For example, weight gain, changes in environment, and progression or regression of disability might warrant an adjustment.
Even age can be a motivator for CoG adjustments.
“Older clients may feel less comfortable with a more tippy chair, while young kids, as they progress and propel better, may require center of gravity more forward,” says Roesler.
Sometimes the adjustment is all about the client’s personal needs as well.
“Personal preferences may also change after a significant time spent in a powered mobility device,” Love says.
Final Thoughts on CoG
Perhaps the most predictable piece of CoG is that it is never predictable. Even after years of adjusting CoG, Adams is surprised occasionally.
“We had a customer about 18 months ago who I had known for over 15 years. She was requesting a chair with a very aggressive rearward CoG and anti-tippers, which I had never known her to use. We were talking about what she wanted in terms of her configuration, and I asked her why she was configuring her chair in such a different way than her previous chairs,” Adams says. “She eventually told me that she was expecting a child and was going to be going through an enormous personal CoG change.
“I promise that in any of the design sessions I’ve done, in all the late nights that I’ve spent trying to understand the ever-elusive perfect combination of theory versus practical when it comes to the design of manual wheelchairs, I didn’t consider getting pregnant.”
Adams’ point: It’s impossible to imagine every possible scenario. There’s always a chance something new will come up in a client’s life that will require an adjustment in CoG.
Ultimately, helping the client live a better life is at the heart of the CoG adjustment.
“Center of gravity combined with proper chair fit and configuration will have a huge impact on everyday efficiency. If we can optimize fit, center of gravity and configuration, the client will be able to maximize their potential and have better skills acquisition,” Roesler says. “It will help them integrate better into their community and enjoy a better quality of life long term.”
This article originally appeared in the Seating & Position Handbook 2014-2015 issue of Mobility Management.