Gravity & Seating
Is It Friend or Foe? How the Forces of Gravity Can Impact Seating for Worse...& Better
- By Laurie Watanabe
- Apr 01, 2014
Industry professionals know that achieving functional seating & mobility for wheelchair users is a many-layered balancing act. For instance, a weight-shifting, pressure-relieving regimen must also be user friendly enough for the consumer to regularly use it. A system durable enough to support bariatric consumers must also be compact enough to maneuver indoors. A pediatric wheelchair needs to fit well now, but also be able to grow with the child. And of course, the seating & mobility system needs to serve both its user and the funding source who pays for it.
And so it goes with the forces of gravity, among the ATP’s most formidable challenges when building a seating & positioning system.
It’s obvious how gravity can literally be a drag to a wheelchair user who struggles to maintain a functional position. Perhaps not as obvious is how gravity, under the right circumstances, can also be used productively by the clinician or ATP who knows how to manipulate it to the client’s advantage.
Gravity & the Ways We Sit
The concept of how gravity works seems obvious and simple even to young children sitting in a classroom. They might offer an explanation similar to the one offered by Kay Koch, OTR/L, ATP, clinical rehab education manager, Invacare Corp.: “Without gravity, all people and other objects would go floating off into space.”
Those same young children would probably squirm and wriggle while discussing gravity, thus proving Koch’s next comment: “Gravity affects posture because even in a seated position, the body has a dynamic response to gravity. Rarely does one sit still for extended periods of time. Our bodies need to exert sufficient force against gravity to keep us upright and midline.”
So when sitting and gravity are both at play in an able-bodied person, what’s happening?
“The better muscle tone is developed, the more efficiently the muscles are able to support an upright position,” Koch points out. “In sitting, the true center of gravity is not at the center of the body, but actually slightly higher with more weight on the top half of the body than the lower half.
“Good sitting posture places the upper body’s center of gravity over the hips, assisting with muscles to balance the body. The head has its own center of gravity and is difficult to position because it lies in front of its major point of support.”
Of course, anyone who’s been trapped for too long in an office chair, a lecture hall or an economy seat on an airplane knows that “textbook” posture truly exists only for short periods.
“There is no single ideal sitting posture beyond the anthropometric reference to a sitting posture of 90°-90°-90°,” Koch says. “Eventually everyone fatigues out of this posture and slides into a position of comfort and support.”
Challenges to the Wheelchair User
As gravity continues to pull on them, and fatigue and discomfort set in, able-bodied sitters begin a non-stop dance of resistance, squirming and shifting and crossing or uncrossing their extremities in a bid to find a temporarily more comfortable position.
But such natural and often subconscious movements are often severely limited for wheelchair users.
“Our muscles work in a combination of continuous and passive contraction,” Koch says. “With individuals who have a disability, this force and functioning can become compromised.
“Postural reactions and adjustments are built of foundations laid down by integration of primitive reflexes. When reflexes are impaired or diminished, the complete development of postural reactions or the ability to adjust may be delayed or absent.”
That’s where the challenge lies for the seating professional, who has to deal not only with gravity’s inexorable pull, but also with the client’s physical challenges.
Jay Doherty, OTR, ATP/SMS, regional manager for Quantum Rehab, explains, “Muscle tone, spasticity and reflex activity will often cause a client to lose their position in the seating system. For example, when a client’s extensor tone kicks in, it may cause the person to slide forward on their seat cushion and down, away from the positioning components on the back. Gravity negatively accentuates this by providing a downward force above the center of mass moving forward and increases the effect of this forward excursion.”
Doherty says the clinician or ATP also needs to consider orthopedic challenges.
“Many types of orthopedic deformity and postural asymmetry cause instability in the body for the seated posture,” he notes. “This instability may significantly impact a functional posture from being achieved and maintained. Initially these orthopedic changes may present as a reducible deformity and cause uneven weight bearing. However, gravity increases the impact of the destructive postural tendency such that it may cause a limitation in range of motion or a non-reducible deformity that will cause the client to shift out of position if not accommodated for by utilizing postural support components in conjunction with a shift in the gravitational pull on the body.”
Gravity & Seating Considerations
Of the give-and-take relationship, Doherty says, “Gravity can be a major challenge, especially for individuals who teeter between using it for stability and moving with or against it for function. While we cannot eliminate gravity, the ATP/clinician can and should use it judiciously to assist the client in maintaining their seated posture in the wheelchair while evaluating the impact of it on the body systems (e.g., circulation, digestion, respiration, etc.), reach and access.”
Koch explains the relationship and major challenge of posture vs. gravity this way: “If the line of gravity falls outside the base of support, then balance is lost. Balance may be achieved with support forces pushing, when those forces exert and balance counter torque. Poor posture requires exertion by the back muscles to counteract the torque caused by the body’s weight.”
Another challenge comes from the ATP or clinician needing to balance not just one part of the client’s body, but rather having to balance parts and the whole.
“The body is a segmented system,” she says. “Stability of the body depends not only on the stability of the individual segments, but the stability of these segments as a whole unit. Any change in the position of those segments will change the overall balance. Reaching forward with your arms, for example, moves your center of gravity forward and needs to be accommodated. Instability in a dependent sitting posture can decrease endurance and comfort in the system and affect circulatory and respiratory efficiency.”
And it’s not just extremities, pelvis and head that are impacted.
“Whole body systems like the somatosensory system, including sense of proprioception and kinesthesia of their joints, pressure, and spatial positioning are involved,” Koch says. “The visual system can also impact seating encompassing not only acuity, but spatial location relative to self and other objects.”
How Technology & Gravity Can Work Together
An improved understanding of what causes postural compromise and advances in assistive technology are making it possible for ATPs and clinicians to harness gravity and make it a powerful ally in the hunt for seating solutions.
Doherty sets up the problem this way: “If a person has weakness throughout their body, it will be very difficult to maintain an upright sitting posture because gravity is going to be pulling on their trunk. This constant downward force on a person’s body cannot be eliminated and constantly works against the person. Often when these individuals are sitting in a straight upright posture, they will eventually end up slouched forward (forward flexed at the trunk). This is typically due to the fact that they’ve fatigued, and eventually gravity pulls them forward into this forward flexed/slouched posture.”
Then how might technology successfully intervene? Doherty says, “I will use the example of the person who fatigues during the day and ends up slouched forward. In this situation we can use a slight amount of tilt to allow gravity to assist the person with maintaining an upright trunk posture. This is achieved by changing the angle to which gravity is working on the body. A slight amount of tilt will allow gravity to assist the person with staying upright against the backrest and decrease the fatigue they may encounter throughout the day.”
As with any seating situation, Koch says finding the solution starts with understanding the challenge.
“In positioning, support, stabilization and balance are key,” she notes. “An evaluation or assessment (see her chart) is needed to see if the support needs to be applied laterally and anteriorly along with the posterior support provided by the back. Balance is achieved in seating using these supports, and can have a static or a dynamic component to provide a balancing counter torque. Gravity can be used to help balance so an individual can be functional while sitting.”
While tilt might be the first technology to come to mind when thinking about gravity, Doherty points out that an array of components and systems, from a pelvic belt (“will assist with preventing a client from sliding forward in their seating system”) to recline (“can allow improved trunk posture by changing gravity’s effect on the position of the trunk”) to headrests (“properly set up along with the tilt or recline angle can off er posterior head support to those individuals with whom gravity impacts the fatigue of the neck muscles”) can help achieve that functional system that clinicians and ATP are seeking.
Koch points out that one of the new position papers from the Rehabilitation Engineering & Assistive Technology Society of North America (RESNA) discusses postural supports.
“In the RESNA position paper (see MMBeat) on the Application of Wheelchairs, Seating Systems and Secondary Supports for Positioning vs. Restraint, the most common and critical primary supports are the wheelchair seat and back support, which are used to enable sitting,” Koch says. “Depending on the postural control and mobility of the occupant, these primary support surfaces may also include foot and leg supports and arm supports.
“There is little doubt that these are essential to maintaining the person in the body support system. Secondary supports are typically those used to help maintain a very specific posture or position of a certain body part or area, such as a person’s upper torso or extremities (legs, arms, and head), while the basic seated position is maintained by the primary supports. Secondary support components are most commonly used to prevent specific movements or postures that are maladaptive, nonfunctional, or unsafe for the user. Utilizing a tilt-in-space or reclining feature on a wheelchair can require less work by the muscles involved, since gravity is supporting the person. These features can also assist with comfort and postural alignment by changing the orientation of the body.”
So while, as Koch says, gravity in combination with the client’s musculoskeletal and neurological systems do impact seating & positioning, “The goals of seating should include positioning to achieve a posture that can be maintained, is functional and clinically beneficial.” And leveraging gravity correctly in conjunction with the right components can be an important part of the seating solution.
She also points out the need to maintain a balance with that other force constantly pulling on ATPs.
“Documentation and justification for these components is needed to ensure the client gets the correct equipment covered that is not only medically needed, but more importantly, needed to maintain function and independence,” Koch says.
Then she quotes German rocket scientist Wernher von Braun: “We can lick gravity, but sometimes the paperwork is overwhelming.”
Addressing Gravity in a Seated Position
By Kay Koch, OTR/L, ATP
|What to Check
||What to Assess
||What to Try/Add/Change
|Pelvic symmetry/level pelvis
PSIS and ASIS Anterior or posterior tilt
|Pelvic belt/different cushion or seating
|Position of the feet
||Hamstring range of motion; are feet bearing weight?
||Footrests djustments to height, angle and depth; calf pad or leg strap
|Position of the thighs
||Are they parallel and bearing weight?
||Adjust cushion or seating system; check footrests
|Seat angle & cushion
||Body weight distributed between the seat and back; check comfort and stability
||Check the cushion for pressure redistribution. May add “squeeze” for stability
||Positioned in midline/and upright
||Laterals or anterior support to assist positioning
||Position of the head/unsupported or supported
||Back height and support may vary by functional activities and propulsion
This article originally appeared in the April 2014 issue of Mobility Management.