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Have you ever sat in a chair and tried balancing yourself by tilt backwards onto the back two legs of the chair? (I am not recommending anyone try this while sitting in a chair - it can be dangerous and you can fall backwards and get hurt! I just realize that it is something I have done in my youth and I have seen my own children do it, too - and, of course, I cautioned them against it.) Perhaps you held onto the table in front of you and used your arms to push yourself backwards to find the balance point? You may remember doing this when you were a child in either grade school or high school. Some of you may have experienced going past the tipping point and having the chair fall behind you or if you were lucky enough, you quickly recovered by moving your weight forward to prevent falling backwards.
I bring up the example of tilting back in a chair to get us thinking about the effect that displacement of one's center of gravity can have on tilting and how to apply this when thinking about the different generic mechanisms for manual wheelchairs with tilt. This month, we will focus on tilt wheelchairs and how the mechanism of tilt affects the base of support, including the overall wheelbase of the wheelchair, potential center of gravity displacement, and the effort required to move a person in and out of tilt.
The first generation of manual wheelchairs with tilt used a pivot mechanism with gas struts to dynamically move the seated position into various degrees of tilt. In fact, these mechanisms continue to exist today in some models of manual wheelchairs with tilt. In a tilt wheelchair with a pivot mechanism, there will be displacement of the center of gravity as the seated person moves through various tilt ranges, just as in the example of pivoting on the two rear legs of a four-legged chair. For a tilt wheelchair with a pivot mechanism, rather than using one's arms to push back on a table to achieve tilt, gas struts, which are activated through a lever mechanism, facilitate the movement.
If we think of the person's center of gravity as roughly being at the level of the belly button when seated, and we think of the example of sitting on a four-legged chair and tilting backwards onto the rear legs, we can visualize that when upright, the person's center of gravity has shifted backwards as the chair shifts backwards. If we draw an imaginary line from the starting point of the center of gravity to the ending point of the center of gravity, we can picture the horizontal displacement, or movement, of the center of gravity. With increasing degrees of tilt when pivoting back on the two rear legs, there is increasing horizontal displacement of the center of gravity.
If we think about the seat tilting over a wheelchair base, as in the case of a manual wheelchair with tilt, we need to think about the base of support. The position of the casters and the rear wheels provides a base of support for the seating in a tilt wheelchair. For a pivot mechanism of tilt, in which there is displacement of the center of gravity with different degrees of tilt, a long base of support is required to provide stability to the wheelchair so that when the seat is tilted to the maximum degree that the system allows, the wheelchair with not tip rearwards.
While a long base of support in a tilt wheelchair with a pivot mechanism provides stability to prevent tipping when in the maximum range, there can be a compromise to maneuverability. Think of the difference between a limousine and a sports car. It is much easier to maneuver and get into parking spots in a vehicle with a shorter wheelbase than a vehicle with a long wheelbase. In terms of a tilt wheelchair, whether it is an assistant pushing the wheelchair or an individual self-propelling, a wheelchair with a shorter wheelbase may be easier to maneuver than one with a longer wheelbase. In addition, the pivot mechanism means that there will be varying degrees of tilt, which affects the ability to propel and maneuver the wheelchair. For these reasons, there have been more developments in tilt mechanisms for tilt wheelchairs.
The second and third generations of tilt wheelchair mechanisms sought to reduce the displacement of the center of gravity through the tilt ranges. By reducing the travel of center of gravity, the wheelbase of the wheelchair could be reduced, thereby improving some of the maneuverability. The second generation of tilt mechanism used a cantilever, or four-bar linkage system, which reduced the amount of travel of the person's center of gravity when tilted between the neutral position and full tilt of the system. To further reduce the travel of center of gravity when using the full tilt range, the third generation of tilt mechanisms used a pivot-and-slide system. With each successive generation of tilt system, the wheelbase of the wheelchair could be reduced while still providing the stability needed. With a reduced wheelbase, maneuverability was improved.
The last generation of tilt mechanism of manual wheelchairs with tilt is a rotational mechanism. Because the person's center of gravity aligns with the center of rotation, there is no displacement of the person's center of gravity. In addition, gas struts are not required due to the physics of the rotational system. See below for a graphic that illustrates this tilt mechanism.
The small red and black circle in the very center of the graphic shows the alignment of the person's center of gravity with the center of rotation of the tilt mechanism. The shaded areas of the seated person illustrate that whatever degree of tilt the seat is in, the person's center of gravity and center of rotation remain aligned. Because there is no horizontal movement of the person's center of gravity with different degrees of tilt, the wheelbase is shorter than wheelchairs with other tilt mechanisms, while still providing stability to the wheelchair in different degrees of tilt. (Adjustments can be made to change the anterior/posterior stability of the wheelchair. However, discussions of these adjustments is beyond the scope of this article.) With a shorter wheelbase, maneuverability is improved. Maneuverability is enhanced further as the weight distribution remains constant over the front casters and the rear wheels, no matter the degree of tilt.
As we have seen, the mechanism of tilt will have an effect on the wheelbase of a manual wheelchair with tilt, which will affect maneuverability and propulsion of the wheelchair. The other effect that the mechanism of tilt has is on the ease of using the tilt mechanism to bring an individual in or out of tilt. Tilt mechanisms in which there is displacement of a person's center of gravity, such as pivot, cantilever, or pivot-and-slide mechanisms, means that there may be differences experienced by the assistant in the ease of putting someone in or out of tilt. For example, if the person's center of gravity is behind the pivot point, when the levers are engaged to activate the tilt mechanism, gravity will force the system downward as the person's center of gravity moves down and towards the back. As the tilt is performed, the person's center of gravity becomes lower and further away from the pivot point, which increases the reaction. Gas struts are required to provide mechanical assistance. However, the assistant will still feel the difference in that it can be easier to put someone into tilt when the center of gravity is behind the pivot point and comparatively more difficult to bring someone up from the tilt position to an upright position.
When the center of gravity aligns with the center of rotation, as in the rotational mechanism, because there is no displacement of center of gravity in different degrees of tilt, there is neutral resistance whether going in or out of tilt. Neutral resistance means that the assistant will experience that it is equally easy to put someone into tilt as it is to being someone up to an upright position. This is true no matter how much the seated person weighs. If the assistant finds that there are issues with either placing the seated individual into tilt or raising the wheelchair from a tilted position in a wheelchair with this tilt mechanism, an adjustment is required to align the person's center of gravity with the center of rotation of the system. (A description of this adjustment is beyond the scope of this article).
Physics is behind manual wheelchairs with tilt. Understanding the science of the various tilt mechanisms available for manual wheelchairs with tilt helps up to understand the possible effects on outcomes for individuals who use manual wheelchairs with tilt and for their assistants. The tilt mechanism of the manual wheelchair will affect: wheelbase, weight distribution between the front casters and rear wheels, maneuverability, potential horizontal displacement of center of gravity, and the ease with which an assistant is able to place an individual in or out of tilt.