The Mathematics of Tilt & Weight Shifting

Studies have shown that users of complex rehab power chairs regularly use tilt to change their position, usually to feel more stable or comfortable. What’s more of a challenge is getting those users to use enough tilt to effectively weight shift to alleviate pressure.1

Consumers who have less inherent core stability often choose to sit in a position that includes a small amount of tilt. For example, sitting while tilted back 10° becomes their new normal because that position allows gravity to more substantially assist them with staying upright.

Jean Sayre, MSOT, ATP, CEAC, VP of Research & Clinical Development for Quantum Rehab, agreed: “There are a lot of people who are permanently in a 5° to 10° tilt while driving.”

These power chair users can run into problems if they don’t consider that tilt amount when deciding how much to tilt to shift weight and relieve pressure. For example, a user might be advised by his clinician to tilt back 45° to relieve pressure — but that number assumes that the user is starting at 0° of tilt. If the user is already tilted back 10° for comfort, then tilts to a total of 45° to relieve pressure, he’s actually only netting 35° of tilt, which is less than what his clinician recommended.

To accommodate those users’ preferences while still making it possible for them to tilt effectively to relieve pressure, Sayre suggests advising clients to “come back out of that 5° to 10° tilt if they’re just sitting on a flat surface and not driving. If they’re watching television or working, just come out of tilt and sit upright.”

Doing that means that clients will start at 0° of tilt, so that when they tilt 45° to relieve pressure and redistribute weight, they are in fact tilting the recommended 45°.

The fact that so many power chair users do prefer to normally sit and drive with some degree of tilt is one reason that Sayre is a proponent of anterior (forward) tilt.

“It’s not just for transfers,” she said. “If you need a forward [seat] slope, if you need to tuck your pelvis forward, with [anterior tilt] you’re extending your spine and changing your position. That’s why Quantum is in the process of developing a power anterior tilt system.”

Going into even a small degree of anterior tilt again changes the posterior tilt math. Instead of starting from zero and tilting back, for example, 45° posterior to weight shift, a client would only have to do a posterior tilt of 35° to still net a 45° weight shift if he starts out from a position of 10° of anterior tilt.

Since many power chair users notoriously dislike tilting too far back, using anterior tilt in conjunction with posterior tilt might be a way to improve compliance while requiring less posterior tilt.

“In powered seating,” Sayre said, “some people only use tilt. I like using tilt and recline to open up those hip angles and to change the hip angles. It’s just [a matter of] changing body positions. We change body positions naturally, and we don’t even think about it. Just in 30 to 60 seconds of sitting, think about how much you shift in your chair.”

Source: (1) Mobility Management, “Tilt by the Numbers,” Feb. 2014, http://tinyurl.com/tiltbythenumbers

This article originally appeared in the May 2019 issue of Mobility Management.

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