Positioning: Safety, Comfort, & Function
Safety first
If one or both of your shoulders is subluxated (a doctor can tell you this), but you are still walking, use a Giv-Mohr or other sling to reduce further damage.
When out and about, in a power wheelchair, PALS should wear some kind of postural support to avoid falls, even if they feel secure, because your head, neck, or torso can weaken in a split second. If someone else is doing the driving, they could be distracted, miss seeing a bump, become ill, etc.
Many PALS have a cheap "seat belt" on their wheelchair that they seldom/never use because it's uncomfortable, obtrusive, or doesn't work well. But extending your body from in an unstable position is dangerous because you could fall or subluxate your shoulder(s), for example, since the loss of muscle weakens the joint.
If you still have some motor function, postural support keeps you stable while reaching/lifting/pulling or even just keyboarding, doing arm exercises passively or actively, or sitting. Of course, for safety, to avoid jolts, injuries, and falls, you also want support if traveling in a motor vehicle, in transit, outdoors, anywhere holding your center of gravity might be at risk. It's also more comfortable not to bounce around. In a bus or train you would also use the transit connectors with your wheelchair's transit slot.
Ordering the Power Wheelchair
It is much harder to get new options for your power wheelchair after it's delivered. When your chair is being ordered, ask whether each of the following items are being ordered considering your environment and progression:
Headrest (the Savant is one to consider if you have a very weak neck)
Arm rests (you can add your own padding, but that makes them less stable)
Seat cushion [normally a ROHO, often the Quadtro Select)
Back cushion (ROHO makes these, too, but they are not routinely supplied with power wheelchairs)
Postural support as described above: at least a hip belt correctly mounted and sized for you, with a small enough buckle that you will wear it
In addition, including "anterior tilt" aka "Active Reach" provides more options if you still have some arm function will also aid in transfers with or without arm use, along with the seat elevation function aka "Active Height" that every PALS should have. Depending on your plan and whether it's Medicare or commercial, you may pay extra for some features.
Function: Time for Robotics?
Why not robotic arms, automatic feeders? These are often advertised as the solutions for reduced arm function.
Robotic and other artificial arm extenders will just have a very limited lifespan for most PALS. But postural support remains important for the entire body, to reduce pain, contractures, and shear stress that can cause skin injuries (sometimes called "bed sores") even when there is no longer mobility.
Use your energy for the big tickets
Arm Support: More than "Parking" Your Arms
There are products such as light switch handles, key turners, and a handle that goes into a car door striker, that can be useful in extending arm function when weakness has begun. There are also arm supports for keyboarding and other desk work -- the ErgoRest line and its clones, as well as a wheelchair arm type ("Dynamic Arm Support," available as an option on Permobil chairs; disclaimer -- I did some early work with it years ago) that helps protect the shoulder from pain and subluxation and hands from contractures when arm function is no longer the issue.
Torso Support: Everything's Connected
Many PALS have a cheap "seat belt" on their wheelchair that they seldom/never use because it's uncomfortable, obtrusive, or doesn't work well. But extending your body from in an unstable position is dangerous because you could fall or subluxate your shoulder(s), for example, since the loss of muscle weakens the joint.
Postural support can be padded or not, and is available in different configurations to wear on any combination of your shoulders, chest, hips, abdomen, calves, ankles depending on your needs, with different kinds of buckles as well.
If you can't easily access an ATP (they work at DME suppliers or clinics and are typically the same person that measures you for the wheelchair) or other professional to fit you with something like that, one DIY solution is the Elastic Mobility Strap, formerly the Universal Elastic Strap, made by Bodypoint, which also makes the other supports mentioned (disclaimer: I worked there). Never use homemade solutions like sashes or pants belts, which can fail/dig in, and cause serious injury.
The Strap is made of a cut-to-length cushioned, machine-wash/dry material allowing for a velcro-like attachment at any point, and wraps around whatever part(s) of you and the wheelchair or lift chair that need connecting, and/or connecting the wheelchair with a seat. It and the extender strap that can make it longer are available on line in different widths.
Of course, tilt/recline/footrest position changes throughout the day help not only in pressure relief, but can be used to help you more easily perform tasks or just read, watch a screen, etc. An adequate wheelchair back, seat cushion, and headrest all work together with any other postural support, just as is true in bed with your pillow, mattress, overlay, and discrete foam cushioning supports for heels, elbows, whatever you need.