1. ## Simple mechanical problem with vectors

Originally Posted by tauble
With a seat back angle of 91 degrees measured from the seat rail
If its not a right angle then its a wrong angle. It actually may need to be less than 90 degrees. Consider an articulated spine (no rods or cables) and hip joint. (I can prepare my hypothetical details of Ms Teflon in glorious FORTRAN comment card ASCII art.) What does gravity do if this angle is greater than 90 degrees? With friction (shear force and decubitis) and without (wooden beaded cover as used by taxi drivers)? You slide out! You also need about 10 degrees dump so you fall back in which you have.

2. Whenever I have tipped or fallen, my hands seems to go "automatically" to floor/gound. I guess as a way to reduce what is going to happen. I do have very strong upper body and have already fallen with hands on floor/ground and rear end still in chair.

Originally Posted by betheny
When you go over a threshold, make a habit of touching chin to chest.Endos hurt but concussions hurt worse. My skull lost in a close encounter w/ a concrete porch.

3. to Zagam--my chair has a 2.5" dump on a 17" deep seat which results in an 8.5 degree seat to floor angle. I'd have to increase the dump to 3" for a 10 degree angle. Hmmm--I would definitely decrease the back angle if I increased the dump. For now, I think I'll keep the smaller dump while making sure my butt is all the way back and adding chin tucks over bumps. I find I'm sliding out, I'll take your advice and increase the dump.

to rlmthrmiles--I imagine I'll assume the hands out/butt in seat position one of these days.

4. Dump should be at least 10 to 15 degrees even though 20 degrees may be ideal. The lumbar support forms an acute angle (less than 90 degrees). If you not stable and tend to slide forward then you have shear force acting on you but and may end up with decubitis ulcers, DVT, etc. Do your feet swell, get AD? Shear force is a killer.

I remember a good basket ball chair where I stuck like glue without strap. Knees were high so it could turn quick. As an every day chair getting though door ways and tolerating grade to side was a problem.

http://journals.lww.com/jnpt/Fulltex...rt_Are.70.aspx

Interventions were adjustment of seat incline angle to 10-15 degrees and placement of lumbar support to achieve slight anterior pelvic tilt, thus restoring lumbar lordosis.
While improved stability can be achieved at twenty degrees of seat inclination, one must be cognizant of the patient's ability to transfer.

5. I think it's rather difficult to really know what seat angle you have because it's not only about frame geometry but also about the cushion. I have 8 degrees frame seat angle but I have a Jay deep contour cushion with higher density at front and I guess it must add few degrees.

6. Zagam: Plowing through the CAD repository revealed most seat angles were in the range of 10-12 degrees. There were two with angles of 13.7/13.8 degrees (JohnC1 and Brianm) and one with a 16.3 angle (Stumpybushman). Four had angles ranging from 5.4 (Jonna) to 8 (EastRando); two of the four were chairs specd by SCI_OTR. That does not count the couple of chairs with ergo seats.

I'm not sure how to interpret the referenced study. It was based on only 4 people all of whom had complete motor and sensory paraplegia with decubitus ulcers that were healed after changing to 10-15 seat angles with lumbar support. The authors mentioned that 20 might be ideal for stability but transfers might be a problem.

As for me, my problem is associated with weakness (and other problems) due to MS rather than SCI and ulcers haven't been a problem. An angle 15 degrees would require a rsh of 14.5 and 20 degrees would drop the rsh to just 13.2 inches. Both would make it harder for me to transfer. At 6 ft tall with long arms, the lower rsh might reduce the efficiency of pushing on the rims more than my weakness can manage.

Maybe there is other research that supports a larger seat incline than I have, but in my case and apparently a few others, a bit less incline may provide a better overall sitting and wheeling position.

I'm leaning toward reducing the back angle a bit to improve stability.

7. Originally Posted by tauble
I'm leaning toward reducing the back angle a bit to improve stability.
Need to make sure on level ground you don't start slumping forwards (kyphosis). If ground is not level then turn so that you are facing uphill.

For ABs there are also papers on this problem. It is distracting to flight crew (Goossens, et al.) and also an OH&S issue for farmers who spend a lot of time on bench seats in their utility vehicles.

If seat angle is inadequate you could periodically lift your self or push on your knees to keep your back straight. However, I think that it is just better to be able to sit properly as my job requires me to sit and pay attention.

The kyphosis in back results in compensating curve in the neck. This is common problem for computer users that I am aware of. I use a computer lots and have to be very careful with my neck. Forever telling users off about their seat adjustments.

http://www.ncbi.nlm.nih.gov/pubmed/9271146

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