How different variables affect wheelchair tipping probability

September 22nd, 2019 | 2 comments

Summary by Grace Hu

Edited by Ana-Maria Oproescu

This is a summary of a research study that was conducted by ICORD researchers Dr. Jaimie Borisoff, Dr. Carolyn Sparrey, and colleague Dr. Thomas. Click here to access the link to the original study. 

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Why is it important to study wheelchair tip probability?

The tipping or falling of wheelchairs can cause an individual to sustain serious injuries, such as traumatic brain injuries, concussions, and bone fractures. The probability of wheelchair tipping can be heightened or reduced by a variety of factors and it is important that we understand these factors in order to increase wheelchair stability. 

What was the purpose of the study?

The purpose of the study was to determine how adjusting different user variables and conditions affects the tip probability of a manual wheelchair during movement (ie. the ‘dynamic tip probability‘). The researchers examined the effects of seat angles, wheelchair speed, user position, user mass, and bump heights. 

How was this study conducted?

The stability of a moving wheelchair was initially tested by using simulations on MADYMO software. The researchers then validated the simulation results by placing test dummies in a wheelchair and rolling them down a slope towards a small block. Each time the wheelchair was rolled down, different variables were adjusted to test the likelihood of the wheelchair tipping. The seat angles were set to values 0-20° below the horizontal, the wheelchair speeds were set to  0-20 km/ hour, the masses of the test dummies varied between 50-115 kg, and the dummy positions varied from 0-10 cm from where the backrest meets the wheelchair. In addition, different bump heights were added along the wheelchair path, varying from 0-4 cm in height. Statistical analysis revealed the effects of these variables on wheelchair tip probability. Different types of tipping included: backward tip, forward tip, rolling over a bump, and stopping due to a bump.

What are the effects of different variables on dynamic tip probability?

 

Wheelchair Speed:	Increasing speed →	Increases probability of wheelchair tipping forwards and backwards Increases probability of wheelchair rolling over bumps instead of stopping due to them
Bump Height:	Increasing bump height above 1.5 cm →	Increases tip probability
Seat Angle:	Increasing the angle below horizontal →	Increases chance of wheelchair’s continued movement over bumps instead of stopping Decreases forward tipping Increases backwards tipping
Backrest Angle:	Reclining →	Has the greatest effect in decreasing forwards tipping

The researchers found that increasing the wheelchair speed increased the probability of the wheelchair tipping forwards and backwards, as well as rolling over bumps instead of stopping due to them. In general, the wheelchairs rolled over bumps less than 1.5 cm in height and the tipping probability increased with greater bump height. Lowering the seat to greater angles below the horizontal also increased the chance of the wheelchair continuing its movement over bumps instead of stopping. Lowering the seat height also decreased forwards tipping while increasing backwards tipping — but overall, reclining the backrest had the greatest effect in decreasing forwards tipping.

What’s the significance of these findings?

These results suggest that forward tipping, which is responsible for the greatest risk of injury, can be reduced by decreasing one’s wheelchair speed, decreasing the size of bumps, reclining the wheelchair backrest, and lowering the wheelchair seat height. However, these findings may not be applicable to all types of wheelchairs, so it is important to remember that adjustments of variables should be made according to each individual wheelchair.