FITTING A WHEELCHAIR BIOMECHANICS AND DESIGN February 24 2009 CORRECT FITTING OF A WHEELCHAIR Complications from improper fitting Sitting habits Correct posture Able bodied person long period of sitting usually 1 2 hours shifting weight all the time Disabled person may sit for 3 to 10 hours per day without repositioning Complications due to poor posture Contractions and deformities Tissue breakdown Reduced performance and tolerance Urinary and respiratory infection Fatigue and discomfort Free Wheelchair Mission Chair www doitfoundation org February 24 2009 CORRECT FITTING OF A WHEELCHAIR Correct anatomical and wheelchair positions Want to distribute weight over butt and thighs Only want 1 25cm clearance between butt and frame Correct body position Figures from Mayall 1995 1 25cm Correct wheelchair position Wheelchair Foundation Chair www kidswithoutborders com February 24 2009 CORRECT FITTING OF A WHEELCHAIR Considerations during assessment Wheelchair Foundation in Tanzania Considerations during prescription Diagnosis and prognosis Age Communication status Cognitive function Perceptual function Physical ability Level of independence in activities during daily living Transfer ability and modality Mobility ambulation and wheelchair mobility Body weight Sensory status Presence of edema Leisure interests Transportation to and from home Roughness of usage Time spent in wheelchair daily Financial resources of patient List from Mayall 1995 Tanzania Big Game Safari Largest donator in Tanzania giving away nearly 7 000 chairs so far Said Wheelchair Foundation will give a chair to anyone who seems to need one a loose requirement that may include people who are crawling on the ground to people who may walk with a crutch Admitted they get so many chairs every year that after the first few hundred have been distributed it is very difficult to find genuinely disabled people to whom they can give them Monduli Rehab Center Criticized the WC Foundation and said wheelchairs should not be given out like candy Because the village terrain is so rough people should be encouraged to walk with crutches or braces and WCs should be a last resort February 24 2009 CORRECT FITTING OF A WHEELCHAIR Cushioning and positioning Pressure Sores Close eyes if squeamish tuberosities Figures from Mayall 1995 February 24 2009 WHEELCHAIR PROPULSION First US wheelchair patent A P Blunt et all 1869 Example state of the art Quickie wheelchair 2006 Wheelchair propulsion 2 10 efficient Woude et al 1986 1998 Optimal human chemical mechanical whole body efficiency 25 Mark s STD Handbook 1978 Occurs at max muscle force and max muscle speed Optimal efficiency and max power output do not occur together Engage more muscles for more power Determine best system Wheelchair propulsion project Determine the upper body motion that yields highest sustainable power at highest efficiency to deterministically design a wheelchair drive system UROP Mario Bollini February 24 2009 WHEELCHAIR PROPULSION RESEARCH Previous work Power output measured from different drive systems Conventional chair Lever powered tricycle Pout 26 5W Pout 39 3W van der Linden et al 1996 van der Woude et al 1997 Motivation To deterministically design a drive system for long and short distance travel the maximum available efficient power should dictate the design Phuman Thuman human Pout Fresist Vdevice Fresist R wheel wheel TBD wheel Gear Ratio human tune through design dictated by environment calculated February 24 2009 WHEELCHAIR PROPULSION RESEARCH Upper body biomechanics data Shigley Mischke 1996 Single arm energy output T Nm F 56 48 3 F 59 1 27J stroke 2 3 rad 5 6 Conventional wheelchair propulsion 2 76 3 T Nm error from van der Linden et al 1996 35J stroke 2 3 rad 5 6 Opposed handrim wheel rotation February 24 2009 WHEELCHAIR PROPULSION RESEARCH Single arm energy output T Nm F 75 6 pull 79 1 71 6 40J stroke 5 6 73J cycle push 56 33J stroke rad 5 6 Rowing motion propulsion Additional questions What unidentified upper body motions can give high power output How different disabilities affect range of motion What type of resistance forces will be encountered depending on the environment Unidentified high power motions February 24 2009 LFC DEVELOPMENT Variable speeds through a fixed geartrain L Wheel projection L Chainring RW DFW DCR Hub Low Gear Geartrain performance Difference between chair velocity VChair and hand velocity VHand High Gear VChair DCR RW VHand DFW L February 24 2009 LFC DEVELOPMENT Lever sizing Pushing power at peak efficiency young male 19 6W 51N and 0 38m s Max pushing force at slow speed both hands 50 male 356N PIn POut PHuman FHandVHand PDrag PRolling PGravity Power balance at peak efficiency 1 FHandVHand CD air A VChair 3 mg VChair cos sin 1 12 0 98 1 31 1 53 0 84 1 09 1 31 0 87 1 09 0 70 Lever length m 0 56 0 66 0 87 0 42 0 44 0 66 0 28 Coeff of rolling resistance 30 15 Coeff of rolling resistance 0 0 00 0 43 0 00 0 22 0 14 0 31 0 22 0 44 1 12 1 26 0 98 1 12 0 84 0 98 0 70 0 84 0 56 0 70 0 42 0 56 0 28 0 42 0 14 0 28 0 00 0 14 0 25 1 53 1 75 1 26 0 01 0 11 0 10 0 08 0 07 0 05 0 04 0 02 1 75 1 97 0 37 Climbing angle deg 0 00 0 01 4 00 8 00 2 19 1 97 1 75 1 53 1 31 Lever 1 09 0 87 length m 0 66 0 44 0 22 0 00 0 19 1 97 2 19 Lever length m for 36T chainring and 20T freewheel 0 13 Lever length m for 36T chainring and 20T freewheel 0 07 Force balance at peak force FRe sist 2 FRolling FGravity mg cos sin Climbing angle deg February 24 2009 LFC DEVELOPMENT Off road testing 17 6 February 24 2009 POWERED TRICYCLE CALCULATIONS February 24 2009 SOLAR ENERGY DISTRIBUTION WWW RISE ORG AU February 24 2009 DESIGN FOR HUMAN USE Safety factors Uncertainties in strength Uncertainties in Loading p y ns Fp Fy nL Where p permissible stress y yield strength ns strength factor of safety typically 1 2 to 1 4 Where Fp permissible load Fy max load nL strength factor of safety ntotal nsnL For machines that can cause injury or death ntotal is typically 4 to 10 When choosing a safety factor consider Does the load come from human activity Does loading come from natural sources terrain etc What are the consequences of failure Is the loading due to a prelaod Does the load come from a power source ex starting vs steady torque Does the load come from driven machinery that can change its output Theory adapted from Shigley 1983 Wikipedia 2007 February 24 2009 EXAMPLE Estimating loading factor in bicycles drop case http www youtube com watch v tMmiN6M7GXs feature PlayList p