Hand posture prediction for power grasp of circular cylinders

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Abstracts

SESSION 12: ERGONOMICS Chairperson: M. Parnianpour PRELIMINARY EVALUATION OF A COMPREHENSIVE BIOIHECHANICAL MODEL USING STRENGTH, STABILITY, AND COEFFICIENT OF FRICTION CONSTRAINTS TO PREDICT HAND FORCE EXERTION CAPABIf..ITY DURIh’G SAGI-ITALLY SWC! STATIC EXERTIONS

Carter J, Kerk(l), Don B. Chaffin( and George B. Page@) (1) Center for Ergonomics, University of Michigan, Ann Arbor, MI 48109-2117 @)Research and Test Dept., Association of American Railroads, Washington, DC 2oool A preliminary laboratory study was undertaken to evaluate a new mathematical, biomechanical model which comprehensively and simultancowsly estimates feasible hand farce exertion capability during sagittally symmetric static exertions. In the model, a set of 15 finear constraint equations have been developed in three classes: coefficient of friction, stability, and saength. This set of consents maps out a feasible solution space for combinations of horizontal and ve.rtical farces at the hands. Inputs to the model include posture, ;IIIthropometry, strength capability, coefficient of friction, and footwear. The pilot subject’s strength and anthropomenic profiles wen: input into the model to aate a predicted feasible hand force solution space for the posture of inter& The p&ict& solution space was then cokpared to the actual hand force values exerted by the subject. The results show $?ra~hicallv that the uredicted feasible solution snace exhibits a reasonable abilitv to pre&ct hand forces. This $&mina& study appears to support the need f& further efforts with this reseakh. There are many hypotheses of interestwhich m currently being studied. The most promising aspect of this model appearsto be its abiIity to com~hensively combine multiple factors that can affect hand force exertion

capability. NORHALWRIST POSITION DURINGl4AXIHALGRIP Shawn W. O’Driscoll, Richard Ness, Tom D. Cahalan, Robfn R. Richards, E.Y. Chao, Kai-Nan An, Orthopaedic Biomechanics Laboratory, Mayo Cl.inic/Mayo Foundation, Rochester, Minnesota, Upper Extremity Reconstructive Service, Division of Orthopaedic Surgery, St. Michael’s Hospital, University of Toronto, Canada Previous attempts to determine the relationship between wrist position and grip strength have involved constraining the wrist with splints. This investigation was designed to determine the position assumed by a normal wrist during unconstrained maximal grip and the relationship between wrist position grip strengths were measured with a Jamar dynamometer in while recording wrist position with unconstraining wrist subject was asked to attempt maximum grip with the elbow

and grip strength. 40 healthy subjects electrogoniometers. flexed 90” and the

Bilateral aged 20-51 Each

forearm in Then grip strength and wrist position were neutral rotation (‘qself-selected” position). recorded while the subjects voluntarily deviated the wrist randomly into either flexion, The self-selected position was 3S”+Zo extension, radial or ulnar deviation of lo-15O. Grip strength was significantly (meankS.E.) of extension and 7Oi 2O of ulnar deviation. less in any position of deviation from this self-selected position (pO.OS, b=3O, R10.1). The dominant wrists were within 5’ of the nondominant ones but were relatively less extended and in more ulnar deviation. HAND POSTURE PREDICTONFOR POWER GRASP OF CIRCULARCYLINDERS Bryan Bucbholz and Thomas J. AmWrong Center for Ergonomics, The University of Michigan, Ann Arbor, MI 48109, U.S.A,

A kinematicmodel of the humanhandhas btwl developedin orderto predictand simulateits prehensilecapabilities. The kinematic skeletonof the handis charactetiredby idealjoints connectedby simplesegments. Fmger-jointangt&ion is characterized by yaw (abduction-adduction), pitch(flexion-extension)androll (axial mtation)angles. The modelis basedon an algorithmthatdetermines contactbetweentwo ellipsoids,which are used to m the surlhcegeometryof the handsegments. Coefficientsfor determining segmentlengths,breadthsanddepthsfromhandlengthand breadthate incorpomMin the model. The modelpredictsthe bandposture for powergasp of ellipsoidalobjectsby “wrapping”the fingersaroundthe object. Handpostureis quantifiedvia thejoint angles. Contactpointsand vectorsalong the hand-objectinterfaceare calculated.The ellipsoid-ellipsoidcontactalgorithmincludesprocedures fordeterminingellipsoidpenetration,so thatsoft tissue deformationmay be simulated. Graphicsproceduresare includedfor visual displayof the m&l. The modelhas been validatedusingcircularcylinders. The validationstudymeasuredflexicmangleson six subjectsthatcoveredthe rangeof handlengthsseen in the generalpopulation.Between41 sad 94% of the variation in measured flexion-extension angles were explained using linear models of hand length and cylinder diametez Increasing cylinder diameter decreased flexion and inclwrsing hand length increased flexkm for both measured and predicted joint angles. Measured joint angles were compared with joint angles predicted by the model for the same size ha& sad cyiinden. Red&ions were relatively consistent fez MCP and PIP, with the largest diffeznces on tht exames of cylinder diameter. Predictions for DIP were most variable, because of this joint’s dependenceon the anglespredictedfor the pmximaljoints. The worstpredictions are seen for DIP-V with the diagonal grasp where larger cylinder angles contribute to prediction errors. On the average,the model overpredicted joint flexionby 5.3% or 2.8 + 12.2 degrees. Information gathered in this nsearch will aid in the desiga of haad tools so that they may be used effWently without adverse effects due to mechanicaltrauma.