Mechanical simulations as a tool for assessing the influence of wheelchair settings on the propulsion efficiency

Abstract

Manual wheelchair (MWC) allows active disabled people to recover mobility but constrains both cardiorespiratory and upper-limb musculoskeletal systems. User morphology, experience and propulsion strategy as well as environment and wheelchair settings can affect the constraints sustained by the upper limbs. Choosing and configuring a wheelchair is a difficult task, which requires the analysis of multiple factors. Due to this difficulty, the process sometimes results in non-optimized wheelchair settings. The present work describes a method which simulates the wheelchair behavior from a mechanical model. The ability of this method to provide a better understanding of the effect of various wheelchair settings on propulsion difficulty is illustrated through an example. At first, appropriate representations (or “models”) need to be developed. In this study, the user was modeled by his center of mass position with respect to drive wheels; the MWC was modeled by an assembly of solids characterized by their weight, geometry and material; and the environment was modeled by a flat surface characterized by roughness and inclination. Through mechanical analysis, the MWC settings and movements were linked to the user efforts and energy losses. As inputs for the simulation, the efforts applied by one subject were previously measured during straightforward propulsion using specific instrumented MWC. Finally, various propulsion scenarios were numerically simulated, in which wheelchair settings were changed, keeping every other experimental conditions constant. Scenarios where instability occurred were discarted. Simulation allowed defining an appropriate wheelchair configuration (mainly defined by a backward position of the user and large castor radii) where energy losses were decreased by 2.5 times with respect to the 3.3 Joules per propulsion cycle observed during experiments. When choosing and configuring a wheelchair, tradeoffs often need to be found between energetic performance and other mandatory aspects, like stability. Through numerical simulation, optimization can be developed to assist in defining a set of MWC setting that takes into account the necessary tradeoffs. Nowadays, similar analysis can be applied to hard smooth/carpet surfaces, turning motion, ramps and cross slopes. More complex subject-specific musculoskeletal models are developed for better precision and understanding.