Biomechanical OptimizationDesign of a Novel Device f. Manual Wheelchair Propulsion

The wheelchair is one of the most commonly used assistive devices for enhancing the mobility of people with disabilities. During manual wheelchair propulsion high loads are repeatedly applied to the arms, leading to a high prevalence of upper-limb pain and pathology in manual wheelchair users. These conditions decrease the quality of life of these patients and are represent a significant burden to the healthcare system. Furthermore, the discontinuous stroke with the push rim in manual wheelchair propulsion is characterized by low mechanical efficiency, typically below 10%. The goal of this project is twofold: firstly, to determine an optimal wheelchair propulsion movement by taking into account the physiological properties of the upper limb, specifically, the anthropometric properties of the skeleton, moment arms of the muscle-tendon units, and the force-length-velocity properties of the muscles; and secondly, to develop two novel optimized mechanisms for manual wheelchair propulsion which provide a circular hand movement and a continuous upper-arm motion pattern. The new designs will involve the use of both flexor and extensor muscles, and hence a larger muscle mass, compared to that which is presently the case in the discontinuous push. The new designs will also spread physical strain over a larger number of muscles and avoid extreme joint excursions that are ergonomically problematic. Boundary conditions will assure that the mechanical devices do not increase the width of the chair, limit the workspace of the upper extremities for other movements or cause an impediment to transfer. To determine the optimized motion patterns, two optimization problems will be solved using an existing and previously validated three-dimensional musculoskeletal model of the upper limb in conjunction with a forward dynamics approach. Two different cost functions will be hypothesized: Cost Function 1 will minimize muscular effort during manual wheelchair propulsion, thereby improving mechanical efficiency; Cost Function 2 will minimize upper-limb joint-contact forces during wheelchair propulsion, which is presumably relevant to patients suffering from musculoskeletal pain or injury. Finally, dynamic optimization theory will be applied to design a mechanism for each of the optimized propulsion paths that generates a motion path for the hand that is as close as possible to the optimal path calculated from the model. Experiments will be performed on a specially-developed instrumented wheelchair ergometer using fifteen healthy subjects and fifteen paraplegic patients with a long history of wheelchair use. The ergometer will be equipped with a standard push-rim drive and the two optimized drives. Upper-limb joint kinematics, three-dimensional forces at the push-rim, and muscle EMG signals will be measured during propulsion. Upper-limb muscle strength will be measured for each subject during maximum isometric contractions using a handheld dynamometer. Musculoskeletal modeling and optimization theory will then be applied, and muscle activations and joint-contact forces determined by inverse dynamics and static optimization. The model-predicted muscle activation patterns will be compared to the measured sequence and timing of muscle EMG. The resulting data from the experiments with the standard push-rim drive and both optimized drives will be evaluated. The results for all three drives will be compared for each subject and between subjects. We expect that wheelchair propulsion using the two optimized drives will result in a significantly higher mechanical efficiency and lower joint-contact loads. Increasing mechanical efficiency will enable lower-limb disabled subjects to ambulate and perform activities of daily living with fewer restrictions. Minimizing joint loads will enable lower-limb disabled subjects to remain more independent, thereby reducing the functional and psychological consequences of upper-limb pain and dysfunction.

The wheelchair is one of the most commonly used assistive devices for enhancing the mobility of people with disabilities. During manual wheelchair propulsion high loads are repeatedly applied to the arms, leading to a high prevalence of upper-limb pain and pathology in manual wheelchair users. These conditions decrease the quality of life of these persons and represent a significant burden to the healthcare system. Furthermore, the discontinuous stroke with the push rim in manual wheelchair propulsion is characterized by low mechanical efficiency, typically below 10%. The goal of this project was twofold: firstly, to computationally determine an optimal wheelchair propulsion movement by taking into account the physiological properties of the musculoskeletal system of the upper limb, that spreads physical strain over a larger number of muscles and avoids extreme joint excursions that are ergonomically problematic; and secondly, to develop a novel optimized mechanism for manual wheelchair propulsion for practical implementation of the computationally determined movement pattern. A three-dimensional musculoskeletal model of the upper limb in conjunction with a forward dynamics approach was used to computationally develop the optimized movement pattern. Then, the novel propulsion mechanism was designed that generates a motion path for the hand that is as close as possible to the optimal path calculated from the model. A specially-developed instrumented wheelchair ergometer was used for experimental investigations on the novel wheelchair propulsion movement with both non-wheelchair users and paraplegic persons with a long history of wheelchair use. The ergometer was equipped with both a standard push-rim drive and the novel optimized drive. Upper-limb joint kinematics, three-dimensional contact forces at the hand, muscle activity signals, and oxygen consumption were measured for each test person during propulsion. Finally, inverse dynamic computer simulations with individually adapted musculoskeletal models and static optimization were applied, to determine muscle activation and joint loads for each individual test person during the experiments. The results show that, in contrary to standard push-rim propulsion, when using the novel optimized propulsion mechanism, joint excursions stay within the ergonomic joint ranges throughout propulsion; muscle activity is distributed over more muscles reducing the physical strain on individual muscles; oxygen consumption and heart beat rates are lower in comparison to push-rim propulsion. This reduction also leads to higher gross mechanical efficiency rates. Use of the novel optimized propulsion mechanism may enable lower-limb disabled subjects to ambulate and perform activities of daily living with fewer restrictions. Minimizing joint loads will enable lower-limb disabled subjects to remain more independent, thereby reducing the functional and psychological consequences of upper-limb pain and dysfunction.