Understanding and modelling upper body strength for people with C5-C7 tetraplegia.

The loss of function following a spinal cord injury (SCI) has a huge impact on a person’s life. Tetraplegia is the result of damage to the cervical region of the spinal cord causing a severe loss of motor and sensory function of the body below the injury. This results in a loss of function of the torso, all four limbs and neck. The severity of the loss of function is dependent on the location where the damage has occurred and whether the injury is classified as complete or incomplete. Complete tetraplegia is defined as an injury that has damaged the entire cross section of the spinal cord. Variation in the injury severity results in considerable differences in the strength capabilities of people with cervical SCI. People living with tetraplegia have a high dependence on assistive and mobility devices to complete the tasks of everyday life. This thesis pertains to the measuring and modelling of the upper body strength of people with tetraplegia. An understanding of the remaining upper body strength capabilities following an SCI is critical to promote the development of appropriate designs to be used by this population. This thesis developed a novel method to measure detailed multidirectional arm strength data of people in a seated position. A purpose built testing apparatus enabled an improved method to capture isometric force measurements rapidly in all directions over a person’s range of motion. Non-disabled participants were used to validate the testing method before testing was completed using participants with tetraplegia A comprehensive multidirectional strength study was completed using 10 partici- pants with C4-C7 tetraplegia and 11 non-disabled participants. First, the study was used to determine the differences in the strength characteristics of people with tetraplegia compared people with no disability. The study was then used to investigate whether each SCI level had a characteristic strength profile in the sagittal plane. The results highlighted an expected reduction in the range of motion and strength capabilities for participants with tetraplegia compared to non-disabled participants. The results found that participants with higher level tetraplegia had more significant reductions in strength in radial (shoulder to hand) direction compared to the circumferential direction resulting in polar plots with a more circular shape. The novel insights gained from the strength study expanded on previous isometric strength studies for people with tetraplegia by measuring isometric force in multiple directions. The statistical modelling section developed fitted models using the results from the strength study to investigate whether consistent strength patterns existed for specific exertion directions. A regression analysis for each participant was completed to fit multidirectional strength data to a fourth order polynomial model. In total 528 contour plots were generated for each direction to compare the empirical results with the corresponding fitted model for each participant. Overall the models had a good fit to the empirical data with an average adjusted R2 value of 0.9232 (4dp) for all participants and directions. The fitted model for each participant enables strength maps generated for each direction. This fitted model compliments the strength study findings by making the strength data more usable. Using the strength maps, designers are able to evaluate the force requirements for user specific designs without a physical prototype. A general model was developed using 7 sets of normalised empirical data from non-disabled participants. The general model had a good fit to the averaged empirical data with an average adjusted R2 value of 0.9427 (4dp) for all directions. The method used to develop a fitted model for the non-disabled participants could also be used for people with tetraplegia if an adequate amount of strength data was available. The theoretical modelling section of this thesis presented a 2D model of the upper body was developed to enable multidirectional force patterns to be predicted and evaluated for people with C5-C7 tetraplegia. Joint torque and static equilibrium limits were used to determine the limiting force over a person’s range of motion in the sagittal plane. The results from the model highlighted that the limiting force is not purely a function of upper limb joint strength but also stability. The model showed that the reduction in joint strength has a large impact on the magnitude of force a person with tetraplegia can apply. The lack of trunk function limited the predicted force a person with tetraplegia could apply due to the increased risk of falling forward due to pivoting at the hip. Comparisons with the results from the strength study were made to evaluate the accuracy of the fitted model and human model. The fitted model was used to make force predictions for two participants. The comparisons using the fitted model revealed that the model had a moderate goodness of fit to the measured data with an average R2 value of 0.6359 (4dp). Further comparisons were not completed due to the variation in the goodness of fit for different directions. The comparisons showed that further development of the fitted model would enable force exertions to be predicted for non-disabled people and people with tetraplegia. The comparison between the results from the human model highlighted the strengths and limitations of the 2D model. The results from the comparison showed that the model was effective in modeling the expected reduction in strength for participants with tetraplegia. The comparisons showed that the model was a useful tool to investigate factors which limit the applied force for each injury level of interest