The high demands of manual wheelchair propulsion put users at risk of additional pain and injury that can lead to further reductions in independence and quality of life. Seat position is an adjustable parameter that has been shown to influence propulsion biomechanics. As a result, a number of studies have attempted to optimize this position. However, due to complexities in quantifying upper extremity demand, seat position guidelines are often based on studies aimed at reducing indirect quantities (e.g., cadence, handrim forces, joint ranges of motion and muscle excitation levels) rather than more direct measures of demand (e.g., muscle stress and metabolic cost). Forward dynamics simulations provide an alternative approach to systematically investigate the influence of seat position on more direct measures of upper extremity demand. The objective of this study was to generate and analyze a set of forward dynamics simulations of wheelchair propulsion across the range of attainable seat positions to identify the optimal seat position that minimizes upper extremity demand (i.e., muscle stress, metabolic cost and muscle antagonism). The optimization results showed both metabolic cost and muscle stresses were near minimal values at superior/inferior positions corresponding to top dead center elbow angles between 110 and 120 degrees while at an anterior/posterior position with a hub-shoulder angle between 10 and 2.5 degrees. These minimal values coincided with a reduction in the level of antagonistic muscle activity, primarily at the glenohumeral joint. Seat positions that deviated from these minimal values increased the level of co-contraction required to maintain a stable, smooth propulsive stroke, and consequentially increased upper extremity demand. These results can provide guidelines for positioning the seat to help reduce upper extremity overuse injuries and pain, and thus improve the overall quality of life for wheelchair users. / text

Unilateral, below-knee amputees have altered gait mechanics, which can significantly affect mobility. For example, amputees often have asymmetric leg loading as well as higher metabolic cost and an increased risk of falling compared to non-amputees. Below-knee amputees lose the functional use of the ankle muscles, which are critical in non-amputee walking for providing body support, forward propulsion and leg-swing initiation. The ankle muscles also regulate angular momentum in non-amputees, which is important for providing body stability and preventing falls. Thus, compensatory mechanisms in amputee walking are developed to accomplish the functional tasks normally provided by the ankle muscles. In Chapters 2 and 3, three-dimensional forward dynamics simulations of amputee and non-amputee walking were generated to identify compensatory mechanisms and their effects on joint loading and metabolic cost. Results showed that the prosthesis provided body support, but did not provide sufficient body propulsion or leg-swing initiation. As a result, compensations by the residual leg gluteus maximus, gluteus medius, and hamstrings were needed. The simulations also showed the intact leg tibio-femoral joint contact impulse was greater than the residual leg and that the vasti and hamstrings were the primary contributors to the joint impulse on both the intact and residual legs. The amputee simulation had higher metabolic cost than the non-amputee simulation, which was primarily due to prolonged muscle activity from the residual leg gluteus maximus, gluteus medius, hamstrings, vasti and intact leg vasti and ankle muscles. In Chapter 4, whole-body angular momentum in amputees and non-amputees was analyzed. Reduced residual leg propulsion resulted in a smaller range of sagittal plane angular momentum in the second half of the gait cycle. Thus, to conserve angular momentum, reduced braking was needed in the first half of the gait cycle. Decreased residual leg braking appears to be an important mechanism to regulate sagittal plane angular momentum in amputee walking, but was also associated with a greater range of angular momentum that may contribute to reduced stability in amputees. These studies have provided important insight into compensatory mechanisms in below-knee amputee walking and have the potential to guide rehabilitation methods to improve amputee mobility. / text

Over one million amputees are living in the United States with major lower limb loss (Ziegler-Graham et al. 2008). Lower limb amputation leads to the functional loss of the ankle plantar flexor muscles, which are important contributors to body support, forward propulsion, and leg swing initiation during walking (Neptune et al. 2001; Liu et al. 2006). Effective prosthetic component design is essential for successful rehabilitation of amputees to return to an active lifestyle by partially replacing the functional role of the ankle muscles. The series of experimental and computer simulation studies presented in this research showed that design characteristics of energy storage and return prosthetic ankles, specifically the elastic stiffness, significantly influence residual and intact leg ground reaction forces, knee joint moments, and muscle activity, thus affecting muscle output. These findings highlight the importance of proper prosthetic foot stiffness prescription for amputees to assure effective rehabilitation outcomes. The research also showed that the ankle muscles serve to stabilize the body during turning the center of mass. When amputees turn while supported by their prosthetic components, they rely more on gravity to redirect the center of mass than active muscle generation. This mechanism increases the risks of falling and identifies a need for prosthetic components and rehabilitation focused on increasing amputee stability during turning. A proper understanding of the effects of prosthetic components on amputee walking mechanics is critical to decreasing complications and risks that are prevalent among lower-limb amputees. The presented research is an important step towards reaching this goal. / text