Kinematic adaptations and invariants couple with stiffness in response to carried load
Caron, Robert R
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The focus of this research was to investigate the biomechanics of walking with load at a constant speed. Two studies were conducted from one experiment using seventeen healthy young volunteers. Subjects walked on a treadmill at their preferred unloaded walking velocity, while nine loads were systematically added to and removed from a backpack. Kinematic data were collected using a 3D motion analysis system. The purpose of the first study was to identify changes in kinematics (adaptations) along with kinematics that do not significantly change (invariants) in the sagittal plane as load was manipulated while walking. Using a within-subject repeated measures design we sought to discover how forward lean changed with load and what function it served relative to center of mass (COM) movement and orientation of the vectors spanning the COM to lower-extremity joints. We found that forward lean changed linearly with load and appears to help maintain the orientation of COM-to-ankle and -knee vectors across a wide range of loads. We also found that the amplitude and shape of the vertical COM trajectory curves for all load conditions remained within a narrow range when compared to the initial load condition, partially maintained by invariant lower-extremity segment orientations and a constant direction of the push-off vector at toe-off. The purpose of the second study was to investigate how the body modulates stiffness in response to changing load at a constant walking velocity. We used two models to evaluate how stiffness altered with load during the stance phase. Linear regression analyses revealed that our estimates of stiffness changed in a highly linear manner in response to the additional gravitational forces due to load. This dissertation showcases a series of kinematic adaptations and invariants that couple with changes in stiffness to meet the demands of changing load during stance. We apply inverted pendulum dynamics to loaded gait and emphasize the importance of constraining COM movement regardless of the substantial perturbation of load while walking. This research has relevance to biomechanics and motor control, with applications to the development of assistive orthoses and exoskeletons, as well as furthering our understanding of human movement.
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