Browsing by Subject "Biomechanics"
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Item Adaptability of stride-to-stride control of stepping movements in human walking and running(2014-05) Bohnsack, Nicole Kristen; Dingwell, Jonathan B.Walking and running are essential tasks people take for granted every day. However, these are highly complex tasks that require significant neural control. This is complicated by the inherent redundancy of the nervous system and by physiological noise. Humans may adopt different control strategies to achieve different goals (environmental or task specific). More specifically, walking/running on a treadmill only requires that one not walk off the treadmill. Of the many possible strategies that can achieve this goal, humans attempt to maintain a constant speed from each stride to the next (Dingwell, John et al. 2010). However, how humans alter the stride-to-stride regulation of their gait when the task goals change (e.g., by maintaining stride length and/or time, during running, or during a predicted walk to run transition speed) has not yet been demonstrated. In the first two of three experiments conducted, healthy adults either walked or ran on a motorized treadmill at a comfortable speed under the following conditions: constant speed, constant speed with the stride length goal (targets on the treadmill), constant speed with the stride time goal (metronome), or constant speed with both stride length and stride time goals. In a third experiment, subjects walked and/or ran at a comfortable speed and also at their predicted theoretical walk to run transition speed. Goal functions derived from the task specifications yielded new variables that defined fluctuations either directly relevant to, or irrelevant to, achieving each goal. The magnitude of the variability, as well as the stride-to-stride temporal fluctuations in these variables, were calculated. During walking, subjects exploited different redundancy relationships in different ways to prioritize certain task goals (maintain stride speed) over others (maintain stride length or stride time) in each different context. In general, subjects made rapid corrections of those stride-to-stride deviations that were most directly relevant to the different task goals adopted in each walking condition. Thus, the central nervous system readily adapts to achieve multiple goals simultaneously. During running, subjects exhibited similar adaptations to walking, but over-corrected to prioritize maintaining stride speed even more strongly. This suggests that stepping control strategies adapt to the level of perceived risk. This purposeful adaptability of these stride-to-stride control strategies could be exploited to developing more effective rehabilitation interventions for patients with locomotor impairments. During the predicted walk-to-run speeds, subjects were able to largely exploit the redundancy within task goal, and effectively operated at “uncomfortable” speeds. These results suggest that the stride speed control is robust even with additional novel tasks and uncomfortable, abnormal speeds of locomotion.Item Assessment of turning performance and muscle coordination in individuals post-stroke(2020-04-29) Lewallen, Lindsey Kate; Neptune, Richard R.Turning is an important activity of daily living and often compromised post-stroke. The fall rate for post-stroke individuals while turning is nearly four times as high compared to healthy adults, with most falls resulting in injury. Thus, there is a need for evidence-based rehabilitation targets to improve turning performance for post-stroke individuals. To produce well-coordinated movements, muscles can be organized into muscle modules (i.e., groups of co-excited muscles). Post-stroke these modules can be merged, leading to impaired muscle coordination and walking performance. However, the relationship between impaired coordination and turning performance is not well understood. Thus, the purpose of this study was to analyze the influence of impaired muscle coordination (i.e., merged modules) on turning performance (i.e., time to complete a turn, number of steps required to complete a turn, smoothness of performing the turn and balance control during a turn). Post-stroke individuals and healthy controls performed three tasks including overground straight line walking, a 90-degree turn and a 180-degree turn. The number of muscle modules during straight line walking were determined using non-negative matrix factorization. As few as two modules were found in post-stroke individuals. Differences in turning performance were only seen in the 180-degree turning performance measures. Those with two modules took longer to turn, used more steps and had less smooth movement. These results suggest obtaining independent modules should be an important aim in locomotor therapies aimed at improving turning performance. In addition, the time it takes to complete a 180-degree turn may be a useful clinical assessment measure of the degree of muscle coordination impairment post-stroke.Item Comparison of muscle coordination between individuals post-stroke and kinematically constrained walking(2019-05-09) Prajapati, Sunil Kishor; Sulzer, James S.Abnormal motor coordination affects motor function following stroke, yet we lack a complete characterization of how such abnormal coordination affects movements such as gait. Previous research found that post-stroke gait exhibited fewer movement primitives, or muscle modules, than healthy individuals, suggesting abnormal coordination may affect gait function. However, aside from abnormal coordination, the reduced number of modules could also be due to compensations in response to other impairments such as increased muscle tone and spasticity. Our previous research compared gait in those with post-stroke Stiff-Knee Gait (SKG) to healthy individuals with kinematically constrained knee flexion. While healthy individuals compensated with pelvic obliquity, those with post-stroke SKG also exhibited greater hip abduction, suggesting the motion may be related to neural impairments such as abnormal coordination. We hypothesize that abnormal coordination, not the compensations due to restricted ranges of motion induced by other impairments, is associated with reduced gait function. In this experiment, we compared muscle coordination patterns emerging from healthy individuals with and without restricted knee kinematics to a cohort of individuals post-stroke, both with and without SKG. We predicted the number of muscle modules would be fewer than healthy individuals with similar gait kinematics and found that mechanical knee restriction reduced the number of modules similar to those with post-stroke SKG in walking. Constraining healthy motions resulted in similar muscular coordination patterns to unrestricted gait suggesting the robustness of muscle recruitment despite a kinematic perturbation. The composition of modules in the pre-swing phase between those with SKG and the mechanically restricted group differed (Spearman’s ρ = -0.024), whereas comparisons between post-stroke individuals without SKG (NSKG) and the healthy group were similar (Spearman’s ρ = 0.833). We found those with SKG relied less on hamstrings than healthy counterparts, suggesting an altered motor command beyond adaptation. Muscle coordination patterns in constrained motions during gait were not similar to SKG while the NSKG group showed greater similarity to normal walking. Thus, our data suggests that abnormal coordination may play a greater role in SKG than those without SKG. The results of this comparison will help develop more accurate interventions for clinical treatment.Item Compensatory mechanisms in below-knee amputee walking and their effects on knee joint loading, metabolic cost and angular momentum(2010-08) Silverman, Anne Katherine; Neptune, Richard R.; Barr, Ronald E.; Dingwell, Jonathan B.; Fernandez, Benito; Longoria, Raul G.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.Item Conquering the charity stripe : a literature review on the biomechanics of free throw shooting(2021-07-24) Williams, Edward Joseph, M.S. in Kinesiology; Hsiao, Hao-YuanBasketball is one of the most popular sports on the planet and free throws are vital component of the game. This literature review aims to assess the existing literature to identify key performance indicators for successful free throw trajectory and the biomechanics of the free throw motion that generate such indicators. Specifically, this review addresses mathematical and computer-generated models regarding free throw shooting trajectory and biomechanical analyses that utilized motion capture. Several motor learning principles are considered in the analysis as well, such as Gentile’s taxonomy of motor skills, Bernstein’s degrees of freedom problem and individual variability. Modeling studies have shown that free throw success is remarkably sensitive to release velocity of the ball. Three other crucial variables were identified as well: release height, release angle and backspin. Biomechanical analyses showed that intersegmental coordination is crucial for success, particularly in the upper arm. Postural control, balance and stability are important as well. However, individual movement patterns can differ significantly and there is some degree of movement variability, even in experts. Therefore, individual analysis is warranted for different players. There are notable gaps in our current understanding of the free throw motion. While the upper extremity is frequently assessed in detail, details regarding the lower extremity are lacking and any relationship between the two is largely ignored. Although postural variables were revealed to play a significant role in consistent success, they are infrequently assessed. Application of models and biomechanical assessments to skill improvement has not yet been investigated in the literature and would be an ideal future direction for this line of research. Future studies should seek to assess the whole body as a unit, rather than the largely segmental approaches utilized thus far. More advanced kinetic data and force plate analysis should be explored by future investigators as well.Item Empirical and musculoskeletal modeling approaches to assess and reshape angular momenta during human locomotion(2022-08-11) Li, Wentao, Ph. D.; Fey, Nicholas Phillip; Neptune, Richard R; Djurdjanovic, Dragan; Hsiao, Hao-YuanDaily ambulatory tasks are complex. As an example, locomotor transitions are transient movements that occur when an individual changes their direction and/or terrain and often require significant neuromotor adjustments to successfully navigate these tasks while remaining balanced. Individuals with pathology-specific changes to the neuro-musculoskeletal system may face additional barriers during these and other ambulatory tasks. It is essential to understand the mechanisms of balance regulation of healthy and fall-prone individuals during daily ambulation to prevent falls, improve performance, inform assistive technologies and enhance the quality of lives of specific individuals. However, a knowledge gaps exists in terms of how specific task-dependent factors and musculoskeletal pathologies affect balance regulation, and how balance can be optimized. In this research, empirical and musculoskeletal modeling studies were performed to investigate the angular momentum (i.e. dynamic balance) regulation of healthy and patient populations. First, we perform experimental studies that compare how task anticipation and complexity influence dynamic balance regulation of healthy individuals during locomotor transitions. We found that when given prior knowledge of the future task, individuals made style-dependent (crossover vs. sidestep) anticipatory adjustments before the transition to maintain dynamic balance. We also show that patients with developmental dysplasia of hip and femoroacetabular impingement syndrome, experienced poor dynamic balance regulation in the sagittal plane during stair ambulation, which is strongly correlated with the numeric pain they experience. Finally, we show that patients with Parkinson’s disease exhibit impaired dynamic balance regulation in stair descent ambulation with increased whole-body and lower-limb angular momentum compared to able-bodied individuals. In a second series of studies, we developed and tested a predictive musculoskeletal simulation framework to explore and guide how specific individuals can improve their performance of locomotor transitions and straightline walking. We show in these studies that altered sequencing and magnitude of joint kinematics, as well as muscle mechanical power were required to optimally reshape (i.e., improve the regulation of or regulate in a predefined profile) dynamic balance of locomotor transitions. We also show that during straight-line walking, optimizing various biomechanically-meaningful performance objectives related to metabolic energy cost, muscle effort, dynamic balance and lower-limb mechanical loading elicit observable changes in walking mechanics. Specifically, decreased hip abduction angle and delayed phase of joint kinematics were associated with simulations that regulated dynamic balance, while increased hip adduction, abduction and subtalar moments were related to simulations that minimized metabolic cost. In all simulations, we show that balance, energy and loading can be optimized in isolation or in combination and result in unique observable changes in walking mechanics. This research provides new insights into the dynamic balance regulation in healthy individuals and patient populations during walking, as well as the mechanics that could be used to improve performance, guide the development of rehabilitation training and assistive device interventions that target specific objectives for specific individuals, during specific tasks.Item Experimental analysis and computational simulation of unilateral transtibial amputee walking to evaluate prosthetic device design characteristics and amputee gait mechanics(2010-05) Ventura, Jessica Dawn; Neptune, Richard R.; Barr, Ronald E.; Crawford, Richard H.; Fernandez, Benito R.; Abraham, Lawrence D.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.Item Exploring blood clot mechanics : scientific and educational tools for comprehensive understanding(2023-12) Sugerman, Gabriella P.; Rausch, Manuel Karl; Hutter, Tanya; Zoldan, Janeta; Parekh, SapunVenous thromboembolism, encompassing deep vein thrombosis and pulmonary embolism, poses a significant global health burden, impacting millions of lives each year. A key determinant of patient outcomes in thromboembolic disease is the occurrence of embolization – or of small clot pieces breaking off. Embolization risk is not currently well understood, and clinical care for thrombi is not informed by clot-specific factors. In order to stratify embolic risk, we must first understand blood clot as a material and the factors which contribute to its ability to resist fracture. This is a challenging problem, as in vivo thrombi are difficult if not impossible to access nondestructively, heterogeneous, and irregular in size and shape. As such, in vitro blood clot mimic models are often used as a proxy for in vivo thrombi. In this dissertation, we first develop an experimental framework to rigorously characterize the mechanics of whole blood clots. We first approximated their behavior as hyperelastic using simple shear testing, simultaneously determining the effects of experimental factors like blood storage and coagulation time on clots’ mechanics. Through this study we determined that blood clot has strongly nonlinear mechanics and its hyperelastic behavior is well described using the Ogden model. Secondly, we examined the dissipative mechanisms which arise prior to fracture in a pure shear geometry. We describe that clots are viscoelastic, demonstrating strain-rate dependence, hysteresis, Mullins-like effect, and nonlinear stress relaxation. Third, we compared the viscoelastic and fracture behavior of clots made from human and bovine blood. We found that, compared to bovine clots, clots made from human blood are weaker, softer, and less resistant to fracture, among other differences. Finally, we developed a low-cost, openly-available mechanical testing device and employed it as a teaching tool in a college-level biomechanics course as well as two elementary-level outreach activities. Altogether, this dissertation contributes to our collective understanding of venous thromboembolism and offers resources for communicating the importance of biomechanics to the public.Item Frontal-plane biomechanical model for predicting peak limb loading during gait in individuals post-stroke(2022-01-25) Smither, Mariah; Hsiao, Hao-Yuan; Freedberg, MichaelBackground: Individuals with hemiparesis exhibit decreased limb loading onto the paretic limb which is negatively associated with walking and balance ability. Decreases in hip abductor strength and delayed hip abductor moments are also associated with diminished limb loading in this population. Additionally, the paretic (affected) foot is often placed more laterally during walking which likely decreases the limb loading force. Overall, this evidence indicates that the hip abduction moment and foot placement are likely limiting factors to limb loading ability. The aim of this study was to construct and validate a biomechanical model using frontal plane variables to predict peak limb loading in older adults and individuals post-stroke. Methods: Older adults (n = 18) and individuals post-stroke (n = 22) walked on a treadmill at their self-selected speed. A biomechanical model was constructed in older adults using the hip abduction moment, center of mass (COM)-foot angle (estimated ground reaction force angle), and the ground reaction force (GRF) moment arm (horizontal distance between the hip joint center and COM) and was subsequently validated in individuals post-stroke. Findings: The model accurately modeled limb loading with three frontal plane gait characteristics explaining 86% and 59% of the variance in limb loading force in the paretic and non-paretic limbs of individuals post-stroke, respectively. Individuals post-stroke were observed to have smaller hip abduction moments and COM-foot angles compared to older adults. However, the GRF moment arm was only smaller in the non-paretic limb compared to older adults, whereas the VGRF was only smaller in the paretic limb compared to older adults. Conclusion: This model can be used to explain the mathematical relationships between the hip abduction torque, foot placement, and limb loading. Future studies may apply this model to quantify the relative contribution of each biomechanical factor to the limb loading force. This will enhance our understanding of the mechanisms of limb loading and provide the framework for designing effective rehabilitation strategies that target limb loading ability, ultimately improving balance and mobility outcomes post-stroke.Item Impact of kinematics and kinetics on classification of dual-task gait(2021-08-02) Chiarello, Mark A.; Sulzer, James S.; Salinas, Mandy McClintock; Hilsabeck, RobinEarly detection of Alzheimer’s Disease and Related Disorders (ADRD) has been a focus of research with the hope that early intervention may improve clinical outcomes. The manifestation of motor impairment in early stages of ADRD has led to the inclusion of gait assessments (typically qualitative or focused only on spatiotemporal gait features) in clinical evaluations for these diseases. This study aims to develop the groundwork for a classification tool to improve early detection of ADRD using biomechanical gait features by determining if machine learning algorithms can decode different levels of cognitive load in healthy individuals. A dual-task paradigm was used to simulate cognitive impairment in 40 healthy adults, with single-task walking trials representing normal, healthy gait. The Paced Auditory Serial Addition Task was administered at two different inter-stimulus intervals of 2.4 s and 1.6 s to manipulate cognitive load (Dual₁ and Dual₂ conditions, respectively). The primary hypothesis of this study is that using kinematic and kinetic gait features will improve classification performance compared to spatiotemporal gait features during single-task and dual-task gait classification among healthy adults. Repeated Measures ANOVA showed significant changes in 13 different gait features across all three levels of cognitive load (Single, Dual₁, and Dual₂). Three supervised machine learning algorithms (Partial Least Square Discriminant Analysis, Linear Discriminant Analysis, and Neural Network Pattern Recognition) were used to classify data points using a series of different gait feature sets, and performance was based on the area under the curve (AUC) of the receiver operating characteristic curve, which offers a combined measure of sensitivity and specificity on a 0-1 scale with a chance level of 0.5. Machine learning classification yielded AUC up to 0.865 for the Single vs Dual classification task (identifying presence of cognitive load), and up to 0.761 for classification across all conditions (identifying level of cognitive load). The results here show the ability to classify gait based on cognitive load with above-chance sensitivity and specificity using gait parameters and machine learning classifiers.Item The influence of altering wheelchair propulsion technique on upper extremity demand(2010-08) Rankin, Jeffery Wade; Neptune, Richard R.; Barr, Ronald E.; Fernandez, Benito R.; Dingwell, Jonathan B.; Richter, William M.Most manual wheelchair users will experience upper extremity injury and pain during their lifetime, which can be partly attributed to the high load requirements, repetitive motions and extreme joint postures required during wheelchair propulsion. Recent efforts have attempted to determine how different propulsion techniques influence upper extremity demand using broad measures of demand (e.g., metabolic cost). However studies using more specific measures (e.g., muscle stress), have greater potential to determine how altering propulsion technique influences demand. The goal of this research was to use a musculoskeletal model with forward dynamics simulations of wheelchair propulsion to determine how altering propulsion technique influences muscle demand. Three studies were performed to achieve this goal. In the first study, a wheelchair propulsion simulation was used with a segment power analysis to identify muscle functional roles. The analysis showed that muscles contributed to either the push (i.e. delivering handrim power) or recovery (i.e. repositioning the hand) subtasks, with the transition period between the subtasks requiring high muscle co-contraction. The high co-contraction suggests that future studies focused on altering transition period biomechanics may have the greatest potential to reduce upper extremity demand. The second study investigated how changing the fraction effective force (i.e. the ratio of the tangential to total handrim force, FEF) influenced muscle demand. Simulations maximizing and minimizing FEF both had higher muscle work and stress relative to the nominal simulation. Therefore, the optimal FEF value appears to balance increasing FEF with minimizing upper extremity demand and care should be taken when using FEF to reduce demand. In the third study, simulations of biofeedback trials were used to determine the influence of cadence, push angle and peak handrim force on muscle demand. Although minimizing peak force had the lowest total muscle stress, individual stresses of many muscles were >20% and the simulation had the highest cadence, suggesting that this variable may not reduce demand. Instead minimizing cadence may be most effective, which had the lowest total muscle work and slowest cadence. These results have important implications for designing effective rehabilitation strategies that can reduce upper extremity injury and pain among manual wheelchair users.Item The influence of prosthetic foot design and walking speed on below-knee amputee gait mechanics(2011-12) Fey, Nicholas Phillip; Neptune, Richard R.; Abraham, Lawrence D.; Barr, Ronald E.; Crawford, Richard H.; Longoria, Raul G.Unilateral below-knee amputees commonly experience asymmetrical gait patterns and develop comorbidities in their intact (non-amputated) and residual (amputated) legs, with the mechanisms leading to these asymmetries and comorbidities being poorly understood. Prosthetic feet have been designed in an attempt to minimize walking asymmetries by utilizing elastic energy storage and return (ESAR) to help provide body support, forward propulsion and leg swing initiation. However, identifying the influence of walking speed and prosthetic foot stiffness on amputee gait mechanics is needed to develop evidence-based rationale for prosthetic foot selection and treatment of comorbidities. In this research, experimental and modeling studies were performed to identify the influence of walking speed and prosthetic foot stiffness on amputee walking mechanics. The results showed that when asymptomatic and relatively new amputees walk using clinically prescribed prosthetic feet across a wide range of speeds, loading asymmetries exist between the intact and residual knees. However, knee intersegmental joint force and moment quantities in both legs were not higher compared to non-amputees, suggesting that increased knee loads leading to joint disorders may develop in response to prolonged prosthesis usage or the onset of joint pathology over time. In addition, the results showed that decreasing ESAR foot stiffness can increase prosthesis range of motion, mid-stance energy storage, and late-stance energy return. However, the prosthetic foot contributions to forward propulsion and swing initiation were limited due to muscle compensations needed to provide body support and forward propulsion in the absence of residual leg ankle muscles. A study was also performed that integrated design optimization with forward dynamics simulations of amputee walking to identify the optimal prosthetic foot stiffness that minimized metabolic cost and intact knee joint forces. The optimal stiffness profile stiffened the toe and mid-foot while making the ankle less stiff, which decreased the intact knee joint force during mid-stance while reducing the overall metabolic cost of walking. These studies have provided new insight into the relationships between prosthetic foot stiffness and amputee walking mechanics, which provides biomechanics-based rationale for prosthetic foot prescription that can lead to improved amputee mobility and overall quality of life.Item An integrated computational-experimental approach for the in situ estimation of valve interstitial cell biomechanical state(2016-05) Buchanan, Rachel Marie; Sacks, Michael S.; Baker, Aaron B; Stachowiak, Jeanne C; Moon, Tess J; Guilak, FarshidMechanical forces are known to regulate aortic valve interstitial cell (AVIC) functional state by modulating their biosynthetic activity, translating to differences in tissue composition and structure and, potentially, leading to aortic valve (AV) dysfunction. While advances have been made toward the understanding of AVIC behavior ex-situ, the AVIC biomechanical state in its native extracellular matrix (ECM) remains largely unknown. Consequently, changes in AVIC behaviors, such as stiffness and contractility, resulting from pathological cues in-situ remain unidentified. We hypothesize that improved descriptions of AVIC biomechanical state in-situ, obtained using an inverse modeling approach, will provide deeper insight into AVIC interactions with the surrounding ECM, revealing important changes resulting from pathological state, and possibly informing pharmaceutical therapies. To achieve this, a novel integrated numerical-experimental framework to estimate AVIC mechanobiological state in-situ was developed. Flexural deformation of intact AV leaflets was used to quantify the effects of AVIC stiffness and contraction at the tissue level. In addition to being a relevant deformation mode of the cardiac cycle, flexure is highly sensitive to layer-specific changes in AVIC biomechanics. As a first step, a tissue-level bilayer model that accurately captures the bidirectional flexural response of AV intact layers in a passive state was developed. Next, tissue micromorphology was incorporated in a macro-micro scale framework to simulate layer-specific AVIC-ECM interactions. The macro-micro AV model enables the estimation of changes in effective AVIC stiffness and contraction in-situ that are otherwise grossly inaccessible through experimental approaches alone. Finally, microindentation studies examining AVIC activation were run in parallel with in-situ studies to emphasize the necessity of an in-situ approach, and the advantage it affords over existing ex-situ methodology. In conclusion, the developed numerical-experimental methodology can be used to obtain AVIC properties in-situ. Most importantly, it can lead to further understanding of AVIC-ECM mechanical coupling under various pathophysiological conditions and the investigation of possible treatment strategies targeting the myofibroblast phenotype characteristic of early signs of sclerotic valvular disease.Item A low-cost volume adjustable lower limb prosthetic socket : design and evaluation(2014-08) Vaughan, Meagan Renee; Crawford, Richard H.An issue of great concern for amputees continues to be lack of proper fit and comfort in their sockets. This lack can often be attributed to changes in the shape of the residual limb that cannot be compensated for by existing prosthetic socket technology. In regions where cost is a prohibitive factor in the replacement of ill-fitting prosthetic sockets, the need for a volume adjustable, and potentially longer lasting, socket design is abundant. This research focuses on designing a volume adjustable lower limb prosthetic socket that accommodates the needs of amputees in underdeveloped countries using collaborative design techniques. Though advocated as a means of accurately identifying and satisfying their needs, including end-users in the design process often adds an additional layer of complexity because of differences in culture, language, or geography among the participants. This research therefore includes a study in which product design techniques were applied to the same volume adjustable socket design problem with a variety of users – typical users, lead users, and new Empathic Lead Users - from different countries, one developed and one developing. To overcome differences among participants, this research includes an alternative strategy to create Empathic Lead Users (ELU) from non-user product design engineers through the use of simulated lead user experiences. As a result of this study, customer needs analysis with ELU helps to identify 95% of traditional and lead user customer needs and 100% more latent needs, and possibly more avenues for product innovations, than interviewing lead or traditional users alone. The concepts generated by all users were also compared. Based on the resulting concepts’ novelty, variety, quality, and quantity, all users were able to satisfactorily complete the concept generation exercises and produced competitive design solutions. Using the concepts generated during this co-design study, a volume adjustable socket was developed. The final socket design, based on the analogous rotational movement of a camera aperture, is pursued through mechanical and subject testing. Early users of the socket liked the design and it has been demonstrated to provide the necessary volume adjustments, but future design iterations to improve its comfort are necessary.Item Merged plantarflexor muscle activity is predictive of poor walking performance in post-stroke hemiparetic subjects(2018-05) Brough, Lydia Gail; Neptune, Richard R.Stroke is the leading cause of long-term disability and individuals post-stroke often experience impaired walking ability. The plantarflexor muscles (PFs) are critical to walking through their contributions to the ground reaction forces. Previous studies have shown muscle activity during walking can be grouped into co-excited muscle sets, or modules. Improper muscle co-activation, or merging of modules, is a common impairment in individuals post-stroke. The purpose of this study was to determine the influence of merged PF modules on walking performance in individuals post stroke by examining balance control, body support, propulsion and walking symmetry. Muscle modules were identified using non-negative matrix factorization to classify subjects as having an independent or merged PF module. The merged group had decreased balance control with a significantly higher frontal plane whole-body angular momentum than both the indepedent and control groups, while the independent and control groups were not significantly different. The merged group also had higher paretic braking and nonparetic propulsion than both the indepdendent and control groups. These results still held when comparisons were limited to subjects who had the same total number of modules, indicating that this was not a general effect due to subjects with merged PF having fewer modules. It is likely that a merged PF module is indicative of general PF dysfunction even when some activation occurs at the appropriate time. These results suggest an independent PF module is critical to walking performance, and thus obtaining an independent PF module should be a crucial aim of stroke rehabilitation.Item Mobility in individuals with traumatic lower-limb injuries : implications for device design, surgical intervention and rehabilitation therapies(2016-05) Ranz, Ellyn Cymbre; Neptune, Richard R.; Barr, Ronald E; Crawford, Richard H; Sulzer, James S; Wilken, Jason MTraumatic injuries to the extremities are commonly observed in emergency room patients and military personnel in combat. Restoring high mobility and functionality is a primary goal post-injury, which may require the use of rehabilitative devices, surgical interventions, and rehabilitation therapies. The research detailed in this dissertation investigates specific elements of these approaches through the use of experimental study and modeling and simulation. In the first study, the influence of passive-dynamic ankle-foot orthosis bending axis on the gait performance of limb salvage subjects was investigated. Bending axis location was altered by fabricating customized orthosis components using additive manufacturing and was tested in a gait laboratory. Altering bending axis location did not result in large or consistent changes in gait measures, however subjects expressed strong preferences for bending axis condition and preference was strongly related to specific gait measures. This suggests that preference and comfort are important factors guiding the prescription of bending axis location. In the second study, musculoskeletal modeling was used to examine the influence of transfemoral amputation surgical techniques on muscle capacity to generate forces and moments about the hip. Muscle reattachment tension and stabilization were shown to be critical parameters for post-amputation capacity, which supports the use of myodesis stabilization (muscle is reattached directly to bone) in amputation procedures. In the third study, a forward dynamics simulation of transfemoral amputee gait was developed and used to examine individual muscle and prosthesis contributions to walking subtasks. The residual hip muscles, and intact ankle, knee, and hip muscles worked synergistically to provide body support, anteroposterior propulsion, mediolateral control, and leg swing. Increased contributions of contralateral muscles to ipsilateral subtasks as well as increased duration of specific muscle contributions were observed in comparison to non-amputee and transtibial amputee walking. These findings can be used to help develop targeted rehabilitation therapies and improve transfemoral amputee locomotion. Through elucidating the influence of PD-AFO bending axis on gait performance as well as the influence of transfemoral amputation surgical techniques on muscle capacity and function, this research provides a foundation for improved rehabilitation outcomes, and thus mobility for individuals who have experienced traumatic lower-limb injuries.Item Modeling and simulating time-dependent changes in soft-tissue derived bioprosthetic heart valve biomaterial in response to cyclic loading(2018-10-12) Zhang, Will, Ph. D.; Sacks, Michael S.; Baker, Aaron; Rylander, Christopher G; Ravi-Chandar, Krishnaswamy; Vyavahare, NarenSoft tissue-derived exogenously cross-linked (EXL) biomaterials continue to be the best choice for the fabrication of bioprosthetic heart valves (BHV). Despite years of use, our understanding of these biomaterials and of the mechanisms leading to their failure remain at an empirical level. The need for advancements in modeling their behavior is further underscored by the development of percutaneously-delivered BHV devices. While these devices offer reduced surgical risk, they also present additional challenges for the design of the leaflets due to limitations in thickness and folding during delivery, resulting in a 2-year mortality rate of 33.9% in general. Thus, we seek to develop a framework for modeling and simulating soft-tissue-derived EXL biomaterials, accounting for the effects of exogenous cross-linking in permanent set and mechanical fatigue. Such approaches can significantly improve the accuracy and reliability of long-term predictions of durability and mechanical function. Firstly, we will establish the form of a nonlinear hyperelastic meso-scale structural constitutive model (MSSCM) for fibrous soft tissues, that can accurately capture the mechanical response of common soft tissues used as a basis for EXL biomaterial. Secondly, we will study the effect of exogenous crosslinks on these tissues using glutaraldehyde (GLUT) EXL bovine pericardium. GLUT-EXLs form polymeric chains through the cross-linking process which more tightly bonds the fibers to the matrix, increasing the non-fibrous matrix stiffness and fiber-fiber interactions. However, GLUT EXLs undergo Schiff-base reactions that lead to scission-healing behaviors that change the geometry of BHVs. We model this effect based on first-order kinetics of the scission healing reaction and validate it using static strain, cyclic strain, and stress control experiments. Next, we will develop a full 3D finite element implementation of the MSSCM with the modifications for EXL for real device applications. We then parametrically examine the changes in geometry and stress distribution of BHVs overtime, exploring initial geometries and material properties which may minimize the risks of the former effects. With this, we aim to develop a better understanding of the underlying process that occurs during long-term cyclic loading through our constitutive modeling approach and device level applications and translate the insights gained to improve BHV design and durability.Item Muscle contributions to balance control, propulsion and leg swing during healthy and post-stroke walking(2021-12-06) Brough, Lydia Gail; Neptune, Richard R.; Kautz, Steven A; Sulzer, James S; Fey, Nicholas PHuman walking requires complex muscle coordination to produce important biomechanical functions such as balance, forward propulsion and leg swing. For healthy individuals, these tasks are often accomplished effortlessly. However, for individuals post-stroke, balance, propulsion and leg swing can be compromised. Thus, the overall goal of this research was to understand how healthy individuals respond to altered balance control via mediolateral foot placement perturbations and how specific muscles contribute to propulsion and leg swing deficits in individuals post-stroke. Controlling mediolateral foot placement is critical to maintaining balance in the frontal plane, but can be difficult post-stroke. Thus, we investigated how healthy individuals maintain their balance after mediolateral foot placement perturbations to compare to individuals post-stroke. We found that participants responded to medial foot placement perturbations using lateral hip and ankle strategies and lateral foot placement perturbations using a lateral ankle strategy, but did not use a plantarflexion strategy following either perturbation. Modeling and simulation analyses further revealed changes in hip and trunk muscle contributions to foot placement, suggesting a coordinated response of the trunk and bilateral hip abductor muscles. On average, changes in muscle contributions to mediolateral ground reaction forces, torso power, and frontal plane external moments were small. These results highlight the responses of healthy individuals to altered balance control via foot placement perturbations. Individuals post-stroke often experience propulsion and knee flexion deficits, leading to slow walking speeds and stiff-knee gait. These deficits may have several underlying causes. Thus, modeling and simulation analyses of individuals post-stroke were used to identify muscle contributions to propulsion deficits, including excess braking from the vasti, plantarflexor braking, low plantarflexor output and reliance on compensatory mechanisms. Moreover, higher vasti contributions to braking in pre-swing predicted lower knee flexion. While the rectus femoris and iliopsoas did not directly contribute to lower knee flexion acceleration in pre-swing compared to contributions from the vasti, in some cases, the rectus femoris absorbed more power and the iliopsoas contributed less power to the paretic leg. These results highlight the heterogeneity of the post-stroke population and the need to identify individual causes of walking deficits to improve rehabilitation outcomes.Item Predictors of shoulder pain in manual wheelchair users(2018-10-08) Walford, Shelby Lee; Neptune, Richard R.Manual wheelchair users rely on their upper limbs to provide independent mobility, which leads to high muscular demand on their upper extremities. This increased demand often results in shoulder pain and injury. However, the specific causes of shoulder pain are unknown. Previous work has shown that decreased shoulder muscle strength is predictive of shoulder pain onset, and others have analyzed joint kinetics, joint kinematics, propulsion technique (e.g. cadence, contact percentage) and intra-individual variability for their relation to shoulder pain or injury. However, one challenge to such studies is that the demand placed on the upper extremity cannot be measured directly, and therefore the causal mechanisms leading to pain and injury are unknown. The purpose of this study was to build upon this previous work and determine in a longitudinal setting whether there are specific kinetic, kinematic, spatiotemporal and intra-individual variability measures that predict whether a manual wheelchair user is likely to develop shoulder pain. All participants were asymptomatic for shoulder pain at the time of initial data collection and were categorized into pain and no pain groups based on who developed shoulder pain at either the 18-month or the 36-month follow-up assessment. Shoulder strength measures, handrim and joint kinetics, kinematics, spatiotemporal measures, individual standard deviations (SDs) and coefficients of variation (CVs) of the aforementioned parameters were evaluated as predictors of shoulder pain using a logistic regression model. The most important predictors of shoulder pain included shoulder adductor strength, positive shoulder joint work during the recovery phase and maximum trunk angle. Individuals who developed shoulder pain had weaker shoulder adductors, higher positive shoulder joint work during recovery, and less trunk flexion than those who did not develop pain. In addition, relative intra-individual variability (CV) was a better predictor of shoulder pain than absolute variability (SD), however future work is needed to determine when increased versus decreased intra-individual variability is more favorable for preventing shoulder pain. Thus, these predictors may provide insight into how to improve rehabilitation training and outcomes for manual wheelchair users and ultimately decrease their likelihood of developing shoulder pain and injuries.Item Redesign of the total wrist prosthesis to address wrist rotation(2013-05) Mehta, Jay Ravi; Crawford, Richard H.The human wrist is a vital joint in daily life, and it is subject to injuries and disease. Currently, severe wrist disease is normally treated with wrist arthrodesis, which is normally reliable but results in a fixed wrist incapable of allowing wrist motion. Another method of treating a nonfunctional or severely painful wrist is wrist arthroplasty where the wrist joint is replaced with an implant that allows wrist movement. As of yet, a suitable wrist implant has not been developed, especially for the case of the post-traumatic, young male wrist, and most current wrist implants fail from failure of the bone-implant interface. Through simulation and literature review, it is concluded that implants that restrict axial rotation are bound to fail overtime. With this conclusion, a new wrist implant prototype is designed that incorporates state of the art materials, fluid film lubrication, proper kinematics, a suitable range of motion, and more. This implant contributes several improvements to the field of wrist arthroplasty.