Biomechanics of Single Stair Climb With Implications for Inverted Pendulum Modeling.

Step-by-step (SBS) stair navigation is used by those with movement limitations or lower-limb prosthetics and by humanoid robots. Knowledge of biomechanical parameters for SBS gait, however, is limited. Inverted pendulum (IP) models used to assess dynamic stability have not been applied to SBS gait. This study examined the ability of the linear inverted pendulum (LIP) model and a closed-form, variable-height inverted pendulum (VHIP) model to predict capture-point (CP) stability in healthy adults executing a single stair climb. A second goal was to provide baseline kinematic and kinetic data for SBS gait. Twenty young adults executed a single step onto stairs of two heights, while attached marker positions and ground reaction forces were recorded. opensim software determined body kinematics and joint kinetics. Trials were analyzed with LIP and VHIP models, and the predicted CP compared to the actual center-of-pressure (CoP) on the stair. Lower-limb joint moments were larger than those reported for step-over-step (SOS) stair gait. Leading knee rather than trailing ankle was dominant. Center-of-mass (CoM) velocity peaked at push-off. The VHIP model accounted for only slightly more than half of the forward progression of the vertical projection of the CoM and was not better than LIP predictions. This suggests that IP models are limited in modeling SBS gait, likely due to large hip and knee moments. The results from this study may also provide target values and strategies to aid design of lower-limb prostheses and powered exoskeletons.

Endocardial Fibroelastosis is Secondary to Hemodynamic Alterations in the Chick Embryonic Model of Hypoplastic Left Heart Syndrome.

BACKGROUND: Endocardial fibroelastosis (EFE) is a diffuse thickening of the ventricular endocardium, causing myocardial dysfunction and presenting as unexplained heart failure in infants and children. One of the postulated causes is persistent and increased wall tension in the ventricles. RESULTS: To examine whether reduced ventricular pressure in a chick model of hypoplastic left heart syndrome (HLHS) induced by left atrial ligation (LAL) at embryonic day (ED) 4 is associated with EFE at later stages, myocardial fibrosis was evaluated by histology and immunoconfocal microscopy and mass spectrometry (MS) at ED12. Immunohistochemistry with collagen I antibody clearly showed a significant thickening of the layer of subendocardial fibrous tissue in LAL hearts, and MS proved this significant increase of collagen I. To provide further insight into pathogenesis of this increased fibroproduction, hypoxyprobe staining revealed an increased extent of hypoxic regions, normally limited to the interventricular septum, in the ventricular myocardium of LAL hearts at ED8. CONCLUSIONS: Abnormal hemodynamic loading during heart development leads to myocardial hypoxia, stimulating collagen production in the subendocardium. Therefore, EFE in this chick embryonic model of HLHS appears to be a secondary effect of abnormal hemodynamics. Developmental Dynamics 247:509-520, 2018. © 2017 Wiley Periodicals, Inc.

Myocardial wall stiffening in a mouse model of persistent truncus arteriosus.

BACKGROUND: Genetic and epigenetic programs regulate dramatic structural changes during cardiac morphogenesis. Concurrent biomechanical forces within the heart created by blood flow and pressure in turn drive downstream cellular, molecular and genetic responses. Thus, a genetic-morphogenetic-biomechanical feedback loop is continually operating to regulate heart development. During the evolution of a congenital heart defect, concomitant abnormalities in blood flow, hemodynamics, and patterns of mechanical loading would be predicted to change the output of this feedback loop, impacting not only the ultimate morphology of the defect, but potentially altering tissue-level biomechanical properties of structures that appear structurally normal. AIM: The goal of this study was to determine if abnormal hemodynamics present during outflow tract formation and remodeling in a genetically engineered mouse model of persistent truncus arteriosus (PTA) causes tissue-level biomechanical abnormalities. METHODS: The passive stiffness of surface locations on the left ventricle (LV), right ventricle (RV), and outflow tract (OFT) was measured with a pipette aspiration technique in Fgf8;Isl1Cre conditional mutant embryonic mouse hearts and controls. Control and mutant experimental results were compared by a strain energy metric based on the measured relationship between pressure and aspirated height, and also used as target behavior for finite element models of the ventricles. Model geometry was determined from 3D reconstructions of whole-mount, confocal-imaged hearts. The stress-strain relationship of the model was adjusted to achieve an optimal match between model and experimental behavior. RESULTS AND CONCLUSION: Although the OFT is the most severely affected structure in Fgf8;Isl1Cre hearts, its passive stiffness was the same as in control hearts. In contrast, both the LV and RV showed markedly increased passive stiffness, doubling in LVs and quadrupling in RVs of mutant hearts. These differences are not attributable to differences in ventricular volume, wall thickness, or trabecular density. Excellent agreement was obtained between the model and experimental results. Overall our findings show that hearts developing PTA have early changes in ventricular tissue biomechanics relevant to cardiac function and ongoing development.

Biomechanics of Step Initiation After Balance Recovery With Implications for Humanoid Robot Locomotion.

Balance-recovery stepping is often necessary for both a human and humanoid robot to avoid a fall by taking a single step or multiple steps after an external perturbation. The determination of where to step to come to a complete stop has been studied, but little is known about the strategy for initiation of forward motion from the static position following such a step. The goal of this study was to examine the human strategy for stepping by moving the back foot forward from a static, double-support position, comparing parameters from normal step length (SL) to those from increasing SLs to the point of step failure, to provide inspiration for a humanoid control strategy. Healthy young adults instrumented with joint reflective markers executed a prescribed-length step from rest while marker positions and ground reaction forces (GRFs) were measured. The participants were scaled to the Gait2354 model in opensim software to calculate body kinematic and joint kinetic parameters, with further post-processing in matlab. With increasing SL, participants reduced both static and push-off back-foot GRF. Body center of mass (CoM) lowered and moved forward, with additional lowering at the longer steps, and followed a path centered within the initial base of support (BoS). Step execution was successful if participants gained enough forward momentum at toe-off to move the instantaneous capture point (ICP) to within the BoS defined by the final position of both feet on the front force plate. All lower extremity joint torques increased with SL except ankle joint. Front knee work increased dramatically with SL, accompanied by decrease in back-ankle work. As SL increased, the human strategy changed, with participants shifting their CoM forward and downward before toe-off, thus gaining forward momentum, while using less propulsive work from the back ankle and engaging the front knee to straighten the body. The results have significance for human motion, suggesting the upper limit of the SL that can be completed with back-ankle push-off before additional knee flexion and torque is needed. For biped control, the results support stability based on capture-point dynamics and suggest strategy for center-of-mass trajectory and distribution of ground force reactions that can be compared with robot controllers for initiation of gait after recovery steps.

Comparison of mechanical testing methods for biomaterials: Pipette aspiration, nanoindentation, and macroscale testing.

Characterization of the mechanical properties of biological materials is often complicated by small volume, irregular geometry, fragility, and environmental sensitivity. Pipette aspiration and nanoindentation testing deal well with these limitations and have seen increasing use in biomaterial characterization, but little research has been done to systematically validate these techniques for soft materials. This study compared the results of pipette aspiration, nanoindentation, and bulk uniaxial tension and compression in determining the small-strain elastic moduli of a range of biomedically-relevant materials, a series of silicone elastomers and polyacrylamide hydrogels. A custom apparatus was developed for pipette aspiration testing, a commercial Hysitron instrument with custom spherical tip was used for nanoindentation, and standard commercial machines were used for tension and compression testing. The measured small-strain elastic moduli ranged from 27 to 368 kPa for the silicones and 11 to 44 kPa for the polyacrylamide gels. All methods detected expected trends in material stiffness, except for the results from one inconsistent silicone. Pipette aspiration and nanoindentation measured similar elastic moduli for silicone materials, but pipette aspiration measured consistently larger stiffness in the hydrogels, which may be explained by the gels' resistance to tension. Despite the difference in size scale among the testing methods, size does not appear to influence the results. These results suggest that both pipette aspiration and nanoindentation are suitable for measuring mechanical properties of soft biomaterials and appear to have no more limitations than bulk techniques.

Effects of age and step length on joint kinetics during stepping task.

Following a balance perturbation, a stepping response is commonly used to regain support, and the distance of the recovery step can vary. To date, no other studies have examined joint kinetics in young and old adults during increasing step distances, when participants are required to bring their rear foot forward. Therefore, the purpose of this study was to examine age-related differences in joint kinetics with increasing step distance. Twenty young and 20 old adults completed the study. Participants completed a step starting from double support, at an initial distance equal to the individual's average step length. The distance was increased by 10% body height until an unsuccessful attempt. A one-way, repeated measures ANOVA was used to determine the effects of age on joint kinetics during the maximum step distance. A two-way, repeated measures, mixed model ANOVA was used to determine the effects of age, step distance, and their interaction on joint kinetics during the first three step distances for all participants. Young adults completed a significantly longer step than old adults. During the maximum step, in general, kinetic measures were greater in the young than in the old. As step distance increased, all but one kinetic measure increased for both young and old adults. This study has shown the ability to discriminate between young and old adults, and could potentially be used in the future to distinguish between fallers and non-fallers.

Stress and strain adaptation in load-dependent remodeling of the embryonic left ventricle.

Altered pressure in the developing left ventricle (LV) results in altered morphology and tissue material properties. Mechanical stress and strain may play a role in the regulating process. This study showed that confocal microscopy, three-dimensional reconstruction, and finite element analysis can provide a detailed model of stress and strain in the trabeculated embryonic heart. The method was used to test the hypothesis that end-diastolic strains are normalized after altered loading of the LV during the stages of trabecular compaction and chamber formation. Stage-29 chick LVs subjected to pressure overload and underload at stage 21 were reconstructed with full trabecular morphology from confocal images and analyzed with finite element techniques. Measured material properties and intraventricular pressures were specified in the models. The results show volume-weighted end-diastolic von Mises stress and strain averaging 50-82 % higher in the trabecular tissue than in the compact wall. The volume-weighted-average stresses for the entire LV were 115, 64, and 147 Pa in control, underloaded, and overloaded models, while strains were 11, 7, and 4 %; thus, neither was normalized in a volume-weighted sense. Localized epicardial strains at mid-longitudinal level were similar among the three groups and to strains measured from high-resolution ultrasound images. Sensitivity analysis showed changes in material properties are more significant than changes in geometry in the overloaded strain adaptation, although resulting stress was similar in both types of adaptation. These results emphasize the importance of appropriate metrics and the role of trabecular tissue in evaluating the evolution of stress and strain in relation to pressure-induced adaptation.