Change search
Refine search result
1 - 40 of 40
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Ambrosi, D.
    et al.
    Ateshian, G. A.
    Arruda, E. M.
    Cowin, S. C.
    Dumais, J.
    Goriely, A.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Humphrey, J. D.
    Kemkemer, R.
    Kuhl, E.
    Olberding, J. E.
    Taber, L. A.
    Garikipati, K.
    Perspectives on biological growth and remodeling2011In: Journal of the mechanics and physics of solids, ISSN 0022-5096, E-ISSN 1873-4782, Vol. 59, no 4, p. 863-883Article, review/survey (Refereed)
    Abstract [en]

    The continuum mechanical treatment of biological growth and remodeling has attracted considerable attention over the past fifteen years. Many aspects of these problems are now well-understood, yet there remain areas in need of significant development from the standpoint of experiments, theory, and computation. In this perspective paper we review the state of the field and highlight open questions, challenges, and avenues for further development.

  • 2. Bartonek, A.
    et al.
    Lidbeck, C. M.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Influence of external visual focus on gait in children with bilateral cerebral palsy2016In: Pediatric Physical Therapy, ISSN 0898-5669, E-ISSN 1538-005X, Vol. 28, no 4, p. 393-399Article in journal (Refereed)
    Abstract [en]

    Purpose: To explore whether focusing a target influenced gait in children with cerebral palsy (CP) and typical development (TD). Methods: Thirty children with bilateral CP (Gross Motor Function Classification System [GMFCS] I-III) and 22 with TD looked at a light at walkway end (Gaze Target) while walking and returned (No Target). Results: During Gaze versus No Target, children with TD reduced temporal-spatial parameters and movements in the sagittal (SPM) and transverse planes. In comparison, during Gaze Target, children in CP1 (GMFCS I) had larger trunk SPM, children in CP2 (GMFCS II) larger neck (SPM), and children in CP3 (GMFCS III) greater head and neck frontal plane movements, and reduced cadence and single support. Conclusions: Focusing a target altered gait in children with CP. Children in CP1 reduced movements similar to children with TD, children in CP2 behaved nearly unchanged, whereas children in CP3 reduced movements and temporalspatial parameters, potentially as a consequence of lack of sensory information from lower limbs.

  • 3.
    Bartonek, Åsa
    et al.
    Karolinska Institutet, Dept. of Women's and Child's Health.
    Wang, Ruoli
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Eriksson, Marie
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Comparison of two carbon fibre spring orthoses' effect on gait in children with myelomeningocele2012In: Gait & Posture, ISSN 0966-6362, E-ISSN 1879-2219Article in journal (Refereed)
  • 4.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Blood flow simulations of the renal arteries - effect of segmentation and stenosis removalIn: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987Article in journal (Refereed)
    Abstract [en]

    Patient specic based simulation of blood flows in arteries has been proposed as a future approach for better diagnostics and treatment of arterial diseases.The outcome of theoretical simulations strongly depends on the accuracy in describing the problem (the geometry, material properties of the artery and of the blood, flow conditions and the boundary conditions). In this study, the uncertainties associated with the approach for a priori assessment of reconstructive surgery of stenoted arteries are investigated. It is shown that strong curvature in the reconstructed artery leads to large spatial- and temporal-peaks in the wall shear-stress. Such peaks can be removed by appropriate reconstruction that also handles the post-stenotic dilatation of the artery. Moreover, it is shown that the effects of the segmentation approach can be equally important as the effects of using advanced rheological models. Unfortunately, this fact has not been recognized in the literature up to this point, making patient specic simulations potentially less reliable.

  • 5.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Flow characteristics and coherent structures in a centrifugal blood pumpIn: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987Article in journal (Refereed)
    Abstract [en]

    Blood clot formation can be initiated by local flow conditions where regions of high shear and long residence time regions, such as flow separation and stagnation, have been identified as risk factors. This study highlights coherent structures,some of which not yet considered in the literature that may contribute to blood clot formation in the ECMO (Extra Corporeal Membrane Oxygenator) circuit. The centrifugal ECMO pump investigated in this study is compact and delivers adequate volume of blood with relatively high pressure in order to compensate for the large pressure drop in the membrane oxygenator. These requirements lead to regions with high shear in several different parts of the pump. In the narrow gap between the pump house and the impeller body (the magnet) a Taylor-Couette-like flow is observed with azimuthally aligned wavy vortices, which are also pushed towards the bottom of the pump-house by the flow generated by the blades. At the bottom gap between the impeller house and the pump house one finds spiraling flow structures, due to the rotation of the former structure. Separation bubbles are found near the tongue of the pump and at the lee sides of the blades. Such vortical structures have in literature been identified as regions where platelets may be activated whereby clots may develop.

  • 6.
    Berg, Niclas
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Influence of red blood cell polydispersity on blood rheology and platelet marginationManuscript (preprint) (Other academic)
  • 7. di Bernardo, M.
    et al.
    Budd, C. J.
    Champneys, A. R.
    Kowalczyk, P.
    Nordmark, Arne B.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Tost, G. O.
    Piiroinen, P. T.
    Bifurcations in Nonsmooth Dynamical Systems2008In: SIAM Review, ISSN 0036-1445, E-ISSN 1095-7200, Vol. 50, no 4, p. 629-701Article, review/survey (Refereed)
    Abstract [en]

    A review is presented of the one-parameter, nonsmooth bifurcations that occur in a variety of continuous-time piecewise-smooth dynamical systems. Motivated by applications, a pragmatic approach is taken to defining a discontinuity-induced bifurcation (DIB) as a nontrivial interaction of a limit set with respect to a codimension-one discontinuity boundary in phase space. Only DIBs that are local are considered, that is, bifurcations involving equilibria or a single point of boundary interaction along a limit cycle for flows. Three classes of systems are considered, involving either state jumps, jumps in the vector field, or jumps in some derivative of the vector field. A rich array of dynamics are revealed, involving the sudden creation or disappearance of attractors, jumps to chaos, bifurcation diagrams with sharp corners, and cascades of period adding. For each kind of bifurcation identified, where possible, a kind of "normal form" or discontinuity mapping (DM) is given, together with a canonical example and an application. The goal is always to explain dynamics that may be observed in simulations of systems which include friction oscillators, impact oscillators, DC-DC converters, and problems in control theory.

  • 8. di Bernardo, M.
    et al.
    Nordmark, Arne B.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Olivar, G.
    Discontinuity-induced bifurcations of equilibria in piecewise-smooth and impacting dynamical systems2008In: Physica D: Non-linear phenomena, ISSN 0167-2789, E-ISSN 1872-8022, Vol. 237, no 1, p. 119-136Article in journal (Refereed)
    Abstract [en]

    A rich variety of dynamical scenarios has been shown to occur when a fixed point of a non-smooth map undergoes a border-collision. This paper concerns a closely related class of discontinuity-induced bifurcations, those involving equilibria of n-dimensional piecewise-smooth flows. Specifically, transitions are studied which occur when a boundary equilibrium, that is one lying within a discontinuity manifold, is perturbed. It is shown that such equilibria can either persist under parameter variations or can disappear giving rise to different bifurcation scenarios. Conditions to classify among the possible simplest scenarios are given for piecewise-smooth continuous, Filippov and impacting systems. Also, we investigate the possible birth of other attractors (e.g. limit cycles) at a boundary-equilibrium bifurcation.

  • 9.
    Dijkstra, Erik J.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Karolinska Institutet, Stockholm, Sweden.
    Computation of ground reaction force using Zero Moment Point2015In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 48, no 14, p. 3776-3781Article in journal (Refereed)
    Abstract [en]

    Motion analysis is a common clinical assessment and research tool that uses a camera system or motion sensors and force plates to collect kinematic and kinetic information of a subject performing an activity of interest. The use of force plates can be challenging and sometimes even impossible. Over the past decade, several computational methods have been developed that aim to preclude the use of force plates. Useful in particular for predictive simulations, where a new motion or change in control strategy inherently means different external contact loads. These methods, however, often depend on prior knowledge of common observed ground reaction force (GRF) patterns, are computationally expensive, or difficult to implement. In this study, we evaluated the use of the Zero Moment Point as a computationally inexpensive tool to obtain the GRFs for normal human gait. The method was applied on ten healthy subjects walking in a motion analysis laboratory and predicted GRFs are evaluated against the simultaneously measured force plate data. Apart from the antero-posterior forces, GRFs are well-predicted and errors fall within the error ranges from other published methods. Joint extension moments were underestimated at the ankle and hip but overestimated at the knee, attributable to the observed discrepancy in the predicted application points of the GRFs. The computationally inexpensive method evaluated in this study can reasonably well predict the GRFs for normal human gait without using prior knowledge of common gait kinetics.

  • 10.
    Engström, Pähr
    et al.
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Bartonek, Åsa
    Karolinska Institutet, Dept. of Women's and Child's Health.
    Tedroff, Kristina
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Orefelt, Christina
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Haglund-Åkerlind, Yvonne
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Does Botulinum toxin A improve the walking pattern in children with idiopathic toe-walking?2010In: Journal of Children's Orthopaedics, ISSN 1863-2521, Vol. 4, no 4, p. 301-308Article in journal (Refereed)
    Abstract [en]

    Background: Numerous recommendations have been made for treating idiopathic toe-walking (ITW), but the treatment results have been questioned. The purpose of this study was to investigate whether botulinum toxin A (BTX) improves the walking pattern in ITW as examined with 3-D gait analysis. Participants and methods: A consecutive series of 15 children (aged 5-13 years) were enrolled in the study. The children underwent a 3-D gait analysis prior to treatment with a total of 6 units/kg bodyweight Botox® in the calf muscles and an exercise program. The gait analysis was repeated 3 weeks and 3, 6, and 12 months after treatment. A classification of toe-walking severity was made before treatment and after 12 months. The parents rated the perceived amount of toe-walking prior to treatment and 6 and 12 months after treatment. Results: Eleven children completed the 12-month follow-up. The gait analysis results displayed a significant improvement, indicating decreased plantarflexion angle at initial contact and during swing phase and increased dorsiflexion angle during midstance at all post-treatment testing instances. According to the parents' perception of toe-walking, 3/11 children followed for 12 months had ceased toe-walking completely, 4/11 decreased toe-walking, and 4/11 continued toe-walking. After 6-12 months, the toe-walking severity classification improved in 9 of the 14 children for whom data could be assessed. Conclusions: A single injection of BTX in combination with an exercise program can improve the walking pattern in children with ITW seen at gait analysis, but the obvious goal of ceasing toe-walking is only occasionally reached.

  • 11. Eriksson, Marie
    et al.
    Bartonek, Asa
    Ponten, Eva
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Karolinska institutet, Sweden.
    Gait dynamics in the wide spectrum of children with arthrogryposis: a descriptive study2015In: BMC Musculoskeletal Disorders, ISSN 1471-2474, E-ISSN 1471-2474, Vol. 16Article in journal (Refereed)
    Abstract [en]

    Background: Arthrogryposis Multiplex Congenita (AMC) is a heterogeneous condition characterized by multiple joint contractures at birth. Greater movements in the trunk and pelvis during walking have been observed in children with AMC using orthoses compared to those wearing only shoes. This study investigated gait dynamics in children with AMC and identified compensatory mechanisms that accommodate walking. Methods: Twenty-six children with AMC who walked with orthoses or shoes and a control group consisting of 37 typically-developing children were evaluated in 3D gait analysis. Children with AMC were divided into subgroups based on which joints needed to be stabilized in the sagittal plane; AMC1 used knee-ankle-foot orthoses (KAFOs) with locked knee joints, AMC2 used KAFOs with open knee joints or ankle-foot orthoses, and AMC3 used shoes. Results: The Gait Deviation Index was lower in AMC groups than in the control group, with the lowest in AMC1. Excessive trunk movements in frontal and transverse planes were observed in AMC2 and especially in AMC1. Lower hip flexion moment was found in AMC1, while AMC2 and AMC3 showed similar hip flexion moments as the control group. Knee extension moments were similar between the groups. In the frontal plane there were only small differences between the groups in hip abduction moment. A joint work analysis indicated greater contribution from the hip muscles to overall positive work in AMC groups, particularly in AMC1, than in the control group. Conclusion: All AMC groups showed less hip extension than the control group, but hip flexion moment was significantly lower only in AMC1, which can be attributed to their gait strategy with bilateral locked KAFOs. AMC1, who had weak knee extensors, were helped by their locked KAFOs and therefore showed similar knee extension moment as the other groups. This finding, together with their gait patterns, demonstrates the children's high reliance on hip muscles and presumably trunk muscles to provide propulsion. Our study shows that with adequate orthotic support, children with AMC and even with severe weakness and contractures can achieve walking.

  • 12. Famaey, Nele
    et al.
    Sommer, Gerhard
    Vander Sloten, Jos
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Arterial clamping: Finite element simulation and in vivo validation2012In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 12, p. 107-118Article in journal (Refereed)
    Abstract [en]

    Commonly used techniques in cardiovascular interventions such as arterial clamping always entail a certain degree of unavoidable iatrogenic tissue damage. Therefore, studies have been directed towards the decrease of undesired intraoperative trauma, for example, through the design of less traumatic surgical instruments. Obviously, the effectiveness of new clamp designs and techniques depends on how well damage mechanisms are understood and how accurate thresholds for safe tissue loading can be set. This information can in part be derived from reliable finite element simulations. This study is the first to describe a finite element simulation of the clamping of a rat abdominal aorta with occlusion and in vivo validation. Material nonlinearity, large deformations, contact interactions and residual strains are hereby taken into account. The mechanical parameters of the model are derived from inflation experiments. The effect of the residual strains, different clamp geometries as well as the effect of variations in material properties are studied. In all simulations, stress concentrations in different regions of the tissue are noticed, especially for a corrugated clamp design. This shows the importance of finite element modeling in understanding the relation between mechanical loading and damage mechanisms. The inclusion of residual strains has its effect not only in the physiological loading regime, but also during clamping. Just as in the physiologic regime, it lowers the stress gradients through the wall thickness. Varying the material properties with the measured standard deviation between specimens leads to an average change of +/- 17% in the maximum and minimum principal stresses. Finally, the model is validated with an in vivo clamping experiment on a Wistar rat in which the clamping force was measured, showing good correspondence with the modeled clamping force.

  • 13. Fiolhais, Miguel
    et al.
    Essén, Hanno
    KTH, School of Engineering Sciences (SCI), Mechanics, Theoretical and Applied Mechanics.
    Providencia, Constanta
    Nordmark, Arne
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Magnetic field and current are zero inside ideal conductors2011In: Progress in Electromagnetics Research B, ISSN 1937-6472, E-ISSN 1937-6472, Vol. 27, p. 187-212Article in journal (Refereed)
    Abstract [en]

    We prove a theorem on the magnetic energy minimum in a system of perfect, or ideal, conductors. It is analogous to Thomson's theorem on the equilibrium electric field and charge distribution in a system of conductors. We first prove Thomson's theorem using a variational principle. Our new theorem is then derived by similar methods. We find that magnetic energy is minimized when the current distribution is a surface current density with zero interior magnetic field; perfect conductors are perfectly diamagnetic. The results agree with currents in superconductors being confined near the surface. The theorem implies a generalized force that expels current and magnetic field from the interior of a conductor that loses its resistivity. Examples of solutions that obey the theorem are presented.

  • 14.
    Forsell, Caroline
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Björck, Hanna M.
    Eriksson, Per
    Franco-Cereceda, Anders
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Biomechanical Properties of the Thoracic Aneurysmal Wall: Differences Between Bicuspid Aortic Valve and Tricuspid Aortic Valve Patients2014In: Annals of Thoracic Surgery, ISSN 0003-4975, E-ISSN 1552-6259, Vol. 98, no 1, p. 65-71Article in journal (Refereed)
    Abstract [en]

    Background. The prevalence for thoracic aortic aneurysms (TAAs) is significantly increased in patients with a bicuspid aortic valve (BAV) compared with patients who have a normal tricuspid aortic valve (TAV). TAA rupture is a life-threatening event, and biomechanics-based simulations of the aorta may help to disentangle the molecular mechanism behind its development and progression. The present study used polarized microscopy and macroscopic in vitro tensile testing to explore collagen organization and mechanical properties of TAA wall specimens from BAV and TAV patients. Methods. Circumferential sections of aneurysmal aortic tissue from BAV and TAV patients were obtained during elective operations. The distribution of collagen orientation was captured by a Bingham distribution, and finite element models were used to estimate constitutive model parameters from experimental load-displacement curves. Results. Collagen orientation was almost identical in BAV and TAV patients, with a highest probability of alignment along the circumferential direction. The strength was almost two times higher in BAV samples (0.834 MPa) than in TAV samples (0.443 MPa; p < 0.001). The collagen-related stiffness (C-f) was significantly increased in BAV compared with TAV patients (C-f = 7.45 MPa vs 3.40 MPa; p = 0.003), whereas the elastin-related stiffness was similar in both groups. A trend toward a decreased wall thickness was seen in BAV patients (p = 0.058). Conclusions. The aneurysmal aortas of BAV patients show a higher macroscopic strength, mainly due to an increased collagen-related stiffness, compared with TAV patients. The increased wall stiffness in BAV patients may contribute to the higher prevalence for TAAs in this group.

  • 15.
    Fuchs, Gabriel
    et al.
    Dept. of Physiology and Pharmacology, Karolinska Institutet ; Department of Cardiology, Sundsvall’s Hospital.
    Berg, Niclas
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
    Broman, Mikael
    Dept. of Physiology and Pharmacology, Karolinska Institutet ; ECMO Centre Karolinska, Pediatric Perioperative Medicine and Intensive Care, Karolinska University Hospital.
    Wittberg, L. Prahl
    KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Modeling sensitivity and uncertainties in platelet activation models applied on centrifugal pumps for extracorporeal life supportIn: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723Article in journal (Refereed)
  • 16.
    Klets, Olesya
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Riad, Jacques
    Department of Orthopaedics, Skaraborg Hospital, Skövde, Sweden.
    Broström, Eva W.
    Karolinska Intitutet, Dept. of Women's and Children's Health, Stockholm, Sweden.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Comparison between a subject-specific and a scaled generic musculoskeletal model of the lower extremities in a subject with unilateral cerebral palsy2011In: Clinical Biomechanics, ISSN 0268-0033, E-ISSN 1879-1271Article in journal (Other academic)
    Abstract [en]

    The purpose of this paper was to develop methods to build a subject-specific musculoskeletal model of the lower extremities based on MRIs of a subject with unilateral CP, and to determine whether a scaled generic musculoskeletal model is accurate enough to characterize MTLs and MALs of 70 muscles in both lower limbs during gait in a subject with unilateral cerebral palsy. We found, that the generic models produced accurate values for changes in MTL during gait for almost all muscles, except adductor longus, adductor magnus, adductor brevis, quadratus femoris, pectineus, extensor digitorum longus, soleus, lateral gastrocnemius, and medial gastrocnemius. MALs computed from the scaled generic model, however, differed considerably from those computed from the subject-specific model. Upon comparison of hip, knee and ankle MALs in affected and non-affected sides of the lower extremities, the scaled generic model generally failed to identify level arm dysfunction in the subject with unilateral CP

  • 17.
    Klets, Olesya
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Riad, Jacques
    Department of Orthopaedics, Skaraborg Hospital, Skövde, Sweden.
    Broström, Eva W.
    Karolinska Institutet, Dept. of Women's and Children's Health, Stockholm, Sweden.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Moment-generating biomechanical factors of hip muscles in persons with unilateral with subject-specific models2012In: Journal of Applied Biomechanics, ISSN 1065-8483, E-ISSN 1543-2688Article in journal (Other academic)
  • 18. Kowalczyk, P.
    et al.
    Di Bernardo, M.
    Champneys, A. R.
    Hogan, S. J.
    Homer, M.
    Piiroinen, P. T.
    Kuznetsov, Yu A.
    Nordmark, Arne B.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Two-parameter discontinuity-induced bifurcations of limit cycles: Classification and open problems2006In: International Journal of Bifurcation and Chaos in Applied Sciences and Engineering, ISSN 0218-1274, Vol. 16, no 3, p. 601-629Article in journal (Refereed)
    Abstract [en]

    This paper proposes a strategy for the classification of codimension-two discontinuity-induced bifurcations of limit cycles in piecewise smooth systems of ordinary differential equations. Such nonsmooth transitions (also known as C-bifurcations) occur when the cycle interacts with a discontinuity boundary of phase space in a nongeneric way, such as grazing contact. Several such codimension-one events have recently been identified, causing for example, period-adding or sudden onset of chaos. Here, the focus is on codimension-two grazings that are local in the sense that the dynamics can be fully described by an appropriate Poincare map from a neighborhood of the grazing point (or points) of the critical cycle to itself. It is proposed that codimension-two grazing bifurcations can be divided into three distinct types: either the grazing point is degenerate, or the grazing cycle is itself degenerate (e.g. nonhyperbolic) or we have the simultaneous occurrence of two grazing events. A careful distinction is drawn between their occurrence in systems with discontinuous states, discontinuous vector fields, or that with discontinuity in some derivative of the vector field. Examples of each kind of bifurcation are presented, mostly derived from mechanical applications. For each example, where possible, principal bifurcation curves characteristic to the codimension-two scenario are presented and general features of the dynamics discussed. Many avenues for future research are opened.

  • 19. Krochak, P.
    et al.
    Fasci, Giuseppe Carmini
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Norman, B.
    Prahl-Wittberg, Lisa
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Bridging chemical dosage, mixing quality and variability in paper sheets2013In: Pap. Conf. Trade Show, PaperCon, 2013, p. 1057-1069Conference paper (Refereed)
  • 20. Lidbeck, Cecilia
    et al.
    Bartonek, Åsa
    Yadav, Priti
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Tedroff, Kristina
    Åstrand, Per
    Hellgren, Kerstin
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Karolinska Inst, Sweden.
    The role of visual stimuli on standing posture in children with bilateral cerebral palsy2016In: BMC Neurology, ISSN 1471-2377, E-ISSN 1471-2377, Vol. 16, no 1, article id 151Article in journal (Refereed)
    Abstract [en]

    Background: In children with bilateral cerebral palsy (CP) maintaining a standing position can be difficult. The fundamental motor task of standing independently is achieved by an interaction between the visual, somatosensory, and vestibular systems. In CP, the motor disorders are commonly accompanied by sensory and perceptual disturbances. Our aims were to examine the influence of visual stimuli on standing posture in relation to standing ability. Methods: Three dimensional motion analysis with surface electromyography was recorded to describe body position, body movement, and muscle activity during three standing tasks: in a self-selected position, while blindfolded, and during an attention-demanding task. Participants were twenty-seven typically-developing (TD) children and 36 children with bilateral CP, of which 17 required support for standing (CP-SwS) and 19 stood without support (CP-SwoS). Results: All children with CP stood with a more flexed body position than the TD children, even more pronounced in the children in CP-SwS. While blindfolded, the CP-SwS group further flexed their hips and knees, and increased muscle activity in knee extensors. In contrast, the children in CP-SwoS maintained the same body position but increased calf muscle activity. During the attention-demanding task, the children in CP-SwoS stood with more still head and knee positions and with less muscle activity. Conclusions: Visual input was important for children with CP to maintain a standing position. Without visual input the children who required support dropped into a further crouched position. The somatosensory and vestibular systems alone could not provide enough information about the body position in space without visual cues as a reference frame. In the children who stood without support, an intensified visual stimulus enhanced the ability to maintain a quiet standing position. It may be that impairments in the sensory systems are major contributors to the difficulties to stand erect in children with CP.

  • 21. Lidbeck, Cecilia M.
    et al.
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Broström, Eva
    Karolinska Institutet, Stockholm, Sweden .
    Bartonek, Åsa
    Karolinska Institutet, Stockholm, Sweden .
    Postural Orientation During Standing in Children With Bilateral Cerebral Palsy2014In: Pediatric Physical Therapy, ISSN 0898-5669, E-ISSN 1538-005X, Vol. 26, no 2, p. 223-229Article in journal (Refereed)
    Abstract [en]

    Purpose: To investigate postural orientation and maintenance of joint position during standing in children with bilateral spastic cerebral palsy (BSCP). Methods: Standing was examined with 3-D motion analysis in 26 children with BSCP, and 19 children typically developing (TD). Two groups of children with cerebral palsy (CP) were analyzed: 15 who were able to maintain standing without support and 11 who needed support. Results: Children with CP stood with more flexion than children TD. In the CP groups, children standing without support stood more asymmetrically with less hip and knee flexion and less movement than those who required support. Conclusion: Children with CP had varying abilities to stand and maintain standing posture with or without support. Both CP groups stood with more flexion than their potential passive joint angle, more obvious in children requiring support. Investigations on how muscle strength and spatial perception influence posture remains to be explored.

  • 22. Löwing, K.
    et al.
    Thews, K.
    Haglund-Åkerlind, Y.
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Karolinska Institutet, Sweden.
    Effects of Botulinum Toxin-A and Goal-Directed Physiotherapy in Children with Cerebral Palsy GMFCS Levels I & II2017In: Physical & Occupational Therapy in Pediatrics, ISSN 0194-2638, E-ISSN 1541-3144, Vol. 37, no 3, p. 268-282Article in journal (Refereed)
    Abstract [en]

    Aims: To evaluate short and long-term effects of botulinum toxin-A combined with goal-directed physiotherapy in children with cerebral palsy (CP). Method: A consecutive selection of 40 children, ages 4–12 years, diagnosed with unilateral or bilateral CP, and classified in GMFCS levels I–II. During the 24 months, 9 children received one BoNT-A injection, 10 children two injections, 11 children three injections, and 10 children received four injections. 3D gait analysis, goal-attainment scaling, and body function assessments were performed before and at 3, 12, and 24 months after initial injections. Results: A significant but clinically small long-term improvement in gait was observed. Plantarflexor spasticity was reduced after three months and remained stable, while passive ankle dorsiflexion increased after 3 months but decreased slightly after 12 months. Goal-attainment gradually increased, reached the highest levels at 12 months, and levels were maintained at 24 months. Conclusion: The treatments’ positive effect on spasticity reduction was identified, but did not relate to improvement in gait or goal-attainment. No long-term positive change in passive ankle dorsiflexion was observed. Goal attainment was achieved in all except four children. The clinical significance of the improved gait is unclear. Further studies are recommended to identify predictors for positive treatment outcome.

  • 23.
    Manda, Krishnagoud
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Mechanics and Growth of Articular Cartilage Around a Localized Metal Implant2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Articular cartilage is a specialized connective soft tissue that resides on the ends of long-bones, and transfers the load smoothly between the bones in diarthrodial joints by providing almost frictionless, wear resistant sliding surfaces during joint articulation. Focal chondral or osteochondral defects in articular cartilage are common and show limited capacity for biological repair. Furthermore, changes in the bio-mechanical forces at the defect site may make the tissue more susceptible to continued degeneration. Alternatively, a contoured focal resurfacing metal implant can be used to treat such full-thickness cartilage defects. Physiological and biomechanical studies on animal models with metal implant have shown good clinical outcomes. However, the mechanical behavior of cartilage surrounding the implant has remained largely unanswered with respect to the joint function.

    First, we developed a simple 3-dimensional finite element model by approximating one of the condyles of a sheep knee joint and parametrically studied the effects of shape, size and positioning of the implant on the mechanical behavior of the cartilage surrounding the implant. The mechanical sealing effect due to the wedge shape of the implant was studied. We also simulated the time dependent behavior of the cartilage surrounding the implant. In the second part, we developed a more sophisticated model accounting for biological growth aspects of the cartilage around the implant together with the in vivo mechanical response of the cartilage in an intact human knee joint. An axisymmetric representation of a human knee condyle including both cartilage layers, meniscus and tibia was considered. A cartilage growth finite element model incorporating dynamic loading from walking, which drives the growth stimulation in the cartilage, was developed. Two individually growing constituents in the solid matrix of cartilage together with the biphasic contacts in the joint were considered in the growth model. From our simulations it is evident that the cartilage near the implant was more stimstimulated, whence the defect edge of the cartilage was growing onto the implant.

    The models developed in the present work are simulation tools and have a potential, in relevant aspects, to predict the physiological behavior of the cartilage surrounding the metal implant.

  • 24.
    Manda, Krishnagoud
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Eriksson, Anders
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Modeling of constrained articular cartilage growth in an intact knee with focal knee resurfacing metal implant2014In: Biomechanics and Modeling in Mechanobiology, ISSN 1617-7959, E-ISSN 1617-7940, Vol. 13, no 3, p. 599-613Article in journal (Refereed)
    Abstract [en]

    The purpose of the present study was to develop a model to simulate the articular cartilage growth in an intact knee model with a metal implant replacing a degenerated portion of the femoral cartilage. The human knee joint was approximated with a simplified axisymmetric shape of the femoral condyle along with the cartilage, meniscus and bones. Two individually growing constituents (proteoglycans and collagen) bound to solid matrix were considered in the solid phase of the cartilage. The cartilage behavior was modeled with a nonlinear biphasic porohyperelastic material model, and meniscus with a transversely isotropic linear biphasic poroelastic material model. Two criteria (permeation and shear), both driven by mechanical loading, were considered to trigger the growth in the solid constituents. Mechanical loading with sixty heavy cycles was considered to represent daily walking activity. The growth algorithm was implemented for 90 days after implantation. The results from simulations show that both cartilage layers were more stimulated near the implant which lead to more growth of the cartilage near the defect. The method developed in the present work could be a powerful technique if more accurate material data and growth laws were available.

  • 25. Naili, J. E.
    et al.
    Broström, E. W.
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, BioMEx.
    Schwartz, M. H.
    The Centre Of Mass Trajectory Is A Sensitive And Responsive Measure Of Compensations For Pain And Weakness Among Individuals With Knee Osteoarthritis Performing A Sit-To-Stand Test2017In: Osteoarthritis and Cartilage, ISSN 1063-4584, E-ISSN 1522-9653, Vol. 25, p. S125-S126Article in journal (Other academic)
  • 26.
    Nordmark, Arne B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Dankowicz, H.
    Champneys, A.
    Discontinuity-induced bifurcations in systems with impacts and friction: Discontinuities in the impact law2009In: International Journal of Non-Linear Mechanics, ISSN 0020-7462, E-ISSN 1878-5638, Vol. 44, no 10, p. 1011-1023Article in journal (Refereed)
    Abstract [en]

    This paper concerns the non-smooth dynamics of planar mechanical systems with isolated contact in the presence of Coulomb friction. Following Stronge [impact Mechanics, Cambridge University Press, Cambridge, 2000], a set of closed-form analytic formulae is derived for a rigid-body impact law based on an energetic coefficient of restitution and a resolution of the impact phase into distinct segments of relative slip and stick. Thus, the impact behavior is consistent both with the assumption of Coulomb friction and with the dissipative nature of impacts. The analysis highlights the presence of boundaries between open regions of initial conditions and parameter values corresponding to distinct forms of the impact law and investigates the smoothness properties of the impact law across these boundaries. It is shown how discontinuities in the impact law are associated with discontinuity-induced bifurcations of periodic trajectories, including non-smooth folds and persistence scenarios. Numerical analysis of an example mechanical model is used to illustrate and validate the conclusions.

  • 27.
    Nordmark, Arne B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Essén, Hanno
    KTH, School of Engineering Sciences (SCI), Mechanics.
    The skipping rope curve2007In: European journal of physics, ISSN 0143-0807, E-ISSN 1361-6404, Vol. 28, no 2, p. 241-247Article in journal (Refereed)
    Abstract [en]

    The equilibrium of a flexible inextensible string, or chain, in the centrifugal force field of a rotating reference frame is investigated. It is assumed that the end points are fixed on the rotation axis. The shape of the curve, the skipping rope curve or troposkien, is given by the Jacobi elliptic function sn.

  • 28.
    Nordmark, Arne B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Kowalczyk, P.
    A codimension-two scenario of sliding solutions in grazing-sliding bifurcations2006In: Nonlinearity, ISSN 0951-7715, E-ISSN 1361-6544, Vol. 19, no 1, p. 1-26Article in journal (Refereed)
    Abstract [en]

    This paper investigates codimension-two bifurcations that involve grazing-sliding and fold scenarios. An analytical unfolding of this novel codimension-two bifurcation is presented. Using the discontinuity mapping techniques it is shown that the fold curve is one-sided and cubically tangent to the grazing curve locally to the codimension-two point. This theory is then applied to explain the dynamics of a dry-friction oscillator where such a codimension-two point has been found. In particular, the presence and the character of essential bifurcation curves that merge at the codimension-two point are confirmed. This allows us to study the dynamics away from the codimension-two point using a piecewise affine approximation of the normal form for grazing-sliding bifurcations and explain the dynamics observed in the friction system.

  • 29.
    Nordmark, Arne B.
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Piiroinen, P. T.
    Simulation and stability analysis of impacting systems with complete chattering2009In: Nonlinear dynamics, ISSN 0924-090X, E-ISSN 1573-269X, Vol. 58, no 1-2, p. 85-106Article in journal (Refereed)
    Abstract [en]

    This paper considers dynamical systems that are derived from mechanical systems with impacts. In particular we will focus on chattering-accumulation of impacts-for which local discontinuity mappings will be derived. We will first show how to use these mappings in simulation schemes, and secondly how the mappings are used to calculate the stability of limit cycles with chattering by solving the first variational equations.

  • 30. Pierce, D. M.
    et al.
    Lilledahl, M. B.
    Ricken, T.
    De Lange Davies, C.
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Graz University of Technology, Graz, Austria .
    Morphological analysis of articular cartilage using multiphoton microscopy as input for constitutive modeling: Experiment and mathematical implementation2010In: 6th World Congress Of Biomechanics (WCB 2010), Pts 1-3, Springer-Verlag New York, 2010, p. 895-898Conference paper (Refereed)
    Abstract [en]

    The 3D structure of collagen fibers in chicken cartilage was quantified using multiphoton microscopy. Samples of fresh chicken cartilage were sectioned in three orthogonal planes using a vibratome. The sections were imaged using multiphoton microscopy, specifically imaging the collagen fibers using the second harmonic signal. Employing image analysis techniques based on Fourier analysis, the primary direction and anisotropy of the collagen fibers were extracted for the superficial layer resulting in a 3D map of the collagen fiber fabric. In the middle layer, image analysis using objective thresholding techniques was employed to extract the volume fraction occupied by extracellular matrix, the rest being occupied by the lacunae and residing chondrocytes. To implement these imaging data in a computational setting, we propose a new, 3D large strain constitutive model for articular cartilage, focused on the essential load-bearing morphology: an inhomogeneous, visco-poroelastic solid matrix reinforced by an anisotropic, viscoelastic dispersed fiber fabric which is saturated by an incompressible fluid residing in strain-dependent pores of the collagen-proteoglycan solid matrix. High-fidelity models, combining advanced imaging and computational biomechanics, will allow us to consider complex problems in structure-function relationships and provide insight to microphysical (mechanobiological) cellular stimuli.

  • 31. Polzer, S.
    et al.
    Gasser, T. Christian
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Bursa, J.
    Staffa, R.
    Vlachovsky, R.
    Man, V.
    Skacel, P.
    Importance of material model in wall stress prediction in abdominal aortic aneurysms2013In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 35, no 9, p. 1282-1289Article in journal (Refereed)
    Abstract [en]

    Background: Results of biomechanical simulation of the abdominal aortic aneurysm (AAA) depend on the constitutive description of the wall. Based on in vitro and in vivo experimental data several constitutive models for the AAA wall have been proposed in the literature. Those models differ strongly from each other and their impact on the computed stress in biomechanical simulation is not clearly understood. Methods: Finite element (FE) models of AAAs from 7 patients who underwent elective surgical repair were used to compute wall stresses. AAA geometry was reconstructed from CT angiography (CT-A) data and patient-specific (PS) constitutive descriptions of the wall were derived from planar biaxial testing of anterior wall tissue samples. In total 28 FE models were used, where the wall was described by either patient-specific or previously reported study-average properties. This data was derived from either uniaxial or biaxial in vitro testing. Computed wall stress fields were compared on node-by-node basis. Results: Different constitutive models for the AAA wall cause significantly different predictions of wall stress. While study-average data from biaxial testing gives globally the same stress field as the patient-specific wall properties, the material model based on uniaxial test data overestimates the wall stress on average by 30. kPa or about 67% of the mean stress. A quasi-linear description based on the in vivo measured distensibility of the AAA wall leads to a completely altered stress field and overestimates the wall stress by about 75. kPa or about 167% of the mean stress. Conclusion: The present study demonstrated that the constitutive description of the wall is crucial for AAA wall stress prediction. Consequently, results obtained using different models should not be mutually compared unless different stress gradients across the wall are not taken into account. Highly nonlinear material models should be preferred when the response of AAA to increased blood pressure is investigated, while the quasi-linear model with high initial stiffness produces negligible stress gradients across the wall and thus, it is more appropriate when response to mean blood pressure is calculated.

  • 32.
    Prahl-Wittberg, Lisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    van Wyk, Stevin
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Mihaescu, Mihai
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Fuchs, Laszlo
    KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
    Gutmark, Ephraim
    Aerospace Engineering, University of Cincinnati.
    Backeljauw, Philippe
    Cincinnati Children's Hospital.
    Gutmark-Little, Iris
    Cincinnati Children's Hospital.
    The Impact of Aortic Arch Geometry on Flow Characteristics2013In: / [ed] AIAA, AIAA, 2013Conference paper (Refereed)
    Abstract [en]

    Cardiovascular defects characterized by geometrical anomalies of the aorta and its eecton the blood ow is the focus of this study. Not only are the local ow characteristicsgeometry dependent, but they are also directly connected to the rheological properties ofblood. Flow characteristics such as wall shear stress are often postulated to play a centralrole in the development of vascular disease.In this study, blood is considered to be a non-Newtonian uid and modeled via theQuemada model, an empirical model that is valid for dierent red blood cell loading.Three patient-specic geometries of the aortic arch are investigated numerically. Thethree geometries investigated in this study all display malformations that are prevalent inpatients having the genetic disorder Turner syndrome. The results show a highly complexow with regions of secondary ow that are enhanced in two of the three aortas. Moreover,blood ow is clearly diverted due to the malformations, moving to a larger extent throughthe branches of the arch instead of through the descending aorta. The geometry havingan elongated transverse aorta is found to be subjected to larger areas of highly oscillatorylow wall shear stress.

  • 33.
    Riad, Jacques
    et al.
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Modlesky, Christopher
    Gutierrez-Farewik, Elena
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Structural Mechanics.
    Broström, Eva W.
    Karolinska Institutet, Dept. of Women's and Children's Health.
    Are Muscle Volume Differences Related to Concentric Muscle Work During Walking in Spastic Hemiplegic Cerebral Palsy?2012In: Clinical Orthopaedics and Related Research, ISSN 0009-921X, E-ISSN 1528-1132, Vol. 470, no 5, p. 1278-1285Article in journal (Refereed)
    Abstract [en]

    Background: Individuals with spastic hemiplegic cerebral palsy are typically high functioning and walk without assistive devices. The involved limb is usually smaller and shorter, although it is not clear whether the difference in muscle volume has an impact on walking capacity. Questions/purposes: We determined the volume of muscles important for propulsion and related that volume to concentric muscle work during walking on the hemiplegic and noninvolved sides in patients with cerebral palsy. Patients and Methods: We studied 46 patients (mean age, 17.6 years; range, 13-24 years) with spastic hemiplegic cerebral palsy. We assessed muscle volume using MRI and concentric muscle work in the sagittal plane from the hip, knee, and ankle using three-dimensional gait analysis. Patients were classified by Winters' criteria to assess the involvement of cerebral palsy and movement pattern during walking. Results: On the hemiplegic side, muscles were smaller, except for the gracilis muscle, and concentric muscle work from the ankle plantar flexors, knee extensors, and hip flexors and extensors was lower compared to the noninvolved side. Hip extensor work was higher on the hemiplegic and the noninvolved sides compared to a control group of 14 subjects without cerebral palsy. Hemiplegic to noninvolved volume ratios correlated with work ratios (r = 0.40-0.66). The Winters classification and previous calf muscle surgery predicted work ratios. Conclusions: Our observations of smaller muscles on the hemiplegic side and changes in muscle work on both sides can help us distinguish between primary deviations that may potentially be treatable and compensatory mechanisms that should not be treated.

  • 34.
    Wang, Ruoli
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Compensatory strategies during walking in response to excessive muscle co-contraction at the ankle joint2014In: Gait & Posture, ISSN 0966-6362, E-ISSN 1879-2219, Vol. 39, no 3, p. 926-932Article in journal (Refereed)
    Abstract [en]

    Excessive co-contraction causes inefficient or abnormal movement in several neuromuscular pathologies. How synergistic muscles spanning the ankle, knee and hip adapt to co-contraction of ankle muscles is not well understood. This study aimed to identify the compensation strategies required to retain normal walking with excessive antagonistic ankle muscle co-contraction. Muscle-actuated simulations of normal walking were performed to quantify compensatory mechanisms of ankle and knee muscles during stance in the presence of normal, medium and high levels of co-contraction of antagonistic pairs gastrocnemius + tibialis anterior and soleus + tibialis anterior. The study showed that if co-contraction increases, the synergistic ankle muscles can compensate; with gastrocmemius + tibialis anterior co-contraction, the soleus will increase its contribution to ankle plantarflexion acceleration. At the knee, however, almost all muscles spanning the knee and hip are involved in compensation. We also found that ankle and knee muscles alone can provide sufficient compensation at the ankle joint, but hip muscles must be involved to generate sufficient knee moment. Our findings imply that subjects with a rather high level of dorsiflexor + plantarflexor co-contraction can still perform normal walking. This also suggests that capacity of other lower limb muscles to compensate is important to retain normal walking in co-contracted persons. The compensatory mechanisms can be useful in clinical interpretation of motion analyses, when secondary muscle co-contraction or other deficits may present simultaneously in subjects with motion disorders.

  • 35.
    Wang, Ruoli
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Centres, BioMEx. Karolinska Institutet, Sweden.
    Herman, Pawel
    KTH, School of Computer Science and Communication (CSC), Computational Science and Technology (CST).
    Ekeberg, Örjan
    KTH, School of Computer Science and Communication (CSC), Computational Science and Technology (CST).
    Gäverth, Johan
    Dept Women's and Children's Health, Karolinska Institutet.
    Fagergren, Anders
    AggeroMedTech AB, Stockholm.
    Forssberg, Hans
    Dept Women's and Children's Health, Karolinska Institutet.
    Neural and non-neural related properties in the spastic wrist flexors: An optimization study2017In: Medical Engineering and Physics, ISSN 1350-4533, E-ISSN 1873-4030, Vol. 47, p. 198-209Article in journal (Refereed)
    Abstract [en]

    Quantifying neural and non-neural contributions to increased joint resistance in spasticity is essential for a better understanding of its pathophysiological mechanisms and evaluating different intervention strategies. However, direct measurement of spasticity-related manifestations, e.g., motoneuron and biophysical properties in humans, is extremely challenging. In this vein, we developed a forward neuromusculoskeletal model that accounts for dynamics of muscle spindles, motoneuron pools, muscle activation and musculotendon of wrist flexors and relies on the joint angle and resistant torque as the only input measurement variables. By modeling the stretch reflex pathway, neural and non-neural related properties of the spastic wrist flexors were estimated during the wrist extension test. Joint angle and resistant torque were collected from 17 persons with chronic stroke and healthy controls using NeuroFlexor, a motorized force measurement device during the passive wrist extension test. The model was optimized by tuning the passive and stretch reflex-related parameters to fit the measured torque in each participant. We found that persons with moderate and severe spasticity had significantly higher stiffness than controls. Among subgroups of stroke survivors, the increased neural component was mainly due to a lower muscle spindle rate at 50% of the motoneuron recruitment. The motoneuron pool threshold was highly correlated to the motoneuron pool gain in all subgroups. The model can describe the overall resistant behavior of the wrist joint during the test. Compared to controls, increased resistance was predominantly due to higher elasticity and neural components. We concluded that in combination with the NeuroFlexor measurement, the proposed neuromusculoskeletal model and optimization scheme served as suitable tools for investigating potential parameter changes along the stretch-reflex pathway in persons with spasticity.

  • 36. Weisbecker, Hannah
    et al.
    Pierce, David M.
    Regitnig, Peter
    Holzapfel, Gerhard A.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Layer-specific damage experiments and modeling of human thoracic and abdominal aortas with non-atherosclerotic intimal thickening2012In: Journal of The Mechanical Behavior of Biomedical Materials, ISSN 1751-6161, E-ISSN 1878-0180, Vol. 12, p. 93-106Article in journal (Refereed)
    Abstract [en]

    Many treatments for cardiovascular diseases include an endovascular insertion of stents or stent grafts into arteries, a procedure which may cause high tissue stresses and even damage in the arterial wall. In order to study such problems by using finite element methods, both appropriate constitutive models and experimental data on human tissue samples are required. Layer-specific experimental data for human tissue tested up to the supra-physiological loading range are rare in the literature. In this study, intact and layer-separated experimental data from uniaxial extension tests are presented for human thoracic and abdominal aortas with non-atherosclerotic intimal thickening undergoing supra-physiological loading. A novel pseudo-elastic damage model, proposed to describe discontinuous softening in aortic arterial tissues, is fit to the obtained experimental data. Fitting of the model with and without consideration of damage accumulation in the non-collagenous matrix material reveals that tissue damage is primarily related to the collagen fiber fabric. By employing the fit model, the effect of aortic tissue pre-conditioning on the material parameters from the resulting data fits is evaluated. Histological examination of the collagen fibers under different applied stretches is used to gain more insights into the structural changes of the tissue under supra-physiological loading.

  • 37.
    Yadav, Priti
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Modelling loading and growth of long bones Modelling loading and growth of long bones2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The long bones grow by the process of endochondral ossification, which occurs at the growth plate. This process is regulated by biological factors and mechanical factors. The biological factors which contribute to endochondral ossification process are genes, hormones, nutrients etc. The mechanical factor is the load acting on the bone. The major forces on the bone are due to joint contact load and muscle forces, which induce stresses in the bone. Carter and Wong proposed in a theory that cyclic or intermittent octahedral shear stress promotes the bone growth and cyclic or intermittent hydrostatic compressive stress inhibits the bone growth. Previously this theory has been used to predict the morphological development of long bones, but with studies using simplified femur and growth plate models. Furthermore, the Carter and Wong theory has a limitation that it does not intrinsically incorporate the resulting growth direction.In the first study, the importance of a subject-specific growth plate over a simplified growth plate has been studied, and growth has been simulated using two different growth direction models: Femoral neck shaft deformation direction and minimum shear stress direction. This study favors the minimum shear stress growth direction model, as it is less sensitive to applied boundary condition than the femoral neck shaft deformation direction model.The second study aims to understand how different muscle groups affect the bone growth tendency. Subject-specific femur and growth plate models of able-bodied children were used. The muscle forces and associated hip contact force from specific muscle groups were applied, and neck shaft angle and femoral anteversion growth tendencies were predicted. This study indicated a tendency for reduction of neck shaft angle and femoral anteversion. Hip abductor muscle forces contribute most, and hip adductor muscle forces least, to bone growth rate.Accurate prediction of bone growth tendency and knowledge of the influence of different muscle groups on bone growth tendency may help in better treatment planning for children at risk of developing bone deformity problems.

  • 38.
    Yadav, Priti
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Shefelbine, Sandra J.
    Gutierrez Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Effect of growth plate geometry and growth direction on prediction of proximal femoral morphologyManuscript (preprint) (Other academic)
  • 39.
    Yadav, Priti
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Shefelbine, Sandra J.
    Gutierrez Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Influence of muscle groups’ activation on proximal femoral growth tendencyManuscript (preprint) (Other academic)
  • 40.
    Yadav, Priti
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics.
    Shefelbine, Sandra J.
    Gutierrez-Farewik, Elena M.
    KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. Karolinska Institutet, Sweden.
    Effect of growth plate geometry and growth direction on prediction of proximal femoral morphology2016In: Journal of Biomechanics, ISSN 0021-9290, E-ISSN 1873-2380, Vol. 49, no 9, p. 1613-1619Article in journal (Refereed)
    Abstract [en]

    Mechanical stimuli play a significant role in the process of endochondral growth. Thus far, approaches to understand the endochondral mechanical growth rate have been limited to the use of approximated location and geometry of the growth plate. Furthermore, growth has been simulated based on the average deflection of the growth plate or of the femoral neck. It has also been reported in the literature that the growth plate lies parallel to one of the principal stresses acting on it, to reduce the shear between epiphysis and diaphysis. Hence the current study objectives were (1) to evaluate the significance of a subject-specific finite element model of the femur and growth plate compared to a simplified growth plate model and (2) to explore the different growth direction models to better understand proximal femoral growth mechanisms. A subject-specific finite element model of an able-bodied 7-year old child was developed. The muscle forces and hip contact force were computed for one gait cycle and applied to a finite element model to determine the specific growth rate. Proximal femoral growth was simulated for two different growth direction models: femoral neck deflection direction and principal stress direction. The principal stress direction model captured the expected tendency for decreasing the neck shaft angle and femoral anteversion for both growth plate models. The results of this study suggest that the subject-specific geometry and consideration of the principal stress direction as growth direction may be a more realistic approach for correct prediction of proximal femoral growth morphology.

1 - 40 of 40
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf