Increased muscle responses to balance perturbations in children with cerebral palsy can be explained by increased sensitivity to center of mass movement.

BACKGROUND: Balance impairments are common in children with cerebral palsy (CP). Muscle activity during perturbed standing is higher in children with CP than in typically developing (TD) children, but we know surprisingly little about how sensorimotor processes for balance control are altered in CP. Sensorimotor processing refers to how the nervous system translates incoming sensory information about body motion into motor commands to activate muscles. In healthy adults, muscle activity in response to backward support-surface translations during standing can be reconstructed by center of mass (CoM) feedback, i.e., by a linear combination of delayed (due to neural transmission times) CoM displacement, velocity, and acceleration. The level of muscle activity in relation to changes in CoM kinematics, i.e., the feedback gains, provides a metric of the sensitivity of the muscle response to CoM perturbations. RESEARCH QUESTION: Can CoM feedback explain reactive muscle activity in children with CP, yet with higher feedback gains than in TD children? METHODS: We perturbed standing balance by backward support-surface translations of different magnitudes in 20 children with CP and 20 age-matched TD children and investigated CoM feedback pathways underlying reactive muscle activity in the triceps surae and tibialis anterior. RESULTS: Reactive muscle activity could be reconstructed by delayed feedback of CoM kinematics and hence similar sensorimotor pathways might underlie balance control in children with CP and TD children. However, sensitivities of both agonistic and antagonistic muscle activity to CoM displacement and velocity were higher in children with CP than in TD children. The increased sensitivity of balance correcting responses to CoM movement might explain the stiffer kinematic response, i.e., smaller CoM movement, observed in children with CP. SIGNIFICANCE: The sensorimotor model used here provided unique insights into how CP affects neural processing underlying balance control. Sensorimotor sensitivities might be a useful metric to diagnose balance impairments.

Assisting walking balance using a bio-inspired exoskeleton controller.

BACKGROUND: Balance control is important for mobility, yet exoskeleton research has mainly focused on improving metabolic energy efficiency. Here we present a biomimetic exoskeleton controller that supports walking balance and reduces muscle activity. METHODS: Humans restore balance after a perturbation by adjusting activity of the muscles actuating the ankle in proportion to deviations from steady-state center of mass kinematics. We designed a controller that mimics the neural control of steady-state walking and the balance recovery responses to perturbations. This controller uses both feedback from ankle kinematics in accordance with an existing model and feedback from the center of mass velocity. Control parameters were estimated by fitting the experimental relation between kinematics and ankle moments observed in humans that were walking while being perturbed by push and pull perturbations. This identified model was implemented on a bilateral ankle exoskeleton. RESULTS: Across twelve subjects, exoskeleton support reduced calf muscle activity in steady-state walking by 19% with respect to a minimal impedance controller (p < 0.001). Proportional feedback of the center of mass velocity improved balance support after perturbation. Muscle activity is reduced in response to push and pull perturbations by 10% (p = 0.006) and 16% (p < 0.001) and center of mass deviations by 9% (p = 0.026) and 18% (p = 0.002) with respect to the same controller without center of mass feedback. CONCLUSION: Our control approach implemented on bilateral ankle exoskeletons can thus effectively support steady-state walking and balance control and therefore has the potential to improve mobility in balance-impaired individuals.

Predictive Simulations of Musculoskeletal Function and Jumping Performance in a Generalized Bird.

Jumping is a common, but demanding, behavior that many animals employ during everyday activity. In contrast to jump-specialists such as anurans and some primates, jumping biomechanics and the factors that influence performance remains little studied for generalized species that lack marked adaptations for jumping. Computational biomechanical modeling approaches offer a way of addressing this in a rigorous, mechanistic fashion. Here, optimal control theory and musculoskeletal modeling are integrated to generate predictive simulations of maximal height jumping in a small ground-dwelling bird, a tinamou. A three-dimensional musculoskeletal model with 36 actuators per leg is used, and direct collocation is employed to formulate a rapidly solvable optimal control problem involving both liftoff and landing phases. The resulting simulation raises the whole-body center of mass to over double its standing height, and key aspects of the simulated behavior qualitatively replicate empirical observations for other jumping birds. However, quantitative performance is lower, with reduced ground forces, jump heights, and muscle-tendon power. A pronounced countermovement maneuver is used during launch. The use of a countermovement is demonstrated to be critical to the achievement of greater jump heights, and this phenomenon may only need to exploit physical principles alone to be successful; amplification of muscle performance may not necessarily be a proximate reason for the use of this maneuver. Increasing muscle strength or contractile velocity above nominal values greatly improves jump performance, and interestingly has the greatest effect on more distal limb extensor muscles (i.e., those of the ankle), suggesting that the distal limb may be a critical link for jumping behavior. These results warrant a re-evaluation of previous inferences of jumping ability in some extinct species with foreshortened distal limb segments, such as dromaeosaurid dinosaurs. Simulations prédictives de la fonction musculo-squelettique et des performances de saut chez un oiseau généralisé Sauter est un comportement commun, mais exigeant, que de nombreux animaux utilisent au cours de leurs activités quotidiennes. Contrairement aux spécialistes du saut tels que les anoures et certains primates, la biomécanique du saut et les facteurs qui influencent la performance restent peu étudiés pour les espèces généralisées qui n'ont pas d'adaptations marquées pour le saut. Les approches de modélisation biomécanique computationnelle offrent un moyen d'aborder cette question de manière rigoureuse et mécaniste. Ici, la théorie du contrôle optimal et la modélisation musculo-squelettique sont intégrées pour générer des simulations prédictives du saut en hauteur maximal chez un petit oiseau terrestre, le tinamou. Un modèle musculo-squelettique tridimensionnel avec 36 actionneurs par patte est utilisé, et une méthode numérique nommée "direct collocation" est employée pour formuler un problème de contrôle optimal rapidement résoluble impliquant les phases de décollage et d'atterrissage. La simulation qui en résulte élève le centre de masse du corps entier à plus du double de sa hauteur debout, et les aspects clés du comportement simulé reproduisent qualitativement les observations empiriques d'autres oiseaux sauteurs. Cependant, les performances quantitatives sont moindres, avec une réduction des forces au sol, des hauteurs de saut et de la puissance musculo-tendineuse. Une manœuvre de contre-mouvement prononcée est utilisée pendant le lancement. Il a été démontré que l'utilisation d'un contre-mouvement est essentielle à l'obtention de hauteurs de saut plus importantes, et il se peut que ce phénomène doive exploiter uniquement des principes physiques pour réussir; l'amplification de la performance musculaire n'est pas nécessairement une raison immédiate de l'utilisation de cette manœuvre. L'augmentation de la force musculaire ou de la vitesse de contraction au-dessus des valeurs nominales améliore grandement la performance de saut et, fait intéressant, a le plus grand effet sur les muscles extenseurs des membres plus distaux (c'est-à-dire ceux de la cheville), ce qui suggère que le membre distal peut être un lien critique pour le comportement de saut. Ces résultats justifient une réévaluation des déductions précédentes de la capacité de sauter chez certaines espèces éteintes avec des segments de membres distaux raccourcis, comme les dinosaures droméosauridés. Voorspellende simulaties van musculoskeletale functie en springprestaties bij een gegeneraliseerde vogel Springen is een veel voorkomend, maar veeleisend, gedrag dat veel dieren toepassen tijdens hun dagelijkse bezigheden. In tegenstelling tot de springspecialisten zoals de anura en sommige primaten, is de biomechanica van het springen en de factoren die de prestaties beïnvloeden nog weinig bestudeerd voor algemene soorten die geen uitgesproken adaptaties voor het springen hebben. Computationele biomechanische modelbenaderingen bieden een manier om dit op een rigoureuze, mechanistische manier aan te pakken. Hier worden optimale controle theorie en musculoskeletale modellering geïntegreerd om voorspellende simulaties te genereren van maximale hoogtesprong bij een kleine grondbewonende vogel, een tinamou. Een driedimensionaal musculoskeletaal model met 36 actuatoren per poot wordt gebruikt, en directe collocatie wordt toegepast om een snel oplosbaar optimaal controleprobleem te formuleren dat zowel de opstijg-als de landingsfase omvat. De resulterende simulatie verhoogt het lichaamszwaartepunt tot meer dan het dubbele van de stahoogte, en belangrijke aspecten van het gesimuleerde gedrag komen kwalitatief overeen met empirische waarnemingen voor andere springende vogels. De kwantitatieve prestaties zijn echter minder, met verminderde grondkrachten, spronghoogtes en spierpeeskracht. Tijdens de lancering wordt een uitgesproken tegenbewegingsmanoeuvre gebruikt. Aangetoond is dat het gebruik van een tegenbeweging van cruciaal belang is voor het bereiken van grotere spronghoogten, en dit fenomeen hoeft alleen op fysische principes te berusten om succesvol te zijn; versterking van de spierprestaties hoeft niet noodzakelijk een proximate reden te zijn voor het gebruik van deze manoeuvre. Het verhogen van de spierkracht of van de contractiesnelheid boven de nominale waarden verbetert de sprongprestatie aanzienlijk, en heeft interessant genoeg het grootste effect op de meer distale extensoren van de ledematen (d.w.z. die van de enkel), wat suggereert dat de distale ledematen een kritieke schakel kunnen zijn voor het springgedrag. Deze resultaten rechtvaardigen een herevaluatie van eerdere conclusies over springvermogen bij sommige uitgestorven soorten met voorgekorte distale ledematen, zoals dromaeosauride dinosauriërs. Prädiktive Simulationen der muskuloskelettalen Funktion und Sprungleistung bei einem generalisierten Vogel Springen ist ein übliches jedoch anstrengendes Verhalten, das viele Tiere bei ihren täglichen Aktivitäten einsetzen. Im Gegensatz zu Springspezialisten, wie Fröschen und einigen Primaten, sind bei allgemeinen Arten, welche keine ausgeprägten Anpassung für Sprungverhalten aufweisen, die Biomechanik beim Springen und die Faktoren, welche die Leistungsfähigkeit beeinflussen, noch wenig untersucht. Computergestützte biomechanische Modellierungsverfahren bieten hier eine Möglichkeit, dies in einer gründlichen, mechanistischen Weise anzugehen. In dieser Arbeit werden die optimale Steuerungstheorie und Muskel-Skelett-Modellierung zusammen eingesetzt, um die maximale Sprunghöhe eines kleinen bodenlebenden Vogels, eines Perlsteisshuhns, zu simulieren und zu prognostizieren. Es wird ein dreidimensionales Muskel-Skelett-Modell mit 36 Aktuatoren pro Bein verwendet, und durch direkte Kollokation wird ein schnell lösbares optimales Steuerungsproblem formuliert, das sowohl die Abstoss- als auch die Landephase umfasst. Die daraus folgende Simulation bringt den Ganzkörperschwerpunkt auf mehr als das Doppelte seiner Standhöhe und entscheidende Aspekte des simulierten Verhaltens entsprechen qualitativ empirischen Beobachtungen für andere springende Vögel. Allerdings ist die quantitative Leistungsfähigkeit geringer, mit reduzierten Bodenkräften, Sprunghöhen und Muskel-Sehnen-Kräften. Beim Abstossen wird ein ausgeprägtes Gegenbewegungsmanöver durchgeführt. Die Durchführung einer Gegenbewegung ist nachweislich entscheidend für das Erreichen grösserer Sprunghöhen, wobei dieses Phänomen möglicherweise nur physikalische Prinzipien auszuschöpfen braucht, um erfolgreich zu sein. Die Verstärkung der Muskelleistung ist daher möglicherweise nicht zwingend ein unmittelbarer Grund für die Verwendung dieses Manövers. Eine Erhöhung der Muskelkraft oder der Kontraktionsgeschwindigkeit über die Nominalwerte hinaus führt zu einer erheblichen Zunahme der Sprungleistung und hat interessanterweise den grössten Effekt bei den weiter distal gelegenen Streckmuskeln der Beine (d.h. bei denjenigen des Sprunggelenks), was darauf hindeutet, dass die distale Gliedmasse ein entscheidendes Element für das Sprungverhalten sein könnte. Diese Ergebnisse geben Anlass zur Überprüfung früherer Schlussfolgerungen hinsichtlich der Sprungfähigkeit einiger ausgestorbener Arten mit verkürzten distalen Gliedmassen, wie beispielsweise bei dromaeosauriden Dinosauriern.

Botulinum toxin injections minimally affect modelled muscle forces during gait in children with cerebral palsy.

BACKGROUND: Children with cerebral palsy (CP) present altered gait patterns and electromyography (EMG) activity compared to typically developing children. To temporarily reduce muscular activity and to correct the abnormal muscle force balance, Botulinum Toxin type A (BTX-A) injections are used. RESEARCH QUESTION: What is the effect of BTX-A injections on dynamic muscle forces during gait, when calculated using an EMG-constrained approach?. METHODS: Retrospective data of ten typically developing (TD) and fourteen children with spastic diplegic CP were used for musculoskeletal modeling and dynamic simulations of gait, before and after BTX-A treatment. Individual muscle forces were calculated using an EMG-constrained optimization, in which EMG of eight muscles was used as muscle excitation signal to constrain the muscle activation patterns. Paired t-tests were used to compare average modelled muscle forces in different phases of the gait cycle pre- and post-BTX-A, summarized in the muscle profile score. Two-sample t-tests were used to determine significant differences between TD and pre- and post-BTX-A modelled muscle forces. RESULTS: For most muscles, the force was decreased in CP compared to TD children in all phases of the gait cycle, both before and after BTX-A treatment. Differences in muscle forces before and after BTX-A treatment were limited, with only few significant differences between pre- and post-BTX-A. Compared to a standard static optimization approach, imposing the EMG activity increased modelled muscle forces for most muscles. SIGNIFICANCE: Our findings indicate that BTX-A treatment has a limited effect on the muscle balance in CP children. Besides that, the use of EMG-constrained optimization is recommended when studying muscle balance in children with CP.

Single-event multilevel surgery, but not botulinum toxin injections normalize joint loading in cerebral palsy patients.

BACKGROUND: Many patients with cerebral palsy present a pathologic gait pattern, which presumably induces aberrant musculoskeletal loading that interferes with natural bone growth, causing bone deformations on the long term. Botulinum toxin interventions and single-event multilevel surgeries are used to restore the gait pattern, assuming that a normal gait pattern restores musculoskeletal loading and thus prevents further bone deformation. However, it is unknown if these interventions are able to restore musculoskeletal loading. Hence, we investigated the impact of botulinum toxin injections and single-event multilevel surgery on musculoskeletal loading. METHODS: Gait data collected in 93 children with bilateral cerebral palsy, which included pre- and post multi-level botulinum toxin (49 children) and single-event multilevel surgery (44 children) assessments, and 15 typically developing children were retrospectively processed using a musculoskeletal modelling workflow to calculate joint angles, moments, muscle and joint contact force magnitudes and orientations. Differences from the typically developing waveform were expressed by a root-mean square difference were compared using paired t-tests for each intervention separately (alpha <0.05). FINDINGS: Botulinum toxin induced significant changes in the joint angles, but did not improve the muscle and joint contact forces. Single-event multilevel surgery induced significant kinematic and kinetic changes, which were associated with improved muscle and joint contact forces. INTERPRETATION: The present results indicate that botulinum toxin injections were not able to restore normal gait kinematics nor musculoskeletal loading, whereas single-event multilevel surgery did successfully restore both. Therefore, single-event multilevel surgery might be protective against the re-occurrence of bone deformation on the longer term.

Ultrasound-Based Optimal Parameter Estimation Improves Assessment of Calf Muscle-Tendon Interaction During Walking.

We present and evaluate a new approach to estimate calf muscle-tendon parameters and calculate calf muscle-tendon function during walking. We used motion analysis, ultrasound, and EMG data of the calf muscles collected in six young and six older adults during treadmill walking as inputs to a new optimal estimation algorithm. We used estimated parameters or scaled generic parameters in an existing approach to calculate muscle fiber lengths and activations. We calculated the fit with experimental data in terms of root mean squared differences (RMSD) and coefficients of determination (R2). We also calculated the calf muscle metabolic energy cost. RMSD between measured and calculated fiber lengths and activations decreased and R2 increased when estimating parameters compared to using scaled generic parameters. Moreover, R2 between measured and calculated gastrocnemius medialis fiber length and soleus activations increased by 19 and 70%, and calf muscle metabolic energy decreased by 25% when using estimated parameters compared to using scaled generic parameters at speeds not used for estimation. This new approach estimates calf muscle-tendon parameters in good accordance with values reported in literature. The approach improves calculations of calf muscle-tendon interaction during walking and highlights the importance of individualizing calf muscle-tendon parameters.

Validation of plantar pressure simulations using finite and discrete element modelling in healthy and diabetic subjects.

Plantar pressure simulation driven by integrated 3D motion capture data, using both a finite element and a discrete element model, is compared for ten healthy and ten diabetic neuropathic subjects. The simulated peak pressure deviated on average between 16.7 and 34.2% from the measured peak pressure. The error in the position of the peak pressure was on average smaller than 4.2 cm. No method was more accurate than the other although statistical differences were found between them. Both techniques are thus complementary and useful tools to better understand the alteration of diabetic foot biomechanics during gait.

Differences in knee adduction moment between healthy subjects and patients with osteoarthritis depend on the knee axis definition.

OBJECTIVE: This study, firstly, investigates the effect of using an anatomical versus a functional axis of rotation (FAR) on knee adduction moment (KAM) in healthy subjects and patients with knee osteoarthritis (KOA). Secondly, this study reports KAM for models with FAR calculated using weight-bearing and non-weight-bearing motion. DESIGN: Three musculoskeletal models were created using OpenSim with different knee axis of rotation (AR): transepicondylar axis (TEA); FAR calculated based on SARA algorithm using a weight-bearing motion (wFAR) and a non-weight-bearing motion (nwFAR). KAM were calculated during gait in fifty-nine subjects (n=20 healthy, n=16 early OA, n=23 established OA) for all models and groups. RESULTS: Significant differences between the three groups in the first peak KAM were found when TEA was used (p=0.038). However, these differences were no longer present when using FAR. In subjects with established OA, KAMs were significantly reduced when using nwFAR compared to TEA models but also compared to wFAR models. CONCLUSION: The presence of excessive KAM in subjects with established KOA showed to be dependent on the definition of the AR: anatomical versus functional. Therefore, caution should be accounted when comparing KAM in different studies on KOA patients. In patients with end-stage knee OA where increased passive knee laxity is likely to exist, the use of weight-bearing motions should be considered to avoid increased variability in the location and orientation of a FAR obtained from activities with only limited joint loading.

Knee contact forces are not altered in early knee osteoarthritis.

OBJECTIVE: This study calculated knee contact forces (KCF) and its relations with knee external knee adduction moments (KAM) and/or flexion moments (KFM) during the stance phase of gait in patients with early osteoarthritis (OA), classified based on early joint degeneration on Magnetic Resonance Imaging (MRI). We aimed at assessing if altered KCF are already present in early structural degeneration. DESIGN: Three-dimensional motion and ground reaction force data in 59 subjects with medial compartment knee OA (N=23 established OA, N=16 early OA, N=20 controls) were used as input for a musculoskeletal model. KAM and KFM, and KCF were estimated using OpenSim software. RESULTS: No significant differences were found between controls and subjects with early OA. In early OA patients, KAM significantly explained 69% of the variance associated with the first peaks KCF but only KFM contributed to the second peaks KCF. The multiple correlation, combining KAM and KFM, showed to be higher. However, only 20% of the variance of second peak KCF was explained by both moments in established OA. CONCLUSION: KCF are not increased in patients with early OA, suggesting that knee joint overload is more a consequence of further joint degeneration in more advanced stages of OA. Additionally, our results clearly show that KAM is not sufficient to predict joint loading at the end of the stance, where KFM contributes substantially to the loading, especially in early OA.

Effect of dexamethasone on nausea, vomiting, and pain in paediatric tonsillectomy.

BACKGROUND: The efficacy of dexamethasone (DEX) to reduce morbidity after paediatric tonsillectomy remains controversial. We evaluated the effect of 0.15 and 0.5 mg kg(-1) DEX on the incidence of postoperative nausea and vomiting (PONV) and on pain intensity after paediatric tonsillectomy. METHODS: A total of 147 children aged 2-8 yr undergoing elective tonsillectomy were included in this prospective randomized double-blind study. At the induction of anaesthesia, subjects received 0.15 mg kg(-1) (DEX 0.15), 0.5 mg kg(-1) (DEX 0.5) DEX, or an equivalent volume of saline solution (placebo). Anaesthetic and surgical techniques were standardized. The incidence of PONV and the need for anti-emetic drugs and additional analgesia (tramadol and/or morphine) were recorded. Postoperative pain was assessed using the Children's Hospital of Eastern Ontario Pain Scale, the visual analogue scale, and the postoperative pain measure for parents. RESULTS: The incidence of early PONV (primary outcome variable) was lower in both DEX groups (DEX 0.15: 21%; DEX 0.5: 22%; placebo: 49%; P=0.001). The incidence of severe pain was reduced in the DEX groups on the second postoperative day (DEX 0.15: 20%; DEX 0.5: 5%; placebo: 47%; P<0.001). The study was not powered to assess a difference between the two DEX dose groups. CONCLUSIONS: A single i.v. injection of DEX at the induction of anaesthesia was effective in reducing the incidence of early and late PONV and the level of pain on the second postoperative day. A 0.15 mg kg(-1) DEX dose appeared to be as effective as a 0.5 mg kg(-1) dose to reduce the incidence of PONV.

Belgian recommendations on perioperative maintenance fluid management of surgical pediatric population.

The European recommendations on perioperative maintenance fluids in children have recently been adapted from hypotonic to isotonic electrolyte solutions with lower glucose concentrations. In Belgium, however, the commercially approved solutions do not match with these recommendations and there is neither consensus nor mandate about the composition and volume of perioperative maintenance fluids in children undergoing surgery despite the continuing controversy in literature. This paper highlights the significant challenges and shortcomings while prescribing fluid therapy for pediatric surgical patients in Belgium. It is sensible to the authors to address these issues with national guidance through an organization such as The Belgian Association for Paediatric Anaesthesiology, and to propose Belgian recommendations on perioperative fluid management in surgical children, with the intention of improving the quality of care in this population.

A physiology-based inverse dynamic analysis of human gait using sequential convex programming: a comparative study.

This paper presents an enhanced version of the previously proposed physiological inverse approach (PIA) to calculate musculotendon (MT) forces and evaluates the proposed methodology in a comparative study. PIA combines an inverse dynamic analysis with an optimisation approach that imposes muscle physiology and optimises performance over the entire motion. To solve the resulting large-scale, nonlinear optimisation problem, we neglected muscle fibre contraction speed and an approximate quadratic optimisation problem (PIA-QP) was formulated. Conversely, the enhanced version of PIA proposed in this paper takes into account muscle fibre contraction speed. The optimisation problem is solved using a sequential convex programing procedure (PIA-SCP). The comparative study includes PIA-SCP, PIA-QP and two commonly used approaches from the literature: static optimisation (SO) and computed muscle control (CMC). SO and CMC make simplifying assumptions to limit the computational time. Both methods minimise an instantaneous performance criterion. Furthermore, SO does not impose muscle physiology. All methods are applied to a gait cycle of six control subjects. The relative root mean square error averaged over all subjects, ε(RMS), between the joint torques simulated from the optimised activations and the joint torques obtained from the inverse dynamic analysis was about twice as large for SO (ε(RMS) = 86) as compared with CMC (ε(RMS) = 39) and PIA-SCP (ε(RMS) = 50). ε(RMS) was at least twice as large for PIA-QP (ε(RMS) = 197) than for all other methods. As compared with CMC, muscle activation patterns predicted by PIA-SCP better agree with experimental electromyography (EMG). This study shows that imposing muscle physiology as well as globally optimising performance is important to accurately calculate MT forces underlying gait.

Functional knee axis based on isokinetic dynamometry data: Comparison of two methods, MRI validation, and effect on knee joint kinematics.

This paper compares geometry-based knee axes of rotation (transepicondylar axis and geometric center axis) and motion-based functional knee axes of rotation (fAoR). Two algorithms are evaluated to calculate fAoRs: Gamage and Lasenby's sphere fitting algorithm (GL) and Ehrig et al.'s axis transformation algorithm (SARA). Calculations are based on 3D motion data acquired during isokinetic dynamometry. AoRs are validated with the equivalent axis based on static MR-images. We quantified the difference in orientation between two knee axes of rotation as the angle between the projection of the axes in the transversal and frontal planes, and the difference in location as the distance between the intersection points of the axes with the sagittal plane. Maximum differences between fAoRs resulting from GL and SARA were 5.7° and 15.4mm, respectively. Maximum differences between fAoRs resulting from GL or SARA and the equivalent axis were 5.4°/11.5mm and 8.6°/12.8mm, respectively. Differences between geometry-based axes and EA are larger than differences between fAoR and EA both in orientation (maximum 10.6°).and location (maximum 20.8mm). Knee joint angle trajectories and the corresponding accelerations for the different knee axes of rotation were estimated using Kalman smoothing. For the joint angles, the maximum RMS difference with the MRI-based equivalent axis, which was used as a reference, was 3°. For the knee joint accelerations, the maximum RMS difference with the equivalent axis was 20°/s(2). Functional knee axes of rotation describe knee motion better than geometry-based axes. GL performs better than SARA for calculations based on experimental dynamometry.

Pathological findings in South American fur seal pups (Arctocephalus australis gracilis) found dead at Guafo Island, Chile.

During four breeding seasons (2004-2008), 78 necropsy examinations were performed on South American fur seal pups (Arctocephalus australis gracilis) found dead on Guafo Island, southern Chile (43°36'S, 74°43'W). Tissue samples from 65 pups were examined microscopically. The primary causes of death were enteritis with microscopical lesions of bacteraemia (28.2%), starvation (23.1%), drowning (21.8%), trauma (19.2%) and stillbirth (2.6%). Those pups with enteritis and microscopical lesions of bacteraemia had haemorrhagic enteritis (100%), interstitial pneumonia (86%), periportal hepatitis (73%) and vasculitis (18%). The pups that died from starvation had atrophy of hepatocytes (61%) and cholestasis (61%). The pups that drowned had bronchoalveolar oedema (65%) and foreign bodies in the airways (65%). In animals that died from trauma, the main lesions were skull fractures (67%). This range of pathological findings is within what would be expected in a healthy otariid breeding colony.

Sensitivity of dynamic simulations of gait and dynamometer experiments to hill muscle model parameters of knee flexors and extensors.

We assessed and compared sensitivities of dynamic simulations to musculotendon (MT) parameters for gait and dynamometer experiments. Our aim with this comparison was to investigate whether dynamometer experiments could provide information about MT-parameters that are important to reliably study MT-function during gait. This would mean that dynamometer experiments could be used to estimate these parameters. Muscle contribution to the joint torque (MT-torque) rather than relative MT-force primarily affects the resulting gait pattern and torque measured by the dynamometer. In contrast to recent studies, therefore, we assessed the sensitivity of the MT-torque, rather than the sensitivity of the relative MT-force. Based on sensitivity of the MT-torque to a parameter perturbation, MT-parameters of the knee flexors and extensors were classified in three categories: low, medium, and high. For gait, classification was based on the average sensitivity during a gait cycle. For isometric and isokinetic dynamometer experiments, classification was based on the highest sensitivity found in the experiments. The calculated muscle contributions to the knee torque during gait and dynamometer experiments had a high sensitivity to only a limited number of MT-parameters of the knee flexors and extensors, suggesting that not all MT-parameters need to be estimated. In general, the highest sensitivity was found for tendon slack length. However, for some muscles the sensitivity to the optimal fibre length or the maximal isometric muscle force was also high or medium. The classification of the individual MT-parameters for gait and dynamometer experiments was largely similar. We therefore conclude that dynamometer experiments provide information about MT-parameters important to reliably study MT-function during gait, so that subject-specific estimates of MT-parameters could be made based on dynamometer experiments.

A physiology based inverse dynamic analysis of human gait: potential and perspectives.

One approach to compute the musculotendon forces that underlie human motion is to combine an inverse dynamic analysis with a static optimisation procedure. Although computationally efficient, this classical inverse approach fails to incorporate constraints imposed by muscle physiology. The present paper reports on a physiological inverse approach (PIA) that combines an inverse dynamic analysis with a dynamic optimisation procedure. This allows the incorporation of a full description of muscle activation and contraction dynamics, without loss of computational efficiency. A comparison of muscle excitations and MT-forces predicted by the classical and the PIA is presented for normal and pathological gait. Inclusion of muscle physiology primarily affects the rate of active muscle force build-up and decay and allows the estimation of passive muscle force. Consequently, it influences the onset and cessation of the predicted muscle excitations as well as the level of co-contraction.

Kalman smoothing improves the estimation of joint kinematics and kinetics in marker-based human gait analysis.

We developed a Kalman smoothing algorithm to improve estimates of joint kinematics from measured marker trajectories during motion analysis. Kalman smoothing estimates are based on complete marker trajectories. This is an improvement over other techniques, such as the global optimisation method (GOM), Kalman filtering, and local marker estimation (LME), where the estimate at each time instant is only based on part of the marker trajectories. We applied GOM, Kalman filtering, LME, and Kalman smoothing to marker trajectories from both simulated and experimental gait motion, to estimate the joint kinematics of a ten segment biomechanical model, with 21 degrees of freedom. Three simulated marker trajectories were studied: without errors, with instrumental errors, and with soft tissue artefacts (STA). Two modelling errors were studied: increased thigh length and hip centre dislocation. We calculated estimation errors from the known joint kinematics in the simulation study. Compared with other techniques, Kalman smoothing reduced the estimation errors for the joint positions, by more than 50% for the simulated marker trajectories without errors and with instrumental errors. Compared with GOM, Kalman smoothing reduced the estimation errors for the joint moments by more than 35%. Compared with Kalman filtering and LME, Kalman smoothing reduced the estimation errors for the joint accelerations by at least 50%. Our simulation results show that the use of Kalman smoothing substantially improves the estimates of joint kinematics and kinetics compared with previously proposed techniques (GOM, Kalman filtering, and LME) for both simulated, with and without modelling errors, and experimentally measured gait motion.

Subject-specific hip geometry affects predicted hip joint contact forces during gait.

Hip loading affects bone remodeling and implant fixation. In this study, we have analyzed the effect of subject-specific modeling of hip geometry on muscle activation patterns and hip contact forces during gait, using musculoskeletal modeling, inverse dynamic analysis and static optimization. We first used sensitivity analysis to analyze the effect of isolated changes in femoral neck-length (NL) and neck-shaft angle (NSA) on calculated muscle activations and hip contact force during the stance phase of gait. A deformable generic musculoskeletal model was adjusted incrementally to adopt a physiological range of NL and NSA. In a second similar analysis, we adjusted hip geometry to the measurements from digitized radiographs of 20 subjects with primary hip osteoarthrosis. Finally, we studied the effect of hip abductor weakness on muscle activation patterns and hip contact force. This analysis showed that differences in NL (41-74 mm) and NSA (113-140 degrees ) affect the muscle activation of the hip abductors during stance phase and hence hip contact force by up to three times body weight. In conclusion, the results from both the sensitivity and subject-specific analysis showed that at the moment of peak contact force, altered NSA has only a minor effect on the loading configuration of the hip. Increased NL, however, results in an increase of the three hip contact-force components and a reduced vertical loading. The results of these analyses are essential to understand modified hip joint loading, and for planning hip surgery for patients with osteoarthrosis.

Oral clonidine vs midazolam in the prevention of sevoflurane-induced agitation in children. a prospective, randomized, controlled trial.

BACKGROUND: This randomized, double-blind study tested the hypothesis that, in comparison with midazolam, premedication with oral clonidine reduces the incidence of emergence agitation in preschool children anaesthetized with sevoflurane. METHODS: Sixty-eight ASA I-II children undergoing circumcision were randomized into three groups to receive different oral premedication given 30 min before anaesthesia: midazolam 0.5 mg kg-1, clonidine 2 microg kg-1, and clonidine 4 microg kg-1. Sevoflurane anaesthesia was administered via a facemask (O2/N2O: 40/60). Analgesia was with penile block (bupivacaine 0.5% 0.3 ml kg-1) and rectal paracetamol (30 mg kg-1). During the first postoperative hour, children were evaluated using a modified 'objective pain scale'. RESULTS: Only the 4 microg kg-1 dose of clonidine was associated with a significant reduction in emergence agitation. Fewer children in the clonidine 4 microg kg-1 group displayed agitation (25%) than in the midazolam group (60%) (P=0.025). Incidence of hypotension and bradycardia, time to first micturition and first drink did not differ among groups. CONCLUSIONS: In comparison with midazolam, clonidine 4 microg kg-1 reduced sevoflurane-induced emergence agitation without increasing postoperative side-effects.

[Postoperative pain management in ambulatory pediatric surgery]. / Prise en charge de la douleur en pédiatrie après chirurgie ambulatoire.

Over the last fifteen years, child's pain has become one of our major concerns. In spite of this evolution, it remains one of the most frequent complications after ambulatory surgery. It is thus essential to implement all the resources we have at our disposal in order to optimize pain management. This can be obtained by basing our strategy on the concept of multimode analgesia. It is consequently essential that each team can achieve its own quality program; the corollary will be the development of clear recommendations for the parents with a systematic analgesics regulation at home and the possibility to resort to the family doctor or to the ambulatory centre in the event of persistence of pain.