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The study of pediatric head injury relies heavily on the use of finite element models and child anthropomorphic test devices (ATDs). However, these tools, in the context of pediatric head injury, have yet to be validated due to a paucity of pediatric head response data. The goal of this study is to investigate the response and injury tolerance of the pediatric head to impact. Twelve pediatric heads were impacted in a series of drop tests. The heads were dropped onto five impact locations (forehead, occiput, vertex and right and left parietal) from drop heights of 15 and 30 cm. The head could freely fall without rotation onto a flat 19 mm thick platen. The impact force was measured using a 3-axis piezoelectric load cell attached to the platen. Age and drop height were found to be significant factors in the impact response of the pediatric head. The head acceleration (14%–15 cm; 103–30 cm), Head Injury Criterion (HIC) (253%–15 cm; 154%–30 cm) and impact stiffness (5800%–15 cm; 3755%–30 cm) when averaged across all impact locations increased with age from 33 weeks gestation to 16 years, while the pulse duration (66%–15 cm; 53%–30 cm) decreased with age. Increases in head acceleration, HIC and impact stiffness were also observed with increased drop height, while pulse duration decreased with increased drop height. One important observation was that three of the four cadaveric heads between the ages of 5-months and 22-months sustained fractures from the 15 cm and 30 cm drop heights. The 5-month-old sustained a right parietal linear fracture while the 11- and 22-month-old sustained diastatic linear fractures. [ABSTRACT FROM AUTHOR]

Abstract Identification of kinetic variables in different masks of foot is important for the evaluation and treatment of chronic low back pain. The aim of this study was to investigate the effect of chronic idiopathic low back pain on kinetic variables of gait in different foot masks. 11 idiopathic chronic low back pain patients and 13 healthy matched controls participated in this study. Using Emed foot-scanner system, the ground reaction force and impulse were measured during barefoot normal walking. Then, the average footprints were divided into 10 masks using the Automask program and the data were extracted using Multimask Evaluation programs. The low back pain disability was measured by Quebec questionnaire. Our results revealed that the ground reaction force and impulse in medial and lateral midfoot and hallux masks of patients were significantly lower than controls. Furthermore, these patients demonstrated greater ground reaction force and impulse in 3–5th metatarsals mask than control group. There was a significant interaction between the low back pain and the foot masks factors. In conclusion, the ground reaction forces and impulses in different areas of foot are affected by low back pain. Therefore, the kinetic gait analysis should be considered as an appropriate tool in evaluation and prescribing proper treatment program in low back pain patients. [ABSTRACT FROM AUTHOR]

The link between gait parameters and foot abnormalities in association with low back pain is not well understood. The objective of this study was to investigate the effects of excessive foot pronation as well as the association of LBP with excessive foot pronation on the GRF components during shod walking. Methods Forty-five subjects were equally divided into a control group, a group of subjects with pronated feet only, and another group with pronated feet and LBP. Ground reaction forces were analyzed during shod walking. Results Foot pronation without low back pain was associated with increased lateral-medial ground reaction force, impulse, and time to peak of all reaction forces in heel contact phase ( p <0.03). In low back pain patients with pronated foot, greater vertical reaction forces ( p =0.001) and loading rate, and time to peak on propulsion force were observed compared to pronated foot without low back pain group. Impulse in posterior-anterior reaction force was smaller in the able-bodied group with normal foot than in the other groups ( p <0.05). Positive peak of free moments of the LBP group was significantly greater than that in other groups ( p <0.05). In conclusion, foot pronation alone was not associated with elevated vertical ground reaction forces. While, low back pain patients with foot pronation displayed higher vertical ground reaction force as well as higher loading rate. Present results reveal that gait ground reaction force components in low back pain patients with pronated foot may have clinical values on the prognosis and rehabilitation of mechanical LBP patients. [ABSTRACT FROM AUTHOR]

Locomotion on an accelerating treadmill belt is not dynamically similar to overground acceleration. The purpose of this study was to test if providing an external force to compensate for inertial forces during locomotion on an accelerating treadmill belt could induce locomotor dynamics similar to real accelerations. Nine males (mean±sd age=26±4 years, mass=81±9 kg, height=1.8±0.05 m) began walking and transitioned to running on an accelerating instrumented treadmill belt at three accelerations (0.27 m s −2 , 0.42 m s −2 , 0.76 m s −2 ). Half the trials were typical treadmill locomotion (TT) and half were emulated acceleration (EA), where elastic tubing harnessed to the participant provided a horizontal force equal to mass multiplied by acceleration. Net mechanical work ( W COM ) and ground reaction force impulses ( I GRF ) were calculated for individual steps and a linear regression was performed with these experimental measures as independent variables and theoretically derived values of work and impulse as predictor variables. For EA, linear fits were significant for W COM ( y =1.19 x +10.5, P <0.001, R 2 =0.41) and I GRF ( y =0.95 x +8.1, P <0.001, R 2 =0.3). For TT, linear fits were not significant and explained virtually no variance for W COM ( y =0.06 x +1.6, P =0.29, R 2 <0.01) and I GRF ( y =0.10 x +0.4, P =0.06, R 2 =0.01). This suggested that the EA condition was a better representation of real acceleration dynamics than TT. Running steps from EA where work and impulse closely matched theoretical values showed similar adaptations to increasing acceleration as have been previously observed overground (forward reorientation of GRF vector without an increase in magnitude or change in spatio-temporal metrics). [ABSTRACT FROM AUTHOR]

Abstract: Humans use equal push-off and heel strike work during the double support phase to minimize the mechanical work done on the center of mass (CoM) during the gait. Recently, a step-to-step transition was reported to occur over a period of time greater than that of the double support phase, which brings into question whether the energetic optimality is sensitive to the definition of the step-to-step transition. To answer this question, the ground reaction forces (GRFs) of seven normal human subjects walking at four different speeds (1.1–2.4m/s) were measured, and the push-off and heel strike work for three differently defined step-to-step transitions were computed based on the force, work, and velocity. To examine the optimality of the work and the impulse data, a hybrid theoretical-empirical analysis is presented using a dynamic walking model that allows finite time for step-to-step transitions and incorporates the effects of gravity within this period. The changes in the work and impulse were examined parametrically across a range of speeds. The results showed that the push-off work on the CoM was well balanced by the heel strike work for all three definitions of the step-to-step transition. The impulse data were well matched by the optimal impulse predictions (R 2>0.7) that minimized the mechanical work done on the CoM during the gait. The results suggest that the balance of push-off and heel strike energy is a consistent property arising from the overall gait dynamics, which implies an inherited oscillatory behavior of the CoM, possibly by spring-like leg mechanics. [Copyright &y& Elsevier]

Abstract: The simplest walking model, which assumes an instantaneous collision with negligible gravity effect, is limited in its representation of the collision mechanics of human gaits because the actual step-to-step transition occurs over a finite duration of time with finite impulsive ground reaction forces (GRFs) that have the same order of magnitude as the gravitational force. In this study, we propose a new collision model that includes the contribution of the gravitational impulse to the momentum change of the center of mass (COM) during a step-to-step transition. To validate the model, we measured the GRFs of six subjects’ over-ground walking at five different gait speeds and calculated the collision impulses and mechanical work. The data showed a significant contribution of the gravitational impulse to the momentum change during collision. To compensate for the gravity, the magnitudes of collision impulse and COM work were estimated to be much greater than in previous predictions. Consistent with the model prediction, push-off propulsion fully compensated for the collision loss, implying the step-to-step transition occurred in an energetically optimal manner. The new model predicted a moderate change in the collision mechanics with gait speed, which seems to be physiologically achievable. The gravitational collision model enables us to better understand collision dynamics during a step-to-step transition. [ABSTRACT FROM AUTHOR]

Abstract: Stresses in a prosthetic heart valve at closure are determined by its geometrical and structural characteristics, by the mechanical support environment, and by the momentum of the valve leaflets or occluder and of the blood at the instant of closure. The mass of blood to be arrested is significantly greater than that of the leaflets or occluder, and is therefore likely to dominate the closure impulse. The kinetic energy of the blood must be transduced into potential energy in the structural components (valve leaflets, aortic root and aorta). This paper presents a methodology for computation and parameterisation of the blood momentum associated with a valve in the aortic position. It is suggested that the influence of physiological parameters, such as systolic waveform and systemic impedance, on the closure characteristics can be investigated based on the fluid dynamic implications. Detailed results are presented for a single leaflet mechanical valve (Bjork-Shiley 60° Convexo-Concave). It is demonstrated that a simple analytical method can yield results that might be adequate for the purposes of valve design. [Copyright &y& Elsevier]

The orientation of the ground reaction force (GRF) vector is a key determinant of human sprint acceleration performance and has been described using ratio of forces (RF) which quantifies the ratio of the antero-posterior component to the resultant GRF. Different methods have previously been used to calculate step-averaged RF, and this study therefore aimed to compare the effects of three calculation methods on two key "technical" ability measures: decline in ratio of forces (D RF) and theoretical maximal RF at null velocity (RF 0). Twenty-four male sprinters completed maximal effort 60 m sprints from block and standing starts on a fully instrumented track (force platforms in series). RF-horizontal velocity profiles were determined from the measured GRFs over the entire acceleration phase using three different calculation methods for obtaining an RF value for each step: A) the mean of instantaneous RF during stance, B) the step-averaged antero-posterior component divided by the step-averaged resultant GRF, C) the step-averaged antero-posterior component divided by the resultant of the step-averaged antero-posterior and vertical components. Method A led to significantly greater RF 0 and shallower D RF slopes than Methods B and C. These differences were very large (Effect size Cohen's d = 2.06 – 4.04) and varied between individuals due to differences in the GRF profiles, particularly during late stance as the acceleration phase progressed. Method B provides RF values which most closely approximate the mechanical reality of step averaged accelerations progressively approaching zero and it is recommended for future analyses although it should be considered a ratio of impulses. [ABSTRACT FROM AUTHOR]