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Joan E. Deffeyes, Regina T. Harbourne, Wayne A. Stuberg, and Nicholas Stergiou

Sitting is one of the first developmental milestones that an infant achieves. Thus measurements of sitting posture present an opportunity to assess sensorimotor development at a young age. Sitting postural sway data were collected using a force plate, and the data were used to train a neural network controller of a model of sitting posture. The trained networks were then probed for sensitivity to position, velocity, and acceleration information at various time delays. Infants with typical development developed a higher reliance on velocity information in control in the anterior-posterior axis, and used more types of information in control in the medial-lateral axis. Infants with delayed development, where the developmental delay was due to cerebral palsy for most of the infants in the study, did not develop this reliance on velocity information, and had less reliance on short latency control mechanisms compared with infants with typical development.

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Juan C. Chicote, Juan V. Durá, Juan M. Belda, and Rakel Poveda

Principal component analysis and functional regression are combined in a model to analyze a time series of pressure maps. The model is tested measuring the pressures over a chair seat while a subject performs a combination of simple movements. A sampling rate of 3 Hz is adequate for applying the model in sitting postures. The model is able to detect patterns of movement over time, although more variables are necessary if the movements produce similar pressure distributions.

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N. Genthon and P. Rougier

This study was aimed at assessing the possible relationship between upright and sitting postures in healthy adults. The center of pressure trajectories from the force platform on which the subjects stood or sat were analyzed through a frequency analysis and modeled through the fractional Brownian motion framework. The same type of control process was involved during sitting and upright posture maintenance. Both upright and sitting posture would be controlled by the same mechanical law and/or by the same type of central process. Conversely, these two postures presented different characteristics and a relative independence. Both postures displayed specific biomechanical constraints and involved specific effectors, in particular along the anterior-posterior axis. Thus, performances in these two postures are completely independent in the anterior-posterior axis, whereas they are slightly linked in the medio-lateral axis. Improved trunk functions to improve postural stability have an interest solely to improve lateral stabilization.

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Jørgen Skotte, Mette Korshøj, Jesper Kristiansen, Christiana Hanisch, and Andreas Holtermann

Background:

The aim of this study was to validate a triaxial accelerometer setup for identifying everyday physical activity types (ie, sitting, standing, walking, walking stairs, running, and cycling).

Methods:

Seventeen subjects equipped with triaxial accelerometers (ActiGraph GT3X+) at the thigh and hip carried out a standardized test procedure including walking, running, cycling, walking stairs, sitting, and standing still. A method was developed (Acti4) to discriminate between these physical activity types based on threshold values of standard deviation of acceleration and the derived inclination. Moreover, the ability of the accelerometer placed at the thigh to detect sitting posture was separately validated during free living by comparison with recordings of pressure sensors in the hip pockets.

Results:

Sensitivity for discriminating between the physical activity types sitting, standing, walking, running, and cycling in the standardized trials were 99%–100% and 95% for walking stairs. Specificity was higher than 99% for all activities. During free living (140 hours of measurements), sensitivity and specificity for detection of sitting posture were 98% and 93%, respectively.

Conclusion:

The developed method for detecting physical activity types showed a high sensitivity and specificity for sitting, standing, walking, running, walking stairs, and cycling in a standardized setting and for sitting posture during free living.

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Kaitlin M. Gallagher, Anita N. Vasavada, Leah Fischer, and Ethan C. Douglas

A popular posture for using wireless technology is reclined sitting, with the trunk rotated posteriorly to the hips. This position decreases the head’s gravitational moment; however, the head angle relative to the trunk is similar to that of upright sitting when using a tablet in the lap. This study compared cervical extensor musculotendon length changes from neutral among 3 common sitting postures and maximum neck flexion while using a tablet. Twenty-one participants had radiographs taken in neutral, full-flexion, and upright, semireclined, and reclined postures with a tablet in their lap. A biomechanical model was used to calculate subject-specific normalized musculotendon lengths for 27 cervical musculotendon segments. The lower cervical spine was more flexed during reclined sitting, but the skull was more flexed during upright sitting. Normalized musculotendon length increased in the reclined compared with an upright sitting position for the C4-C6/7 (deep) and C2-C6/7 (superficial) multifidi, semispinalis cervicis (C2-C7), and splenius capitis (Skull-C7). The suboccipital (R 2 = .19–.71) and semispinalis capitis segment length changes were significantly correlated with the Skull-C1 angle (0.24–0.51). A semireclined reading position may be an ideal sitting posture to reduce the head’s gravitational moment arm without overstretching the assessed muscles.

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Nicholas Stergiou, Yawen Yu, and Anastasia Kyvelidou

Movement variability is considered essential to typical motor development. However, multiple theoretical perspectives and measurement tools have limited interpretation of the importance of movement variability in biological systems. The complementary use of linear and nonlinear measures have recently allowed for the evaluation of not only the magnitude of variability but also the temporal structure of variability. As a result, the theoretical model of optimal movement variability was introduced. The model suggests that the development of healthy and highly adaptable systems relies on the achievement of an optimal state of variability. Alternatively, abnormal development may be characterized by a narrow range of behaviors, some of which may be rigid, inflexible, and highly predictable or, on the contrary, random, unfocused, and unpredictable. In the present review, this theoretical model is described as it relates to motor development in infancy and specifically the development of sitting posture.

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Robert James

This article reviews the role of augmented biofeedback as a treatment aid for selected neuromuscular problems in children and adolescents with cerebral palsy. Neuromuscular dysfunction often prevents those inflicted with cerebral palsy from performing even simple tasks of daily activity. Research has evaluated the role of augmented biofeedback in reducing this neuromuscular dysfunction. Augmented feedback, on the whole, has been successful in improving head and neck posture, reducing hypertonicity, and improving weight-bearing during gait, hand-eye coordination, sitting posture, and drooling. However, most studies have shown that the carry-over without feedback was limited. Moreover, the generalization to real-life situations often was not demonstrated. The sample size in most studies was very small. Future research should address the adequate number of training sessions needed to produce an improvement and consider the mode and type of feedback appropriate for a given subject. Augmented biofeedback appears to have important implications in the treatment of those with cerebral palsy.

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André Luiz Felix Rodacki, Neil Edward Fowler, and Simon Bennett

The aim of this study was to compare the kinematic pattern and the segmental movement co-ordination when the trunk segment was constrained in different positions during plyometric rebound jumps. Nine skilled volleyball players, experienced in plyometric training, were asked to perform a random series of maximal rebound jumps, using three different seat arrangements (90°, 135°, and 180°) in a pendulum swing device. From two-dimensional filming, performed in a right sagittal plane at 200 Hz, it was possible to calculate ankle, knee, and hip displacements; velocities; and muscle-tendon lengths. The subjects showed similar ankle and knee angles between experimental conditions. The hip joint angle differed significantly between conditions. Only the muscle-tendon lengths of the biarticular muscles spanning the knee/hip were affected by the seat arrangement variations. Significantly greater knee angular velocities were observed in the upright sitting posture (90°). The hip was consistently the first joint to extend. The ankle and knee joint reversals were not invariant, regardless of the seat arrangement. The movement co-ordination strategy did not differ across postural variations.

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T. Adam Thrasher, Vivian W. Sin, Kei Masani, Albert H. Vette, B. Cathy Craven, and Milos R. Popovic

Understanding how the human body responds to unexpected force perturbations during quiet sitting is important to the science of motor behavior and the design of neuroprostheses for sitting posture. In this study, the performance characteristics of the neck and trunk in healthy individuals were assessed by measuring the kinematic responses to sudden, unexpected force perturbations applied to the thorax. Perturbations were applied in eight horizontal directions. It was hypothesized that displacement of the trunk, settling time and steady-state error would increase when the perturbation direction was diagonal (i.e., anterior-lateral or posterior-lateral) due to the increased complexity of asymmetrical muscle responses. Perturbation forces were applied manually. The neck and trunk responded in a synchronized manner in which all joints achieved peak displacement simultaneously then returned directly to equilibrium. Displacement in the direction of perturbation and perpendicular to the direction of perturbation were both significantly greater in response to diagonal perturbations (p < .001). The center of mass returned to equilibrium in 3.64 ± 1.42 s after the onset of perturbation. Our results suggest that the trunk sometimes behaves like an underdamped oscillator and is not controlled by simple stiffness when subjected to loads of approximately 200 N. The results of this study are intended to be used to develop a neuroprosthesis for artificial control of trunk stability in individuals with spinal cord injury.

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Pascal Madeleine

This laboratory study investigated seated computer work before and after prolonged constrained sitting. Discomfort ratings and kinetic and kinematics data were recorded in nine healthy males performing computer work for 5 min before and after 96 min of sitting. The displacement of the center of pressure (CoP) in anterior-posterior and medial-lateral directions and lumbar curvature (LC) were calculated. The root mean square, standard deviation, and sample entropy values were computed from the CoPs and LC signals to assess the magnitude, amount of variability, and regularity of sitting dynamics, respectively. The discomfort increased for the buttocks (p = .02).The standard deviation and sample entropy values of the CoPs and LC signals, respectively, increased (p < .04) and decreased (p < .004) whereas the root mean square remained unchanged (p > .15) after prolonged constrained sitting compared with before. This present study showed that during seated computer work, prolonged constrained sitting affected the amount of variability and the regularity of sitting postural control, whereas the magnitude was not affected. The importance of the dynamics of sitting control may challenge the idea of a static and ideal seated posture at work.