The mechanical role of the trunk and lower extremities in a seated weight-moving task in the sagittal plane

Reaching in sitting: The effect of seat design and body manipulations

BACKGROUND

Sitting is a common and familiar position used daily as a platform for many motor activities in the workplace, at school, or at home.

OBJECTIVE

To investigate how difference in the chair design and selected sitting manipulations contribute to reach distance in sitting.

METHODS

Ten healthy subjects were required to reach forward as far as possible while sitting in an adjustable chair with 0°, 10° forward or 10° backward inclination of the seat, with and without footrest and leg support, with legs crossed, and when holding the edge of the seat with the contralateral arm.

RESULTS

In comparison to sitting with feet on the footrest, the maximal reaching distance decreased significantly when sitting on either forward or backward inclined seat (p <  0.05) and it increased when the subjects held the edge of the seat while seated with footrest and the posterior leg support (p <  0.05). There was no major effect of crossing the legs or the use of anterior leg support on the maximal reach distance.

CONCLUSIONS

Modification of the chair design could increase or decrease reaching distance in sitting. The outcome of the study provides a background for future investigations of the effect of sitting positions on reaching distance in the workplace, at home, or at school.

Predicting independence of gait by assessing sitting balance through sitting posturography in patients with subacute hemiplegic stroke

BACKGROUND

Post-stroke sitting balance is a known predictor of independence of gait after stroke. However, previous studies used only qualitative scales or measured static or dynamic sitting balance alone.

OBJECTIVES

To investigate whether quantitative parameters of sitting posturography at post-stroke 1 month can predict independent gait.

METHODS

In this prospective cohort study, we enrolled patients with first-ever stroke who could hold a sitting posture at post-stroke 1 month. Sitting balance was assessed using posturography at post-stroke 1 month. Independence of gait was assessed using functional ambulation categories at post-stroke 2 months. We predicted mobility independence at post-stroke 2 months according to sitting balance at post-stroke 1 month. We also assessed the correlation between sitting posturography parameters and clinical scales.

RESULTS

We enrolled 27 patients. The limit of stability deviation predicted independent gait at post stroke 2 months (cutoff, 78.4%). Further, there was a high degree of correlation between sitting posturography parameters (weight-bearing distribution deviation and limit of stability deviation) and Berg Balance Scale (ρ = 0.763, ρ = 0.777; p < .001, respectively), Scale for Assessment and Rating of Ataxia (ρ = -0.853, ρ = -0.929; p < .001, respectively), and Fugl-Meyer Assessment scale (upper extremities: ρ = 0.520, ρ = 0.480 [p = .005, p = .011, respectively]; lower extremities: ρ = 0.744, ρ = 0.564 [p < .001, p = .002, respectively]) scores.

CONCLUSIONS

Sitting posturography parameters is clinically useful because they can quantitatively assess post-stroke balance and neurological impairment and predict post-stroke independence of gait even when patients cannot reach their arms forward or stand upright.

A NEW STABILITY CRITERION FOR HUMAN SEATED TASKS WITH GIVEN POSTURES

In digital human modeling (DHM), the analysis of postural stability has five main goals: to determine if a posture is stable or unstable through an explicit criterion; to quantify the level of stability or provide a margin of stability that accounts for the height of the center of mass (COM) above the support plane(s); to be valid in the presence of externally applied forces and moments; be able to assess stability when multiple noncoplanar support planes exist, as is the case with seated postures; and to give insight into the support reaction force (SRF) distribution. To date, there is not a method for analyzing stability that can effectively meet each goal. This paper presents a new stability criterion and stability analysis that accomplishes each intended goal. The stability analysis is derived from the calculation of joint torque using the recursive Lagrangian dynamic formulation. A 56-degree-of-freedom (DOF) articulated digital human model is used to model seated postures to demonstrate the proposed stability criterion. Different given postures with different external load cases are presented.

Physics-Based Seated Posture Prediction for Pregnant Women and Validation Considering Ground and Seat Pan Contacts

An understanding of human seated posture is important across many fields of scientific research. Certain demographics, such as pregnant women, have special postural limitations that need to be considered. Physics-based posture prediction is a tool in which seated postures can be quickly and thoroughly analyzed, as long the predicted postures are realistic. This paper proposes and validates an optimization formulation to predict seated posture for pregnant women considering ground and seat pan contacts. For the optimization formulation, the design variables are joint angles (posture); the cost function is dependent on joint torques. Constraints include joint limits, joint torque limits, the distances from the end-effectors to target points, and self-collision avoidance constraints. Three different joint torque cost functions have been investigated to account for the special postural characteristics of pregnant women and consider the support reaction forces (SRFs) associated with seated posture. Postures are predicted for three different reaching tasks in common reaching directions using each of the objective function formulations. The predicted postures are validated against experimental postures obtained using motion capture. A linear regression analysis was used to evaluate the validity of the predicted postures and was the criteria for comparison between the different objective functions. A 56 degree of freedom model was used for the posture prediction. Use of the objective function minimizing the maximum normalized joint torque provided an R2 value of 0.828, proving superior to either of two alternative functions.

Does postural chain mobility influence muscular control in sitting ramp pushes?

This study was conducted under the hypothesis that voluntary movement involves a perturbation of body balance and that a counter-perturbation has to be developed to limit the perturbation effects, which is a condition necessary to perform the movement efficiently. The stabilising action is produced in body segments that constitute the "postural" chain, and the voluntary movement by the segments said to constitute the "focal" chain. In order to deepen the understanding of how the postural chain contributes to the motor act, isometric transient efforts were considered. Seven adults in a sitting posture were instructed to exert bilateral horizontal pushes on a dynamometric bar, as rapidly as possible, up to their maximal force (Fx). Two sitting conditions were considered: full ischio-femoral contact (100 BP) and one-third ischio-femoral contact (30 BP), the latter being known to yield greater pelvis and spine mobility, that is greater postural mobility. Each session consisted of ten maximal pushes for each sitting condition. In order to explore the influence of postural mobility on muscular control and push force, surface EMGs of 14 postural and focal muscles were recorded. In addition, reaction forces (Rx) and displacement (Xp) of the centre of pressure (along the anteroposterior axis) were measured, as well as iliac crest acceleration (xh and zh, along the anteroposterior and vertical axes, respectively). The results showed that push force varied abruptly during the task ramp effort. When the ischio-femoral contact was limited, push force was enhanced, as well as the rate of push force rise (Fx/Deltat, Deltat being the force rise duration), suggesting a greater perturbation to balance. Also, there were significant increases in the Rx reaction forces, indicating body segment acceleration: "dynamic" phenomena occurred in the articulated body chain in response to increases in Fx. In addition, even though muscular contraction was isometric, postural EMGs, as well as focal EMGs, were phasic, a feature which characterises transient force exertion. The Rx reaction forces were associated with backward displacement of the centre of pressure, Xp. The centre of pressure displacement was interpreted as a backward pelvis rotation, an interpretation which was confirmed by backward and upward iliac crest accelerations. When ischio-femoral contact was reduced, the backward pelvis rotation was significantly increased, resulting from an increased pelvis and spine mobility. Distinct focal and postural EMG sequences were found to be associated with the effort. Two different sets of muscles were observed when considering recruitment order, the focal and the postural muscles. The ankle muscles were activated before the pelvis, the back and the scapular girdle, with the upper limb muscles activated only after the onset of the primum movens of push action (serratus anterior): the activation process followed a distal to proximal progression order. Moreover, the postural EMG sequence was anticipatory, that is there were anticipatory postural adjustments (APAs). Modifying the ischio-femoral contact did not induce a change in either the postural muscle set or in the recruitment order. There were significant increases in the level of activation (integrated EMG) of the postural muscles when ischio-femoral contact was reduced. They did not result from an increase in EMG duration but only from a modulation of EMG amplitude, suggesting that postural control for different ischio-femoral contacts involves adapting the motor program according to the postural requirements, rather than changing the postural strategy. Moreover, as APA amplitude was increased when ischio-femoral contact was reduced, it could be assumed that the postural chain is programmed in relation to postural chain mobility. In addition, the increase in postural EMGs was interpreted as an increased counter-perturbation opposed to an increased push force. It is concluded that greater mobility of the postural chain favours a greater dynamic counter-perturbat chain favours a greater dynamic counter-perturbation, which, in turn, allows the development of a greater push force; the ability to develop such a counter-perturbation (termed PKC: posturo-kinetic capacity) is enhanced when postural chain mobility is greater. Postural chain mobility appears to be a task parameter, and postural control appears to involve adapting the motor program according to the postural requirements, rather than changing the postural strategy.

Kinematic modeling for the assessment of wheelchair user's stability

A computer kinematic model was developed to simulate the lateral and transverse stabilities of wheelchair users in order to compare the effect of different backrests. This model is composed of ellipsoids and parallelepipeds representing the main components of the human body, the seating devices and the wheelchair. A fifteen-segment three-dimensional (3-D) model linked by spherical and revolute joints was created using the ADAMS software (Mechanical Dynamics, Inc.). Torsional springs and dampers are used at the joints to represent four sets of articulation stiffness. Seating devices are represented with 45 rectangular surface patches. The interface between human body and seating devices is modeled by contact elements, which included the specification of stiffness, damping, and deformation of cushions and buttocks. Simulations of a user and his wheelchair moving at 1.4 m/s on a tilted pathway were performed. Different indexes [trunk lateral tilt (TLT) and trunk transverse rotation (TTR)] were measured and compared to those of a similar experimental study on four subjects. The effect of joint stiffness was quantified and a sensitivity study showed the importance of the hip, neck, lumbar, and thoracic joint stiffness on model response (between 16% and 68%). Two backrests (standard and highly contoured) were tested with the kinematic model and their stability compared. Overall, the coherence between the simulations and the experiments shows that this approach is appropriate to compare various seating devices (maximal difference of 1.3 degrees between the simulated and experimental curves for the intermediate joint stiffness sets). The smallest rotations of the highly contoured backrest (6.3 degrees versus 8.9 degrees for TLT and 3.9 degrees versus 6.7 degrees for TTR) suggest that the contouring of the mid torso is more efficient than the lower torso to provide stability to the wheelchair user. This model is an adequate tool to test and improve the design of seating aids.

Simulation of the Seated Postural Stability of Healthy and Spinal Cord-Injured Subjects Using Optimal Feedback Control Methods

A two-dimensional, biomechanical computer model was developed, using the software package Working Model(TM), to simulate the postural control of seated individuals. Both able-bodied and spinal cord-injured subjects were represented. The model incorporated active control of the upper body through full-state feedback. Specifically, a linear quadratic regulator scheme was implemented in the model. Nonlinearities were included in the torque computations to mimic physiological constraints and disability. Interactions between the subject and the wheelchair were also included in the model. Simulation results were compared with those obtained from experiments in which the subjects had attempted to remain stable during the application of significant disturbance moments, similar to those experienced during braking in a vehicle. While subjects exhibited more complex control schemes, the model was able to simulate overall stability. Therefore, it is believed that the model could prove beneficial to future research examining the effects of various restraints on stability.

Effect of seat cushion on dynamic stability in sitting during a reaching task in wheelchair users with paraplegia

OBJECTIVES

To examine the effects of seat cushions on dynamic stability in sitting during a controlled reaching task by wheelchair users with paraplegia.

DESIGN

A randomized, controlled test.

SETTING

Rehabilitation center.

PARTICIPANTS

Nine wheelchair users with paraplegia.

INTERVENTIONS

Three types of cushions--an air flotation, a generic contoured, and a flat polyurethane foam--were tested during a controlled reaching task in ipsilateral and contralateral directions, at 45 degrees from the sagittal plane in the anterolateral direction. Center of pressure (COP) coordinates were monitored by using a pressure measurement system as well as a force platform under seat.

MAIN OUTCOME MEASURES

Trajectory of COP, maximal distance covered by COP, maximal velocity of COP; and the index of asymmetry between right and left maximal pressure under ischial tuberosities.

RESULTS

The generic contoured cushion allowed the COP to cover significantly (p <.02) a larger distance (81 +/- 28mm) when compared with the air flotation (63 +/- 25mm) or the flat foam (61 +/- 29mm) cushions. The COP velocity was significant (p <.05) for the generic contoured cushion (.14 +/-.05m/s) versus the air flotation (.10 +/-.04m/s) or the flat-foam (.10 +/-.03m/s) cushions. The index of asymmetry was higher for the generic contoured and the flat foam cushions. During reaching, maximal pressure under ipsilateral ischial tuberosity was significantly higher for the flat foam (275 +/- 70mmHg) and the generic contoured (235 +/- 81mmHg) cushions, when compared with the air flotation cushion (143 +/- 51mmHg).

CONCLUSION

Seat cushions can significantly affect sitting balance during reaching tasks. This study provided an objective method to assess the dynamic stability of wheelchair users when they perform activities of daily living requiring reaching. These findings have implications for wheelchair seating recommendations, especially seat cushion selection.

Knee joint motion: description and measurement

Knee joint motion has been described in various ways in the literature. These are explained and commented on. Two methods for describing knee joint motion with 6 degrees of freedom (DOF)--Euler angle and the helical axis of motion--are discussed. Techniques to measure joint motion which can either approximate the motion to less than 6 DOF or fully measure the spatial motion are identified. These include electrical linkage methods, radiographic and video techniques, fluoroscopic techniques and electromagnetic devices. In those cases where the full spatial motion is measured, the data are available to describe the motion in simpler terms (or with less DOF) than three rotations with three translations. This is necessary for clinical application and to facilitate communication between the clinician and the engineer.

Task-related training improves performance of seated reaching tasks after stroke. A randomized controlled trial

BACKGROUND AND PURPOSE

After stroke, the ability to balance in sitting is critical to independence. Although impairments in sitting balance are common, little is known about the effectiveness of rehabilitation strategies designed to improve it. The purpose of this randomized placebo-controlled study was to evaluate the effect of a 2-week task-related training program aimed at increasing distance reached and the contribution of the affected lower leg to support and balance.

METHODS

Twenty subjects at least 1 year after stroke were randomized into an experimental or control group. The experimental group participated in a standardized training program involving practice of reaching beyond arm's length. The control group received sham training involving completion of cognitive-manipulative tasks within arm's length. Performance of reaching in sitting was measured before and after training using electromyography, videotaping, and two force plates. Variables tested were movement time, distance reached, vertical ground reaction forces through the feet, and muscle activity. Subjects were also tested on sit-to-stand, walking, and cognitive tasks. Nineteen subjects completed the study.

RESULTS

After training, experimental subjects were able to reach faster and further, increase load through the affected foot, and increase activation of affected leg muscles compared with the control group (P < .01). The experimental group also improved in sit-to-stand. The control group did not improve in reaching or sit-to-stand. Neither group improved in walking.

CONCLUSIONS

This study provides strong evidence of the efficacy of task-related motor training in improving the ability to balance during seated reaching activities after stroke.