The balance control effects on sitting posture induced by lumbosacral orthosis wear vary depending on the level of stability.

This study aimed to assess the differential impacts of lumbosacral orthosis (LO) wear in different sitting conditions through posturographic measurements. Twelve healthy subjects sat on a force platform with three variable stability levels (stable and on seesaws with a long and short radius, inferring slightly and highly unstable sitting, respectively) and three orthosis conditions (no LO, neutral LO, lordotic LO). Using fractional Brownian motion modelling of the centre of pressure (CoP) displacements, it appears that a stable sitting position did not highlight any particular differences between the LO models. With the lordotic LO, a slightly unstable sitting position decreased the mean time by 72% (p < 0.002) before postural corrective mechanisms took over. In contrast, in highly unstable sitting conditions, the lordotic LO induced larger CoP displacements (increasing variance by 162%, p < 0.038). Thus, depending on the amount of perturbation and the device design, wearing an LO may have a neutral, positive or negative impact on postural control in the sitting position.

Biomechanical analysis of postural strategies over the first two months following anterior cruciate ligament reconstruction.

To assess the postural strategies developed over the first 2 months following surgery by ACL patients during rehabilitation and highlight the sensory-motor impairment recovery, 21 patients were measured at three timeframes. Three two-legged standing conditions were assessed: with the eyes open, with the eyes closed either wearing or not wearing a knee orthosis. The results indicate that the weight-bearing asymmetry, initially observed (i.e., 56-44% of body-weight), disappeared progressively during rehabilitation (51-49%). The comparison of the plantar center-of-pressure displacements under both sound and operated legs demonstrated noticeable differences that also tended to decrease but without reaching a matched behavior during the last measures. These effects were seen in both eyes open and eyes closed conditions with the greatest effects in the latter condition. Wearing a knee orthosis inferred no particular changes in the postural control behaviors. These data could be used as benchmarks for highlighting the effects on undisturbed postural control of various surgery techniques and/or rehabilitation protocols.

Estimation of centre of gravity movements in sitting posture: application to trunk backward tilt.

The aim of this study was to highlight, in sitting posture, the value of distinguishing between the movements of the vertical projection of the centre of gravity (CG(v)) and its difference from the centre of pressure (CP-CG(v)). A protocol for healthy, young, trained adults, consisting in tilting their trunk backward or keeping it vertical was used. A frequency analysis shows that statistically significant effects were only seen on CP-CG(v) movements: the RMS increased by 37% (p = 0.004), while the MPF decreased by 5% (p = 0.016), suggesting an increased muscular activity in these tilting postures. In contrast, no statistically significant effects on CP and CG(v) were reported. These data highlight the advantage, in sitting posture, of splitting overall CP displacements into basic components (i.e. CG(v) and CP-CG(v)), each of them having a biomechanical significance.

Short-term effects on postural control can be evidenced using a seesaw.

Postural control strategies are frequently assessed through posturography on a firm surface. Their motor efficiency is related to the coordination between center-of-gravity (CG) and center-of-pressure (CP) movements, which results from long-term training. However, when standing on a seesaw favoring pitching body motions, a new coordination, requiring short-term training, needs to be learned again. On this type of device, somesthetic cues from the ankle joints become invalid and motor command is amplified because of the curvature of the contact ridges. To highlight the mechanisms involved in short-term improvements of postural control, 11 healthy young adults were trained for 20 min by standing on a seesaw with their eyes closed. Two series of posturographic measurements, before and after the training, were recorded whilst the subjects stood on the seesaw with eyes closed. The results indicate a reduction in the horizontal CG displacements along the anteroposterior axis and CP-CG displacements along both anteroposterior and mediolateral axes. Fractional Brownian motion (fBm) analysis further explains these lessened CG movements by highlighting a reduced distance covered before the corrective mechanisms take over. The other fBm parameters, the contribution of stochastic activity during the shortest and longest time intervals or the time interval of the transition points (which expresses the mean delay before the corrective mechanisms take over), remained unchanged. These results could likely be explained by reduced tonic muscular activity of the lower limbs and/or recalibration of the sensory systems in order to improve the detection of the CG movements.

In healthy subjects, the sitting position can be used to validate the postural effects induced by wearing a lumbar lordosis brace.

AIM OF THE STUDY: To assess the validity of the sitting position when testing lumbar braces for the maintenance of lordosis. PATIENTS AND METHODS: Twelve young adult subjects participated in the experiment, in which they were seated on force platform. The four experimental conditions (with or without a brace and with or without enforced lordosis) were chosen in order to distinguish between the roles played by lordosis and the brace, respectively. The trajectories of the centre of pressure (CP) were analyzed and compared, in order to assess postural orientation and stabilisation processes. RESULTS: Although no effect was seen in terms of orientation, our data showed that use of a lumbar brace led to a notable reduction in CP displacement along the mediolateral and anteroposterior axes. Lordosis barely affected postural performance and only an increase in the mean CP velocity was observed. Lastly, an analysis of variance failed to reveal an interaction between the "lordosis" and "brace" factors. CONCLUSION: A lumbar brace (in the absence or presence of lordosis) helps subjects to improve their sitting performance. In contrast to previous studies based on the standing posture, the fact that significant differences were found as a function of brace wear emphasises the discriminant power of the sitting position. This task should therefore be applied more widely in the development of more appropriate, validated equipment for lower back pain sufferers.

Lateral ankle sprain alters postural control in bipedal stance--part 1: restoration over the 30 days following the injury.

The time evolution of the postural behavior of 23 lateral ankle sprain patients (degrees I and II) were evaluated 14 h and 10 and 30 days on average after their injury and compared with those of 30 age-matched healthy individuals. The patients were tested with separate measurements of the reaction forces under each limb to highlight the possible compensatory mechanisms between the sound and the injured legs. Their postural behavior in bipedal stance was characterised by a weight-bearing asymmetry with more weight on the sound leg and an asymmetry of the postural stabilisation mechanisms, which are limited and perturbed under the injured leg. Pain appears to be the main factor for explaining these postural asymmetries. Despite these asymmetries, the patients were nonetheless more unstable than the individuals constituting the group control. Ten days later, only the weight-bearing asymmetry was still observed whereas 30 days later, the postural behavior was totally normal once again. Lateral ankle sprain perturbs the contribution of the injured leg in postural stabilisation, inducing a larger involvement of the sound leg in the postural stability process. These characteristics are largely reduced 10 days after the injury.

Lateral ankle sprain alters postural control in bipedal stance: part 2 sensorial and mechanical effects induced by wearing an ankle orthosis.

To investigate the effects induced by wearing an orthosis during the rehabilitation process, 23 ankle sprain patients (degrees I and II) were evaluated in three conditions (reference, with an elastic compression stocking and with an orthosis), 14 h, 10 and 30 days on average after their injury and compared with those of 30 age-matched healthy individuals. The patients were tested with separate measurements of the reaction forces under each limb to highlight the possible compensatory mechanisms between the sound and the injured legs. Their postural stability was enhanced during unilateral orthosis wear, explained by a bilateral effect involving both feet. Wearing a compression stocking induced comparably mild intermediate effects compared with the effects observed with the orthosis. These effects were constant throughout the next month. Following lateral ankle sprain, wearing an orthosis allows patients to improve postural function a few hours after the injury to 1 month later. Only cutaneous pressure intervening without mechanical maintenance induced mild effects, indicating that orthosis effects on postural control could partly result from its sensorial stimulation. It, therefore, seems relevant to prescribe orthosis wear for at least 1 month.

Wearing shoes increasing dorsiflexion improves short-term but not long-term balance control in young healthy adults.

Wearing sport shoes inducing ankle dorsiflexion has been shown to alter the biomechanical specificities of the stretched muscles. The possible effect over the short and long term upon the sensorial capacities induced by such stretching has not been addressed yet. Fourteen healthy individuals were involved to assess the proprioceptive repercussion and their effects upon postural control strategies. Postural control and proprioceptive assessment were measured twice: when receiving sport shoes inducing ankle dorsiflexion and 18 days later. Proprioceptive effects were assessed using an ad-hoc device through which the seated and blindfolded subjects were required to reposition their feet in a starting position after the ankles were passively displaced to dorsiflexed and plantarflexed positions. Center-of-gravity horizontal displacements (CG(v)), estimated from center-of-pressure (CP) displacements, and CP-CG(v) displacements were measured through a force platform during upright quiet stance maintenance. The initial session was recorded with the subjects barefoot and wearing the shoes with a set of chocks with 0 degrees (horizontal) and -5 degrees (dorsiflexion) tilting angles. The second session included only barefoot performance in horizontal and dorsiflexion conditions. Dorsiflexion had no immediate effect on the postural control strategies along the anteroposterior axis. In contrast, barefoot or wearing shoes, stability was increased along the mediolateral axis during the dorsiflexion conditions. No ankle proprioceptive or postural change was observed after wearing the shoes for 18 days. Wearing dorsiflexion sport shoes induces short-term effects probably by inducing a backward tilt of the pelvis. A muscular adaptation likely prevents this effect from being prolonged.

Biomechanical analysis of postural control of persons with transtibial or transfemoral amputation.

OBJECTIVE: In human subjects, leg amputation impairs upright quiet-stance control. The inability to exert appropriate force reactions under the amputated leg and a slight weight-bearing asymmetry cause the amputee to develop compensatory mechanisms through the sound leg. This study is aimed at assessing these mechanisms and the influence of the level of amputation. DESIGN: The postural strategies of transtibial and transfemoral amputees for maintaining quiet stance were investigated using a dual-force platform. With this device, mean body weight distribution and plantar and resultant center of pressure trajectories can be measured and used to assess the likely compensatory mechanisms. RESULTS: Compensatory strategies were found in both transtibial and transfemoral groups, with a forward shift of the mean positions of the center of pressure under the amputated leg and greater resultant center of pressure displacements appearing along the anteroposterior axis, respectively. The transfemoral group presents a larger center of pressure displacements under the amputated leg than the transtibial group. CONCLUSION: These data emphasize the role played by the differences between the center-of-pressure magnitudes intervening under the sound and amputated legs and the size of the prosthesis-stump contact area in these postural strategies.

Dynamical assessment of weight-bearing asymmetry during upright quiet stance in humans.

Distributing asymmetrically the body weight (BW) has been previously described through static parameters such as the mean position of the resultant centre of pressure (CP(Res)) along the medio-lateral axis or mean BW distribution on the two legs. However, neither the respective dynamic contribution of each leg in the CP(Res) displacements nor the role of an ankle mechanism and a load/unload hip mechanism was established. The main goal of this study is to investigate whether asymmetric postural control can be better assessed through such information. To this aim, 14 healthy adults were required to stand with or without weight-bearing asymmetry. The recorded CP(Res) trajectories were re-computed by substituting for each time the average value of the left or the right foot plantar CP trajectory. The contribution of ankle and hip mechanisms, consisting in modifying either the pressure distribution under the feet or the loading-unloading of the BW on each leg, respectively, was also assessed through an identical substitution principle. Distributing asymmetrically the BW reinforces the contribution of the CP displacements under the loaded foot in the generated CP(Res) movements, especially along the antero-posterior axis. The predominant role played by the ankle mechanisms, observed along the antero-posterior axis, is significantly decreased by the BW asymmetry. The reduced correlations with the traditional parameters used for characterizing BW asymmetry along the medio-lateral axis let suggest that this novel approach could be useful to differently assess the effects on postural control of a BW asymmetrically distributed in normal and impaired individuals.

What insights can be gained when analysing the resultant centre of pressure trajectory?

This article reviews the main aspects of the interpretation of centre of pressure trajectories measured from a force platform during undisturbed upright stance control. This objective measure has been used for more than 40 years by both clinicians and fundamental scientists to assess the postural stability of both healthy and disabled individuals. Besides rough summary statistics, which provide global information on the position of the body relative to the base of support, various techniques have been proposed such as frequency analysis, fractional Brownian motion modelling, and chaos systems, to give more relevant insights from neurophysiological and biomechanical points of view. Concurrently, the contribution of the joints and muscles of the lower legs has been studied trough EMG measures as well as indices computed from data from separate measures under each foot.

Relative contribution of the pressure variations under the feet and body weight distribution over both legs in the control of upright stance.

The resultant centre of pressure (CP(Res)) trajectories are aimed at controlling body movements in upright stance. When standing on two legs, these trajectories are generated by exerting reaction forces under each foot and by loading-unloading mechanisms intervening at the hip level. To assess the respective contribution of each of these factors in stance maintenance, a group of healthy individuals were tested in several conditions including standing quietly and voluntarily producing under each foot larger CP displacements in phase and in opposite phase along medio-lateral (ML) and antero-posterior (AP) axes. The results, based on the computation of coefficients of correlation between CP(Res) trajectories and various time series including the relative body weight applied to one leg and plantar CP trajectories, highlight some differences according to the axes along which the displacements take place and the amplitudes of the movements. Furthermore, the comparison of the CP(Res) trajectories resulting from each one of these two factors reveals the predominant role played by the loading-unloading mechanisms intervening at the hip level for the movements along the ML axis and those of the plantar CP displacements along the AP axis. Increasing the plantar CP displacements in phase or in opposite phase substantially modifies these contributions although without inferring a shift to the benefit of the other mechanism. The specific morphology of the ankle and hip joints implicated in this postural task plainly explains this postural control organisation. In particular, the link between the segmental configuration of the lower limbs and these mechanisms are discussed.