How proprioceptive impairments affect quiet standing in patients with multiple sclerosis

To assess if multiple sclerosis patients with proprioceptive impairment are specifically affected during quiet standing with eyes open and how they can develop motor compensatory processes, 56 patients, classified from sensory clinical tests as ataxo-spastic (MS-AS) or only having spasticity (MS-S), were compared to 23 healthy adults matched for age. The postural strategies were assessed from the centre-of-pressure trajectories (CP), measured from a force platform in the eyes open standing condition for a single trial lasting 51.2 s. The vertical projection of the centre of gravity (CGv) and its vertical difference from the CP (CP-CGv) were then estimated through a biomechanical relationship. These two movements permit the characterization of the postural performance and the horizontal acceleration communicated to the CG and from that, the global energy expenditure, respectively. Both MS-AS and MS-S groups demonstrate larger CGv and CP-CGv movements than healthy individuals of the same age. Whilst similar CGv values are noticed in both MS subgroups, suggesting similar postural performances, statistically significant differences are observed for the CP-CGv component. Biomechanically, this feature expresses the necessity for the MS-AS group to develop augmented neuro-muscular means to control their body movements, as compared to the MS-S group. By demonstrating for both groups of patients similar postural performance accompanied by a varying degree of energy expenditure to maintain undisturbed upright stance, this study reveals that MS-AS patients which are affected by proprioceptive loss can compensate for this deficit with more efficient control strategies, when standing still with their eyes open.     

Analysis of center-of-pressure data during unipedal and bipedal standing using fractional Brownian motion modeling

To question the relation between uni-and bipedal postural skills, 21 subjects were required to stand on a force platform through uni- and bipedal conditions. These two protocols are commonly used paradigms to assess the balance capacities of healthy and disabled patients. The recorded displacements of the center of pressure (CP) were decomposed along mediolateral and anteroposterior axes and assessed through variance positions and parameters obtained from fractional Brownian motion (fBm) modeling to determine the nature and the spatiotemporal organization of the successive controlling mechanisms. The variances underline the relative independence of the two tasks. Nevertheless, as highlighted by the fBm framework, postural correction is initiated for the unipedal stance after shorter time delays and longer covered distances. When compared to bipedal standing, one of the main characteristics of unipedal standing is to induce better-controlled CP trajectories, as deduced from the scaling regimes computed from the fBm modeling. Lastly, the control of the CP trajectories during the shortest time intervals along the anteroposterior axis appears identical for both uni- and bipedal conditions. Unipedal and bipedal standing controls should thus be viewed as two complementary tasks, each providing specific and complementary insights into the postural control organization.     

How proprioceptive impairments affect quiet standing in patients with multiple sclerosis

To assess if multiple sclerosis patients with proprioceptive impairment are specifically affected during quiet standing with eyes open and how they can develop motor compensatory processes, 56 patients, classified from sensory clinical tests as ataxo-spastic (MS-AS) or only having spasticity (MS-S), were compared to 23 healthy adults matched for age. The postural strategies were assessed from the centre-of-pressure trajectories (CP), measured from a force platform in the eyes open standing condition for a single trial lasting 51.2?s. The vertical projection of the centre of gravity (CG_v) and its vertical difference from the CP (CP-CG_v) were then estimated through a biomechanical relationship. These two movements permit the characterization of the postural performance and the horizontal acceleration communicated to the CG and from that, the global energy expenditure, respectively. Both MS-AS and MS-S groups demonstrate larger CG_v and CP-CG_v movements than healthy individuals of the same age. Whilst similar CG_v values are noticed in both MS subgroups, suggesting similar postural performances, statistically significant differences are observed for the CP-CG_v component. Biomechanically, this feature expresses the necessity for the MS-AS group to develop augmented neuro-muscular means to control their body movements, as compared to the MS-S group. By demonstrating for both groups of patients similar postural performance accompanied by a varying degree of energy expenditure to maintain undisturbed upright stance, this study reveals that MS-AS patients which are affected by proprioceptive loss can compensate for this deficit with more efficient control strategies, when standing still with their eyes open.     

Effect of an additional manual motor task on the maintenance of equilibrium in the frontal and sagittal planes in standing subjects

The postural oscillations of a standing subject during an additional manual motor task consisting in holding a movable ball in the center of a flat box were studied. The movements of the center of pressure (CP) in the frontal and sagittal planes were studied when subjects were standing on a stable rigid support and on a movable unstable support. The effect of the additional motor task on the movement of the CP depended on the stability of the support. When the additional task was performed, the sagittal movements of the CP increased in the case a movable support and did not increase when the support was stable. The additional task decreased the frontal movements of CP in the case of a stable support, and it did not change the frontal movements of CP when the support was unstable. Thus, the performance of an additional motor task led to a reduction of the efficiency of the postural control system in maintaining equilibrium on an unstable support. This decrease may be due to a greater cortical influence on the posture control system in subjects standing on a movable support in comparison with this influence in the case of a stable support.     

Self versus Environment Motion in Postural Control

To stabilize our position in space we use visual information as well as non-visual physical motion cues. However, visual cues can be ambiguous: visually perceived motion may be caused by self-movement, movement of the environment, or both. The nervous system must combine the ambiguous visual cues with noisy physical motion cues to resolve this ambiguity and control our body posture. Here we have developed a Bayesian model that formalizes how the nervous system could solve this problem. In this model, the nervous system combines the sensory cues to estimate the movement of the body. We analytically demonstrate that, as long as visual stimulation is fast in comparison to the uncertainty in our perception of body movement, the optimal strategy is to weight visually perceived movement velocities proportional to a power law. We find that this model accounts for the nonlinear influence of experimentally induced visual motion on human postural behavior both in our data and in previously published results.     

Locomotory mechanisms in Antarctic pycnogonids

Patterns of walking, modes of joint movement, and individual limb diversity were analysed with the aid of ciné film of several living Antarctic pycnogonids, including the 8-legged Colossendeis australis, C. angusta, Pallenopsis patagonica , and Nymphon sp., the 10-legged Decolopoda australis , and the 12-legged Dodecolopoda mawsoni. Appendage musculature of several of these species and also of the 10-legged Pentapycnon charcoti and Pentanymphon antarcticurn was dissected. At least two distinct morphotypes were identified: a short-legged, crawling variety ( P. charcoti ); and the more typical long-legged, large bodied, walking forms. No gross differences in musculature of joints were noted in the species examined. All joints are, at least superficially, hinge joints. The coxa-body joint is largely immobile, the coxa 1-coxa 2 joint alone exhibits promotion-remotion and all other joints are flexion-extension joints. The 8-legged forms move in an imprecise manner, there being irregularity of leg raising and lowering and where legs touch down in relation to the body and to other legs. The 10- and 12-legged forms exhibit more precise patterns of metachronal leg movements. Although legs move in a basic promotion-remotion, extension-flexion mode, there is a certain degree of twisting of a leg as it is picked up, brought forward, and set down; models indicating how such joint movement occurs were constructed. The possibility that hydrostatic pressure is employed in extension is considered and is found to be remote. Lateral placement of legs, orientated in almost all directions in the horizontal plane of the trunk, achieves a versatility of movement similar to that in crabs. Comments on pycnogonid taxonomic affinities are offered.     

Anticipatory postural adjustments in children with hemiplegia and diplegia

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of standing posture. However, little is known about if and how children with cerebral palsy (CP) generate APAs. Two groups of children with CP (hemiplegia and diplegia) and a group of children with typical motor development performed arm flexion and extension movements while standing on a force platform. Electromyographic activity of six trunk and leg muscles and displacement of center of pressure (COP) were recorded. Children with CP were able to generate anticipatory postural adjustments and produce directionally specific APAs and COP displacements similar to those described in adults and typically developing children. However, children with diplegia were unable to generate APAs of the same magnitude as children with typical development and hemiplegia and had higher baseline muscle activity prior to movement. In children with diplegia, COP was posteriorly displaced and peak acceleration was smaller during bilateral extension compared to children with hemiplegia. The outcomes of the study highlight the role of APAs in the control of posture of children with CP and point out the similarities and differences in anticipatory control in children with diplegia and hemiplegia. These differences may foster ideas for treatment strategies to enhance APAs in children with CP.     

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.     

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.     

Undisturbed stance on a double seesaw: Interaction between asymmetries of the center-of-pressure patterns under each foot and weight-bearing

Both center-of-pressure (CP) displacements under each foot and relative body-weight distribution intervene in the production of resultant CP movements. To better understand their respective involvement, a protocol was set up for young healthy individuals consisting in standing on a double seesaw, favoring pitch motions and laying on a dual-force platform. The postural control effects induced by two types of asymmetry, weight-bearing and the CP movement patterns, were investigated. These asymmetries were achieved by associating two seesaws with two different lengths for the radii of the ridges and by requiring specific body-weight distributions. The results indicate that the postural strategies, aimed at controlling anteroposterior sway, are related to the subjects’ capacity to minimize the CP displacements under the less stable support, whatever load is applied. In contrast, the degree of involvement of the more stable support must be viewed as a complement used to secure the appropriate motor output, i.e., the resultant CP movements. Within this objective, both the applied load and the CP amplitudes under the more stable support are taken into account. These data provide additional insights into the compensatory mechanisms between the interactions between the two feet, which are used to produce the adequate resultant CP movements and therefore upright stance control. The specificity of the double seesaw that can induce asymmetric CP patterns and/or asymmetric body-weight distribution makes it a legitimate contender to be used as a rehabilitation device for patients with neurological and/or traumatic diseases.     

Influence of an asymmetrical body weight distribution on the control of undisturbed upright stance

Postural asymmetry in humans is generally associated with different pathologies. However, its specific influence on undisturbed upright stance is poorly understood. To evaluate its separate effects on each support, the centre of pressure (CP) displacements were recorded through two force platforms. In a second step, the complex resultant centre of pressure trajectories (CP(Res)) were computed and decomposed into two elementary components: the horizontal displacements of the centre of gravity (CG(h)) and the difference in the plane of support between the vertical projection of CG(h) and CP(Res) (CP-CG(v)). These motions were then processed through a frequency analysis and modelled as fractional Brownian motion to gain some additional insight into their spatio-temporal organisation. Ten healthy adults were tested in three conditions consisting of various weight distributions. The quality of the mechanism involved in the control of the unloaded support CP motions appears to decrease as the asymmetry becomes more pronounced. To be precise, larger increases of the CP displacements are observed for the unloaded support compared to the loaded one. As a result, the CP(Res) motions are themselves augmented in the ML direction, inducing in turn larger CG(h) and CP-CG(v) motions. Postural asymmetry thus constitutes an important constraint on the control of upright undisturbed stance by generating changes in the control of both supports and by reducing the efficiency of the hip load/unload mechanisms. On the other hand, by inducing larger body sways, postural asymmetry necessitates higher energy expenditure and the setting of particular control mechanisms.     

Effects of excessive body weight on postural control

Research that evaluated both static and dynamic stability was performed, to clarify the impact of excessive body weight on postural control. The spontaneous center of foot pressure (CP) motion during quiet stance and a range of forward voluntary CP displacements were studied in 100 obese, and 33 lean women. Characteristics of postural sway were acquired while the subjects were standing quiet on a force plate with eyes open (EO) and with eyes closed (EC). Their anterior range of CP voluntary displacements was assessed upon a range of maximal whole body leanings which were directed forward. A substantial reduction of postural sway was observed in all patients which had increased body weight. Main postural sway parameters i.e., the total path length as well as its directional components were negatively correlated with the body mass and body mass index (BMI). The range of a whole body voluntary forward leaning, did not exhibit any significant change in patients with an obesity grade of I and II. Such a deficit was, however, found in subjects with a body mass index above 40. In conclusion, the increased body weight imposed new biomechanical constraints, that resulted in functional adaptation of the control of the erect posture. This functional adaptation was characterized by a reduced postural sway associated with a substantial reduction of the dynamic stability range in subjects with BMI>40.     

Unilateral lower limb muscle fatigue induces bilateral effects on undisturbed stance and muscle EMG activities

The study investigated the effects of an unilateral ankle muscle fatigue onto independent postural control parameters including the trajectories of the estimated resultant CoP (CoPres) and his components: the centre of gravity (CG) and CoP–CG trajectories.Nine healthy men realized series of 10 toe-lift immediately followed by 10 knee flexions until exhaustion with one (Ex) leg. Maximal isometric voluntary contractions, postural sway measures of each leg, and muscular activities of the ankle muscles were recorded before and immediately after the fatiguing exercise.As expected, the latter induced a decrease in maximal voluntary peak force associated with a greater variability of the relative contribution of each leg on the CoPres, enhanced all postural parameters of the non-exercised leg. A significant decreased of the tibialis anterior EMG activity for the Ex leg and an increased one for the NoEx leg. Finally, following unilateral fatigue, the body sway destabilisation seemed to occur only along the medio-lateral (ML) axis.The enhanced and greater variability of the variance along ML axis might be explained by the recourse at the loading–unloading strategy choice and suggests a central attempt to compensate for pain sensation.     

Influence of an additional movement task for the upper extremities on equilibrium maintenance in frontal and sagittal level in standing man

The postural oscillations of standing man were studied during additional manual motor task that consisted of maintaining of the moving ball in the center of flat box. The movement of a center of pressure (CP) in frontal and sagittal plane were analyzed during standing on stable rigid support and on moving unstable support. The influence of the additional motor task on CP movement depend on level of support stability. Sagittal CP movement increased while the additional task was executed during standing on moving support but it did not when the support was stable. Frontal CP movement decreased when the additional task was executed during standing on stable support but it did not while the support was unstable. Thus execution of the additional motor task execution led to the reduction of efficacy of the postural control on the moving unstable support. This result suggests that the cortical influence on the postural mechanism was stronger during standing on moving support in comparison to the standing on the stable support.     

Rhythmic movement and rhythmic auditory cues enhance anticipatory postural adjustment of gait initiation

Abstract

The purpose of this study was to determine whether the rhythmic movements or cues enhance the anticipatory postural adjustment (APA) of gait initiation. Healthy humans initiated gait in response to an auditory start cue (third cue). A first auditory cue was given 8?s before the start cue, and a second auditory cue was given 3?s before the start cue. The participants performed the rhythmic medio-lateral weight shift (ML-WS session), rhythmic anterior-posterior weight shift (AP-WS session), or rhythmic arm swing (arm swing session) in the time between the first and second cues. In the rhythmic cues session, rhythmic auditory cues with a frequency of 1?Hz were given in this time. In the stationary session, the participants maintained stationary stance in this time. The APA and initial step movement preceded by those rhythmic movements or cues were compared with those in the stationary session. The temporal characteristics of the initial step movement of the gait initiation were not changed by the rhythmic movements or cues. The medio-lateral displacement of the APA in the ML-WS and arm swing sessions was significantly greater than that in the stationary session. The anterior–posterior displacement of the APA in the rhythmic cues and arm swing sessions was significantly greater than that in the stationary session. Taken together, the rhythmic movements and cues enhance the APA of gait initiation. The present finding may be a clue or motive for the future investigation for using rhythmic movements or cues as the preparatory activity to enlarge the small APA of gait initiation in the patients with Parkinson’s disease.     

Method to Measure Tone of Axial and Proximal Muscle

The control of tonic muscular activity remains poorly understood. While abnormal tone is commonly assessed clinically by measuring the passive resistance of relaxed limbs¹, no systems are available to study tonic muscle control in a natural, active state of antigravity support. We have developed a device (Twister) to study tonic regulation of axial and proximal muscles during active postural maintenance (i.e. postural tone). Twister rotates axial body regions relative to each other about the vertical axis during stance, so as to twist the neck, trunk or hip regions. This twisting imposes length changes on axial muscles without changing the body''s relationship to gravity. Because Twister does not provide postural support, tone must be regulated to counteract gravitational torques. We quantify this tonic regulation by the restive torque to twisting, which reflects the state of all muscles undergoing length changes, as well as by electromyography of relevant muscles. Because tone is characterized by long-lasting low-level muscle activity, tonic control is studied with slow movements that produce "tonic" changes in muscle length, without evoking fast "phasic" responses. Twister can be reconfigured to study various aspects of muscle tone, such as co-contraction, tonic modulation to postural changes, tonic interactions across body segments, as well as perceptual thresholds to slow axial rotation. Twister can also be used to provide a quantitative measurement of the effects of disease on axial and proximal postural tone and assess the efficacy of intervention.     

Comparative analysis of vertical posture control in athletes differing in expertise

The balance control was studied in athletes with different degrees of expertise in sports: masters of sports (MSs, n = 18) and candidates for masters of sports (CMSs, n = 13). The balance function was assessed by means of a stabilographic system (Rhythm, Russia) in static tests, including the simple bipedal stance (BS) and squat (SQ) positions, as well as in dynamic tests, including Involute, evaluating the tracking movement and, Step Input, assessing the response of the whole body to a visuomotor task. It was found that MSs did not differ in the linear or angular velocity of oscillations of the center of pressure (CP) in the BS or SQ positions from CMSs with the same anthropometric data, PWC₁₇₀, and trunk strength. MSs exhibited a relative dominance of low-frequency oscillations in spectral analysis in the BS test with the eyes open. In the Step Input test, MSs had a lower latent period of response and a greater speed and accuracy of the forward and backward body movements in response to the visual signals and exhibited a relative dominance of high-frequency oscillations. The results showed that MSs had an improved postural control, which was mainly expressed in the dynamic test for the speed and accuracy of the whole-body response to visual signals in the vertical posture.     

Postural control in healthy young adults using a double seesaw device

Postural control on single and double seesaws was investigated in young healthy adults required to stand as still as possible on two side-by-side seesaws favoring pitch motion and lying on two separate force platforms. The device offers the possibility to get associated or dissociated seesaws and, if dissociated, to induce asymmetric patterns for the centers-of-pressure (CP) under both left and right feet by using different radii for the two seesaws. Substituting a parallelepiped volume to one seesaw offering a firm contact to one foot is also possible. The results indicated that dissociating the two seesaws led to increased resultant CP (CP_Res) and vertically projected center-of-gravity movements (CG_v) only along the mediolateral axis, whereas a slight decreasing tendency characterized these movements along the antero-posterior axis. When standing on two independent seesaws with different radii, significantly larger CP displacements were seen along the antero-posterior axis under the foot lying on the more stable support, i.e., the seesaw with the longer radius or the parallelepiped volume. In these two asymmetrical conditions, the CP_Res output results from a compensatory mechanism, i.e. larger movements under one foot to compensate for the decreased movements occurring under the opposite foot. This postural control strategy is aimed at allowing sufficient CP_Res displacements in order to appropriately secure balance. Because of the complex sensorimotor coordination induced, involving differentially in certain cases both legs, the double seesaw device can be viewed as a possible tool for challenging postural control by inducing asymmetrical patterns between left and right feet CP movements.     

Postural reactions of humans in upright stance: Dependence on different visual conditions and the level of stiffness in the ankle joints

We checked on the supposition that the magnitude of postural reactions to an unexpected postural disturbance in upright stance in humans can be determined to a considerable extent by the level of background stiffness in the ankle joints. For this purpose, we estimated changes in the joint stiffness under different conditions of visual control; these values were estimated within the period of background body oscillations (i.e., before the beginning of a compensatory motor reaction) and compared with those in the course of postural reactions evoked by vibrational stimulation of the ankle (shin) muscles. Experiments were carried where the subjects stood with open and closed eyes (OE and CE, respectively) and while standing wearing spectacles with frosted glass passing only diffuse light (DL). In the course of the tests, the subjects stood in the usual comfortable vertical position (hereafter, standard stance) or in the same position but with the possibility to lightly touch an immobile object by a finger (stance with additional support). Such technique was used to weaken the effects of CE and DL on background sways of the body and to lead these sways close to the level typical of OE conditions. The joint stiffness was estimated using an approach based on frequency filtration of oscillations of the center of pressure of the feet (CPF) that allowed us to select signals proportional to displacements of the total center of gravity (CG) of the body and to calculate the difference between oscillations of the CPF and CG (a CPF-CG variable). The CPF-CG variable is proportional to the horizontal acceleration of the CG and, therefore, can be used for estimation of the changes in stiffness in the ankle joints. Under conditions of standard stance, the usual conditions rather similarly influenced both variables (CG and CPF-CG) in the course of both background body oscillations and a postural response. The examined variables were the greatest under CE conditions, decreased under conditions of perception of DL, and became smallest with OE. At standing with additional support, the dependence of the examined variables on visual conditions disappeared within the period of background body oscillations (before the beginning of postural reactions). In this case, the magnitude of oscillations of the CPF-CG variable under CE and DL conditions decreased to the level observed at standing under OE conditions. The magnitude of CG displacements induced by vibrational stimulations of the muscles remained, nevertheless, clearly dependent on visual conditions (the same regularities were observed as in the case of standing with no additional support). Thus, our findings demonstrate that the correlation between the characteristics of postural reactions in the upright stance and the level of ankle joint stiffness is not single-valued. Neirofiziologiya/Neurophysiology, Vol. 39, No. 2, pp. 146–153, March–April, 2007.     

Mechanisms of Motor Adaptation in Reactive Balance Control

Balance control must be rapidly modified to provide stability in the face of environmental challenges. Although changes in reactive balance over repeated perturbations have been observed previously, only anticipatory postural adjustments preceding voluntary movements have been studied in the framework of motor adaptation and learning theory. Here, we hypothesized that adaptation occurs in task-level balance control during responses to perturbations due to central changes in the control of both anticipatory and reactive components of balance. Our adaptation paradigm consisted of a Training set of forward support-surface perturbations, a Reversal set of novel countermanding perturbations that reversed direction, and a Washout set identical to the Training set. Adaptation was characterized by a change in a motor variable from the beginning to the end of each set, the presence of aftereffects at the beginning of the Washout set when the novel perturbations were removed, and a return of the variable at the end of the Washout to a level comparable to the end of the Training set. Task-level balance performance was characterized by peak center of mass (CoM) excursion and velocity, which showed adaptive changes with repetitive trials. Only small changes in anticipatory postural control, characterized by body lean and background muscle activity were observed. Adaptation was found in the evoked long-latency muscular response, and also in the sensorimotor transformation mediating that response. Finally, in each set, temporal patterns of muscle activity converged towards an optimum predicted by a trade-off between maximizing motor performance and minimizing muscle activity. Our results suggest that adaptation in balance, as well as other motor tasks, is mediated by altering central sensitivity to perturbations and may be driven by energetic considerations.