Posture, dynamic stability, and voluntary movement.

This paper addresses the question of why voluntary movement, which induces a perturbation to balance, is possible without falling down. It proceeds from a joint biomechanical and physiological approach, and consists of three parts. The first one introduces some basic concepts that constitute a theoretical framework for experimental studies. The second part considers the various categories of "postural adjustments" (PAs) and presents major data on "anticipatory postural adjustments" (APA). The last part explores the concept of "posturokinetic capacity" (PKC) and its possible applications.

Postural sway increase in low back pain subjects is not related to reduced spine range of motion.

This study questioned whether postural sway increase in low back pain subjects was related to impaired spine mobility, and especially to a decrease in the range of motion, which was assumed to represent structural spine stiffness. Ten low back pain subjects and ten healthy control subjects performed spine flexion-extension and spine side bending tests, and standing posturographic examination in different experimental conditions. Low back pain subjects showed increased postural sway along the antero-posterior axis and reduced side bending, i.e. posturographic and range of motion parameters varied in the opposite direction. Moreover, no correlation was found between these two types of parameters. Although significant, the slight decrease in spine side bending did not seem sufficiently great to disturb the low amplitude movements that maintain postural equilibrium. Hence, it was concluded that postural sway increase in low back pain is not related to a reduced spine range of motion, but might be linked to an increase in muscular active tension, which reduces dynamic mobility capacity.

Does respiration perturb body balance more in chronic low back pain subjects than in healthy subjects?

OBJECTIVE: To determine whether body balance is perturbed more in low back pain patients than in healthy subjects, under the concept of posturo-kinetic capacity. DESIGN: Comparison of posturographic and respiratory parameters between low back pain and healthy subjects. BACKGROUND: It has been demonstrated that respiratory movements constitute a perturbation to posture, compensated by movements of the spine and of the hips, and that low back pain is frequently associated with a loss of back mobility. METHOD: Ten low back pain patients and ten healthy subjects performed five posturographic tests under three different respiratory rate conditions: quiet breathing (spontaneous), slow breathing (0.1 Hz) and fast breathing (0.5 Hz). RESULTS: Intergroup comparison showed that the mean displacements of the center of pressure were greater for the low back pain group, especially along the antero-posterior axis, where respiratory perturbation is primarily exerted. Inter-condition comparison showed that in slow and fast breathing relatively to quiet breathing, the mean displacement of the center of pressure along the antero-posterior axis was significantly increased only for the low back pain group. CONCLUSION: According to the results, respiration presented a greater disturbing effect on body balance in low back pain subjects. RELEVANCE: This study provides information on the causes of the impaired body balance associated with chronic low back pain, which could be used to improve treatment strategy.

Does body stability depend on postural chain mobility or stability area?

The purpose of this study was to examine whether postural stability depends only on the support base perimeter, that is the stability area, when body balance is perturbed by respiration. To this end, seven normal subjects were asked to breathe quietly, breathe deeply and to hold their breath (apnoea). They were asked to maintain a standing posture (Sta), and two sitting postures differing by the ischio femoral contact with the seat (Sit100 and Sit30). In other words, these three postures differed not only by the stability area, but also by pelvis mobility. The thoracic perimeter, displacement of the centre of pressure (CP) and iliac crest acceleration (Ah), taken as an index of pelvis mobility, of seven normal subjects were recorded. The results showed that the sway path (SP) was longer in seated subjects than in standing ones, and in Sit100 than in Sit30. The distance between the CP extreme positions (Delta Xp) varied in the opposite direction to SP. Iliac crests and thoracic displacements were shown to be in phase in Sit condition, and did not display any particular pattern in Sta. It was concluded that postural steadiness depends on the postural chain mobility in addition to stability area. As pelvis and lumbar column mobility are related, it is proposed that both contribute to postural chain mobility, owing to respiratory perturbation being compensated.

A sequence of postural muscle excitations precedes and accompanies isometric ramp efforts performed while sitting in human subjects.

The purpose of this study was to explore how the muscles which control postural body segments are activated during bilateral isometric ramp pushes exerted with the upper limbs by seated subjects. The paradigm under study presents the advantage that the subject is in a quasi-static posture, and since upper limbs are stretched out, the dynamic phenomena, which might occur, can only originate from the rest of the body, which means from the postural chain. A dynamometer was used to measure the horizontal force, Fx, exerted on the bar, and a custom-designed force-plate was used to measure global reaction forces (Rx) and displacement of the centre of pressure (Xp) along the antero-posterior axis. Electromyograms (EMGs) were picked up by bipolar surface electrodes from 14 muscles crossing the lower limb, pelvis, trunk and upper limb joints. It was shown that transient push efforts require monotonous EMG increase in postural as well as in focal muscles. The EMG sequence starts with the postural muscles and ends with the focal ones. The postural EMG sequence is anticipatory. It is concluded that the EMG sequence is programmed according to the task parameters, and that its role is to counteract in advance the perturbing effect of the effort, in order to allow the effort to be performed efficiently. It is suggested that excitation between the postural muscles is distributed according to their biomechanical role in relation to the supports.

Anticipatory EMG patterns associated with preferred and non-preferred arm pointing movements.

Rapid arm movements generate balance perturbations, which are anticipated and counteracted by postural adjustments. The effect of lateral preference on the control of the postural preparation to maximal velocity upperlimb pointing movements was investigated in right-handers. The muscular activities characterizing the postural adjustments were compared for preferred and non-preferred upper-limb movements. Movements were performed in two sitting conditions differing by their stability ("full on seat" and "edge of seat"). The electromyographic activity of the arm-movement prime mover and of several trunk and hip muscles involved in postural control was recorded with surface electrodes. Results indicate the presence of a reproducible pattern of anticipatory postural adjustments (APAs) involving trunk and hip muscles preceding the activation of the prime mover. In the "full on seat" condition, APAs started earlier and movement velocity was higher for preferred than for non-preferred arm movements. In the "edge of seat" condition, maximal velocity of movement did not differ significantly between both sides, but a higher excitation level of postural muscles was required to achieve this similar performance when the non-preferred upper limb was used. These findings are consistent with the hypothesis that handedness involves differences in the postural control associated with upperlimb movements, in other words that lateral preference is associated with a postural laterality.

Do anticipatory postural adjustments occurring in different segments of the postural chain follow the same organisational rule for different task movement velocities, independently of the inertial load value?

Dynamic phenomena, termed anticipatory postural adjustments (APA), are known to precede the onset of voluntary movement. Their anticipatory nature confers a particular status on APAs: as they cannot be triggered reflexly by afferent signals induced by a voluntary movement, it can be asked whether the APA parameters are centrally programmed as a function of some task movement parameters or are only the peripheral consequence of control variables. To this end the present study aims to determine whether the APAs occurring at the different sites of the postural chain yield the same accelerometric patterns and follow the same organisational rules when the task movement velocity changes, independently of the inertial load value. Subjects performed unilateral shoulder flexions at maximal and submaximal velocities, with (IUF) and without (OUF) an additional inertial load. Accelerometers were attached to the wrist and trunk, and on both sides of the body at shank, thigh, hip and shoulder. The results show that: 1) there was evidence of anticipatory acceleration in all segments of the postural chain; 2) each acceleration profile for the anticipatory phase was maintained over different focal movement velocities whether there was an additional load or not; 3) there were significant linear relationships between the amplitude of each segmental anticipatory acceleration and the square of the task movement velocity, the slope of which differs as a function of the inertial load; 4) there were close intersegmental correlations between these anticipatory accelerations which did not depend on the inertial load. In addition the correlation between the lower limbs and the opposite side of the body was positive, suggesting a diagonal postural pattern. A comparison of the present kinematic data with the corresponding EMG data reported in the literature argues in favour of a centrally determined kinematic pattern. It is proposed that the diagonal postural pattern between postural segments be considered as one of the order rules which could simplify the control process of asymmetrical movement. The anticipatory kinematics of each of the body segments would be calibrated with the velocity and the inertial load and scaled to the other segments to counteract the perturbing effect of the asymmetrical focal movement on body balance.

Are there anticipatory segmental adjustments associated with lower limb flexions when balance is poor in humans?

For a leg raising task performed in a sagittal plane, it has been shown that body balance instability can suppress anticipatory postural adjustments (APAs). The aim of this study was to determine whether the global (centre of mass) postural adjustments were replaced by local (segmental) ones, which were compensating each other and resulting in a lack of global APAs. Six healthy subjects must perform a lower limb flexion from two initial postures, corresponding to a bipedal (Fbu) and an unipedal (Fuu) stance. Kinematics of postural adjustments were recorded with accelerometers. The results showed that in Fbu the kinematics have large durations of APAs, contrary to Fuu where there are none. They showed also that during the voluntary movement the magnitudes of the segmental postural accelerations were equal or superior in Fuu than in Fbu on the anteroposterior and lateral axes, where balance is poor. Also while, on the contrary, the magnitudes are reversed on the vertical axis. These results suggest that firstly: (1) the absence of APAs can correspond to a strategic response for weak postural base configuration and secondly; (2) the local postural accelerations, depending to the axes, are linked to two different functions: to maintaining the balance and to performing the focal movement.

Are amplitude and duration of anticipatory postural adjustments identically scaled to focal movement parameters in humans?

Anticipatory postural adjustments (APA), are known to precede the onset of voluntary movement. The aim of this study was to determine whether APA amplitude and duration are programmed identically according to the voluntary movement parameters. Subjects performed shoulder flexions at maximal and sub-maximal velocities, with and without an additional inertial load. APA amplitude and duration were plotted against kinetic energy and work of the forthcoming voluntary limb movement and showed a significant linear relation. When APA duration was plotted against mechanical work, two distinct APA duration values corresponded to a given value of work depending on the weight of the load, but this was not the case for APA amplitude. It is suggested that the APA amplitude and duration are scaled according to the same movement parameters, but not in the same way, APA duration being additionally sensitive to the postural effect which results from an additional weight.

[Dynamic postural adjustments associated with the development of isometric forces in sitting subjects]. / Ajustements posturaux dynamiques associés au développement de forces isométriques chez des sujets assis.

The present study was undertaken to determine whether dynamic postural adjustments in sitting subjects occur during the development of isometric forces and whether the performance, i.e., the maximal isometric force, is related to the surface of contact of the ischio-femoral region with the seat. The subjects were asked to exert maximal horizontal two-handed isometric pushes on a bar. External force (Fx), antero-posterior reaction force (Rx) and antero-posterior displacement of the center of pressure (Xp) were measured continuously during the development of isometric forces, the subsequent hold state lasting 5 s. Two postural conditions were studied that differed by the ischio-femoral contact with the seat (100 or 30%), the foot support being bipodal (BP). It was observed that: a) Fx and Rx increased during the development of isometric forces while Xp moved backwards; b) the instantaneous variations in Fx, in relation to Rx and Xp were linearly related; c) the maximal force reached at the end of the ramp effort and the maximal values of Rx and Xp were significantly increased when the ischio-femoral contact was reduced from 100 to 30 BP. It is concluded that isometric ramp efforts are accompanied by dynamic postural adjustments and that the greater these adjustments, the larger the maximal isometric force. These observations are interpreted as supporting the view that any variation in the external force perturbs the balance of the subject, and that the maximal value of this force depends on the intensity of the dynamic postural counter-perturbation.

Is body balance more perturbed by respiration in seating than in standing posture?

The perturbing influence of respiration on balance was studied in sitting and standing subjects. The pneumograms and displacements of the centre of pressure of 10 normal subjects were recorded during quiet breathing, deep breathing and apnoea. The usual stabilometric parameters were measured, and a power spectrum density and time-locked averaging were used. The sway path was longer in seated subjects than in standing ones, suggesting that instantaneous compensatory phenomena are less efficient. The respiratory component of the sway path was larger in seated (0.16) than in standing subjects (0.09). It is concluded that respiration is a significant input for postural control, and that sitting entails less instantaneous steadiness. The concept of respiratory synergy is discussed.

Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable?

Anticipatory postural adjustments (APA) were studied in maximum velocity flexion of lower limb from two initial postures, a bipedal stance (Fbu) and unipedal stance (Fuu). In Fbu, the dynamics of center of gravity (CG) and ankle and hip muscle EMG activity showed large APA. In contrast, in Fuu there were no APA, the CG dynamics and the ankle EMG activity started at the same time as the intentional movement while the hip EMG activity started some 30 ms before the thigh flexion. The knee flexion velocity was lower in Fuu than in Fbu (7 rd/s versus 12 rd/s). These results suggest that fast voluntary movements do not require APA when the postural equilibrium is unstable, and that an alternative strategy is used. The absence of APA in Fuu, in contrast to the presence of APA in Fbu, suggests that the postural command and the focal one are time-locked and organized in a parallel process.

[Relationship between postural support and intentional movement: biomechanical approach]. / Relation entre support postural et mouvement intentionnel: approche biomécanique.

The presentation is in two parts. The first, more theoretical, section reviews the basic biomechanical features underlying the movement of all bodies, both animate and inanimate, being in contact with the ground. Application of biomechanical laws shows that intentional movement can only be executed if the external environment can react to the movement of members relative to the body. Thus, it is theoretically necessary that the part of the body, including the body segment(s) that are located between the ones which are voluntarily moved and the supporting surfaces, ie, the "postural support", is involved in the motor activity. The general equilibrium conditions of a complex mechanical system such as the human body are then examined to show why movement itself disturbs balance stability and that postural reactions are, a priori, required to preserve this stability. The second sections includes an analysis of postural activities for several types of sensory-motor tasks. They are shown to be synergistically organized, and the major features of this organisation are described. The anticipatory postural adjustments (APA) which occur BEFORE the start of voluntary movement, and are thus "preprogrammed", are then examined. It is shown that they depend on the parameters of the planned movement, posture and the uncertainty about the tasks. This is followed by a discussion of the reasons why APA may be considered as a counter-perturbation, opposing, in advance the perturbation of equilibrium brought about by the voluntary movement. The presentation concludes with several hypotheses for explaining the organisation of postural activity associated with voluntary movement. The most likely appears to be a process of "parallel" command of the voluntary and postural components.

Is balance or posture at the end of a voluntary movement programmed?

Two leg movements, differing only by the final balance conditions, were analysed to determine whether the final body posture or the equilibrium were taken into account by the motor programming. The test movement was a flexion-extension of a lower limb executed at maximal velocity. The initial body posture was bipedal, while the final one was either bipedal or unipedal. The biomechanical and electromyographic data showed significant differences between parameters of the two movements. The results indicate that the final body equilibrium is the major parameter controlled.

Posturo-kinetic organisation during the early phase of voluntary upper limb movement. 1. Normal subjects.

The nature and organisation of anticipatory postural adjustments (APA) associated with the early phase of a voluntary upper limb movement were studied. Upper limb elevations, performed at maximal velocity, were studied according to three conditions: bilateral flexions (BF) and unilateral flexions without and with an additional inertia (respectively OUF and IUF). Activities of the anterior part of the deltoid (DA) and of main muscles of the lower limbs, pelvis, trunk and scapular girdle were recorded by surface electromyography. Miniature-accelerometers enabled the recording of the tangential acceleration of the arm at wrist level (Aw) and the antero-posterior accelerations of various body links. Systematic investigations allow a precise description of the segmental phenomena which precede the onset of the voluntary movement. Before the activation of the anterior deltoid, a sequence of EMG modifications occurred in muscles of lower limbs, pelvis and trunk. The onset of Aw was preceded by anticipatory local accelerations of all the body links. Anticipatory EMG activities and local accelerations were organised according to patterns which were specific to the forthcoming voluntary movement. By comparing anticipatory EMG activities with anticipatory local accelerations, the nature of anticipatory postural movements can be determined. They appear to counteract the disturbing effects of the forthcoming voluntary movement. Because of their reproducibility and specificity, the anticipatory postural movements can be considered as preprogrammed. Postural adjustments and voluntary movement appear to be parts of the same motor program. Anticipatory postural movements should result from muscular functional synergies selected from a pre-evaluation of the perturbative aspects of the forthcoming movement.

Compensatory reactions in forward fall: are they initiated by stretch receptors?

Electromyographic activities from soleus and tibialis anterior muscles of 12 healthy subjects and 3 patients with vestibular syndrome, together with onset of movement of the head and leg were recorded during balance recovery. The disequilibrium was induced from initial forward inclination of the body. Soleus of the oscillatory foot showed brisk activity starting 59 msec (mean latency) after the perturbation and lasting 100-120 msec. Soleus of the stance foot showed similar activity, except that its duration was longer. The antagonist muscle activities started 5-20 msec later and showed similar time courses but of smaller amplitude. Ischaemia of the leg did not modify the latency. The onset of head movement was 10-20 msec after the perturbation, and those of leg were 70-90 msec. The results suggest that the early motor responses of the balance recovery were neither triggered by discharges from soleus group I and group II afferents, nor from vestibular cues, but could possibly originate in receptors located at the abdominal or lumbar level.

Postural adjustments associated with rapid voluntary arm movements in patients with Parkinson's disease.

To determine the extent to which deficits in coordination between posture and movement are influenced by postural disorders, postural adjustments associated with rapid voluntary arm movement were studied in PD patients and controls. EMG activity in postural muscles of the lower limbs and the trunk and local anteroposterior accelerations of the upper part of the leg were recorded in subjects who rapidly raised their arms to a horizontal position in response to a visual signal. The arm movement was characterized electromyographically by EMG activity from the deltoid muscle (anterior portion) and kinetically by acceleration of the arm. Study of characteristics of voluntary movement showed a nonsignificant increase in RT (simple-reaction time task) and an important increase in MT. There were important differences between PD patients and control subjects with regard to postural adjustments. Timing between voluntary movement and postural movement was anticipatory in 5% of PD patients, whereas it was anticipatory in 100% of control subjects. In PD patients, organization of early postural adjustments was not specific to voluntary movement; in control subjects, organization of early postural adjustments was specific to the forthcoming movement. Last, a possible functional relation between the "quality" of postural adjustments and a reduction in motor performance of normal subjects is suggested.

Biomechanical study of the programming of anticipatory postural adjustments associated with voluntary movement.

The present research concerns anticipatory postural adjustments (APA), with the purpose of determining whether they are preprogrammed and of specifying their biomechanical finality. The experimental situation allowed us to distinguish between the voluntary movement itself (an upper limb elevation) and the postural adjustments associated with it. To this aim, the upper limb kinematics, evaluated from an accelerometer fixed at wrist level, were compared to the whole body dynamics, recorded by means of a force platform. Movements, executed in series of five, were studied according to three conditions: bilateral flexions (BF) and unilateral flexions (UF), with (IUF) and without (OUF) an additional inertia, of the stretched upper limb(s). Six right handed adults were tested twice. Results showed that the ground reaction resultant forces as well as the ground reaction resultant moment about the vertical axis presented reproducible variations before and after the onset of upper limb acceleration. The biomechanical organization of APA corresponded, for the three experimental conditions, to an upward and forward acceleration of the body center of gravity, and also, for UF, to a resultant moment directed towards the contralateral side. The duration of APA varied with the characteristics of the forthcoming voluntary movement, increasing significantly from BF to OUF and from OUF to IUF. It is concluded that APA correspond to dynamic phenomena which are centrally preprogrammed. The inertia forces associated with APA may, when the time comes, balance the inertia forces due to the movement of the mobile limb therefore counteracting the disturbance to postural equilibrium.(ABSTRACT TRUNCATED AT 250 WORDS)

Are dynamic phenomena prior to stepping essential to walking?

The aim of our research was to examine the function of the pre-gait weight shifts in generating the dynamic forces needed to start walking at different speeds. Five subjects participated in the experiment, and a total of 105 gait initiation movements, executed on a large force plate, for three speed conditions (slow, normal, and fast), were examined. Results, which related to durations of the anticipation and of the step execution phases and to biomechanical parameters (progression velocity of the center of gravity, backward shift of the center of foot pressure, and magnitude of propulsive forces at heel-off time), suggested that dynamic phenomena prior to stepping are essential to walking as far as they contribute to the creation of convenient conditions for progression. The configuration of the support basis prior to stepping limits the progression velocity reached at the end of the first step.

Chronometric analysis of the posturo-kinetic programming of voluntary movement.

Posturo-kinetic programming was investigated in a simple reaction time paradigm. Standing subjects performed voluntary upper limb elevations under three conditions differing by the importance of their destabilizing effect on the initial balance. Three indexes were considered: the reaction time (RT) corresponding to delay between the response signal (RS) and the onset of the upper limb acceleration at wrist level (Aw); the motor latency (ML) corresponding to delay between RS and onset of the earlier of the two accelerations at shank level (As) which was previously shown to indicate the onset of postural adjustments; and the postural anticipation (PA) corresponding to the delay between the onset of the earliest As and the onset of Aw. The results showed that the RT varied in relation to parameters of the forthcoming movement. RT variations were essentially due to differences in the PA. It appeared that the ML depended on cognitive factors while the PA depended on biomechanical ones. it is concluded that ML corresponds to the "true" reaction time while PA covers the earliest part of the motor response.