Simon Bouisset

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)

A sequence of postural movements precedes voluntary movement

Abstract Voluntary movement of the upper limb is preceded by postural movements occurring in the lower limbs and the pelvis. These movements are organized according to a consistent pattern, which can be related to the pattern of EMG activity. They are specific to the forthcoming voluntary movement and therefore can be considered as preprogrammed. The sequence of the anticipatory activity may represent part of a central motor program and tends to reduce the early perturbations of the bodys center of gravity.

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.

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.

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.

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.

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.

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?

Abstract. 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.

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.