Serge Le Bozec

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.

Postural dynamics in maximal isometric ramp efforts.

Abstract. A global biomechanical model of transient push efforts is proposed where transient efforts are taken into consideration, with the aim to examine in greater depth the postural adjustments associated with voluntary efforts. In this context, the push effort is considered as a perturbation of balance, and the other reaction forces as a counter-perturbation which is necessary for the task to be performed efficiently. The subjects were asked to exert maximal horizontal two-handed isometric pushes on a dynamometric bar, as rapidly as possible. They were seated on a custom-designed device which measured global and partitive dynamic quantities. The results showed that the horizontal reaction forces and the horizontal displacement of the centre of pressure increased quasi-proportionally with the perturbation. In addition, it was established that vertical reaction forces increased at seat level whereas they decreased at foot level, resulting in minor vertical acceleration and displacement of the centre of gravity. On the contrary, the anteroposterior reaction forces increased both at foot and at seat levels. Based on a detailed examination of the various terms of the model, it is concluded that transient muscular effort induces dynamics of the postural chain. These observations support the view that there is a postural counter-perturbation which is associated with motor activity. More generally, the model helped to specify the effect of postural dynamic phenomena. It makes it possible to stress the importance of adherence at the contact level between the subject and the seat in the course of transient efforts.

Respiratory disturbance to posture varies according to the respiratory mode.

The purpose of this study was to determine whether respiratory disturbance to posture varies as a function of the respiratory mode, i.e. thoracic or abdominal. To this aim, 10 healthy male subjects underwent a posturographic examination associated with a measurement of respiratory kinematics. Experimental conditions varied posture (sitting, standing) respiratory amplitude (quiet breathing, deep breathing) and respiratory mode (thoracic, abdominal). In addition to classical posturographic parameters, original peak detection algorithm and emergence parameter calculated from the Fast Fourier Transform were used to assess the respiratory component in CP displacements. Results showed that along the antero-posterior axis, time domain and frequency domain parameters were both significantly greater in thoracic breathing mode than in abdominal mode. It was concluded that respiratory kinematics have a more prominent disturbing effect on posture when they involve the rib cage rather than the abdomen.

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.

Do bimanual isometric push efforts in humans stop as a consequence of postural muscle exhaustion

The purpose of this study was to explore whether global efforts stop as a consequence of postural muscle exhaustion. To this end, seated adults were asked to exert 75% maximal voluntary contractions bimanual push efforts until exhaustion. A dynamometer was used to measure the horizontal force exerted on a bar (Fx) and a custom-designed force plate measured the antero-posterior displacement of the centre of pressure (Xp). Electromyograms were picked up by bipolar surface electrodes from the primum movens (serratus anterior) and four postural muscles (trapezius superior, erectores spinae, rectus abdominis, rectus femoris). Root mean square and mean power frequency were calculated over 2-s intervals and compared to corresponding Fx and Xp values. It was shown that the effort stops as a consequence of exhaustion of postural muscles (rectus abdominis and rectus femoris), and not of the primum movens. It is concluded that postural muscles make a major contribution to global efforts, in that they allow compliance to biomechanical requirements, that is, to preserve the distance between the centre of pressure and the centre of gravity, which must be proportional to the external force.

Postural control in isometric ramp pushes: the role of Consecutive Postural Adjustments (CPAs)

Postural adjustments, which occur after the end of a voluntary movement (termed Consecutive Postural Adjustments: CPAs), were studied and compared to the corresponding Anticipatory Postural Adjustments (APAs). Seven right-handed male adults were asked to perform horizontal two-handed maximal ramp pushes as quickly as possible, while sitting. A dynamometric bar measured the reaction to push force (Fx) and a custom-designed device measured the resultant reaction forces along the antero-posterior axis (Rx). Two ischio-femoral contacts (100 BP: full ischio-femoral contact of the ischio-femoral length; and 30BP: one-third contact) were considered. Each session consisted of ten pushes. The reaction forces, as well as push force, increased continuously, displaying similar time course profiles. However, Rx continued to increase after the end of push rise, which ascertained CPAs. CPAs were showed to be consistent kinetic phenomena, using a biomechanical analysis, based on time courses of reaction forces and CoG kinematics. Their coherence was checked precisely, by comparing theoretical and experimental occurrences of remarkable points (extrema and zero crossings). CPA durations and peak amplitudes (dCPA and pCPA) were significantly greater than the corresponding APA values (dAPA and pAPA). Moreover, dAPAs and dCPAs increased (p < 0.001), as did pCPAs (p<0.001) and pAPAs (p < 0.05) when the peak push force was greater (30 BP), showing that the probability of finding a statistically significant difference is greater for APA duration than amplitude, unlike CPAs. Finally, the present results were discussed in relation to the hypothesis according to which the focal and the postural components are parts of the same motor program.

Does increased muscular tension along the torso impair postural equilibrium in a standing posture

This paper focused on the relationship between active muscular tension along the torso and postural equilibrium while standing. Eleven healthy male subjects underwent a posturographic examination associated with a bimanual compression of a dynamometric bar, which was used to set the torso muscular activity at three different levels (0MVC, 20MVC, 40MVC). Electromyographic pre-tests identified the main superficial muscles of the compressive load as: pectoralis major, latissimus dorsi, thoracic and lumbar erector spinae. Kinematics of the chest wall was recorded by means of two sensing belts, in order to assess the respiratory component of the center of pressure (CP) signal. The analysis of time-domain stabilometric parameters showed that CP displacements were larger and faster in 40MVC that in 20MVC, with no variation between 0MVC and 20MVC. The respiratory component of the CP signal was not sensitive to the compressive load. It was concluded that increased muscular tension along the torso is likely to disturb postural equilibrium, but only when it exceeds a given level.

Does increased muscular tension along the torso disturb postural equilibrium more when it is asymmetrical

The aim of this study was to determine whether increased muscular tension disturbs postural equilibrium more when it is asymmetrical. Ten healthy male subjects underwent a posturographic examination associated with an original uni and bilateral compressive load paradigm designed to set the active muscular tension at different controlled levels along each side of the torso. Respiratory kinematics were recorded by means of two sensing belts. Two electromyographic pre-tests were used to map out the main motor muscles of the task and to quantify the level of asymmetry induced by unilateral loads. The posturographic examination revealed that the mean deviation of the CP along the medial-lateral axis was significantly greater in unilateral than in bilateral compressive loads. It was suggested that increased muscular tension along the torso induces a more disturbing effect on posture when it is asymmetrical.

Are simultaneous postural adjustments (SPA) programmed as a function of pointing velocity

This paper deals with the influence of velocity on the postural adjustments that occur during the course of a voluntary movement, that is to say, simultaneous postural adjustments (SPA). To this aim, a pointing task performed at different velocities (V) was considered. Upper limb kinematics and body kinetics were recorded. Using a 2-DOF model, the body was divided into two parts: the right upper limb (termed the “focal” chain) and the rest of the body (termed the “postural” chain). This model allowed us to calculate the kinetics of both subsystems (−Fx and