Christian Ribreau

Electromyographic and biomechanical characteristics of segmental postural adjustments associated with voluntary wrist movements

Abstract. This study re-investigates the characteristics of segmental postural adjustments associated with rapid mono-articular movements and analyses their dependence on initial postural conditions. Subjects performed rapid voluntary wrist flexions and extensions while maintaining their upper limb posture as stable as possible, with or without an elbow support. Surface electromyographic activity (EMG) was recorded from Flexor carpi ulnaris, Extensor carpi radialis, Biceps brachii, Triceps brachii and Deltoideus anterior. The kinematics of the three joints and kinetics in the support condition were also recorded. A planar mechanical model was used to determine the muscle torque required to keep the upper limb posture constant while performing wrist movements. All subjects showed anticipatory postural adjustments (APA) which, unlike those described for whole-body postural control, could not counteract in advance the perturbing inter-segmental forces created by the movement. Postural muscles were activated before the wrist movement with a chronology specific to the direction of the wrist movement. Some postural muscular activities anticipated that of the prime-movers in accordance with muscle torque, which had to be applied to the joints to keep the upper limb posture constant. These results reveal that the central nervous system (CNS) uses the same organization of the motor command for the control of both segmental and whole-body posture: APA and corrective postural adjustments (CPA), which are based on well-organized anticipatory postural muscle activities (APMA), except that APA can be non-efficient in segmental postural control. The presence or absence of an elbow support influenced the level of activation of postural muscle but not their chronology. This result suggests that the CNS uses a sequence of APMA: a postural muscle synergy which is predetermined as a function of the intended direction of the movements and modulates the gain towards certain muscles, in accordance with the gravitational effects, and supports reaction changes.

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.

Chronology of upper limb anticipatory postural adjustments associated with voluntary wrist flexions and extensions in humans

Six sitting healthy subjects were instructed to keep a constant upper limb posture while performing wrist flexions and extensions. Acceleration of the wrist, elbow and shoulder joints, and surface electromyograms (EMGs) of the upper limbs main flexors and extensors were studied. Results indicated the existence of anticipatory (APA) and corrective postural adjustments. The APAs were based on a reproducible directional chronology of postural muscle activations. As shown by a simple mechanical model, this chronology was in accordance with the muscular torque which should be applied to the joints to keep the upper limb posture constant. All these data indicate that APA are involved in segmental posture, where their general organization is similar to those of APA associated with whole-body movements. The use of constant directional postural synergies well agrees with a simplification of the motor control according to Bernsteins theory.

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.

A simple method to compare body and upper limb kinetics in the course of pointing task

The subject’s initial posture was standardised before each trial, with the upper arm in a vertical direction, and the forearm in a horizontal direction, with the index finger outstretched. The instruction was to hit, at maximal velocity, a target located in front of the subject, at shoulder level and at the maximal reach distance. Two postural conditions were considered: 100% (100 bipedal (BP)) and 30% (30 BP) of the ischio-femoral length contact with the seat. Three male subjects carried out seven pointing trials for each condition. The subjects were seated on a custom-designed device, which consisted of a seat force plate and two foot force plates from GITECH Robotics. These three force plates allowed measurement of the force component Rx (and also Rz) from the support onto the subjects. Kinematics data were collected from a two-optokinetic camera recording system (Selpot II). Active markers (LEDs) were placed on both ends of two rigid aluminium rods. The kinematics data were collected at 500 Hz, processed, stored in a PC microcomputer and synchronised with the force plate. These data were used to calculate two joint angles in the shoulder sagittal plane: at the shoulder, between the upper arm and the vertical, and, at the elbow, between the upper arm and the forearm. They were used to implement a two-link model in order to calculate the quantity of acceleration of the upper limb.

Adherence and Postural Control: A Biomechanical Analysis of Transient Push Efforts

This chapter focuses on the question of the interface between the body and its physical environment, namely adherence and friction. First, a short survey of literature is presented and some basic statements on adherence reviewed. They help define the adherence constraints associated with different motor tasks. Then, a new paradigm is presented, the transient push paradigm, which offers manifold facilities. In particular, it makes it possible: i) to exert transient external force in the absence of external movement; ii) to divide the body into a focal and a postural chain; and iii) to manipulate the surface contacts between the body and its supports, without perturbing body balance.

Effets de la contention élastique sur le flux veineux au cours de mouvements simples et de contractions musculaires du membre inférieur chez le volontaire sain: Étude pilote

Resume Objectif Etude du flux veineux femoral au cours de gestes du membre inferieur (mouvements simples, contractions musculaires volontaires) chez le sujet sain ; evaluation des effets de la contention elastique sur la chasse veineuse induite. Methodologie Travail experimental portant sur dix volontaires de 21 a 25 ans examines par echo-doppler apres avoir ete equipes de capteurs permettant de caracteriser la cinematique des mouvements et la mise en jeu des muscles. Mouvements etudies En decubitus strict : contraction de la cuisse et flexion dorsale du pied. En orthostatisme bipodal : contraction de la cuisse, tip-toe, transfert de charge vers l’avant, extension de la jambe sur le genou, mise en charge du membre, et flexion dorsale du pied. Signaux recueillis via une chaine d’acquisition informatisee pour une analyse minutieuse secondaire. Correlation de la vitesse moyenne en veine femorale commune a l’activite musculaire. Repetition des memes tests apres mise en place d’un bas de contention de classe II francaise (Varisma cuisse). Resultats et conclusion Tous les gestes etudies accroissent le flux veineux et la contention augmente la chasse veineuse. Il apparait que les differents sujets n’ont pas le meme niveau d’activite musculaire dans un meme geste. Ceci induit une variabilite des effets hemodynamiques d’un individu a l’autre. (J Mal Vasc 2005 ; 30 : 98-102)

The role of consecutive postural adjustments (CPAs) during postural control in isometric ramps pushes

Postural adjustments involved in the stabilising reactions, related to voluntary movement, have been described at least since Babinski’s (1899) observations. Since then, many experimental studies have been run. They focused mainly on anticipatory postural adjustments (APAs), following the pioneering work of Hopf and Hufschmidt (1963) and Belenkii et al. (1967). On the other hand, very little attention has been paid to the consecutive postural adjustments (CPAs), which occur after the end of voluntary movement. The aim of this paper was to stress the role of CPAs, as compared to APAs, and to specify certain aspects of postural control. To this end, maximal isometric ramp pushes were studied. They were performed by subjects in two sitting postures differing in the ischio-femoral contact with the seat. This task was chosen because it offers many advantages, which have already been developed (Le Bozec and Bouisset 2004). In this work, two of them can be emphasised: (i) since push originates in the focal chain, and as the upper limbs are fully extended, the kinetics should be located in the postural chain; and (ii) since the subjects are seated, a change in ischio-femoral contact with the seat is not difficult to obtain, which produces two different peak push force values, each of them resulting from a maximal effort, without perturbing gross body balance.

Developing Steady Laminar Flow through Uniform Straight Tubes with Varying Wall Cross Curvature

Numerical calculations are used to determine not only the wall shear stress but also the entry length in a laminar steady flow of an incompressible Newtonian fluid. The fluid is conveyed through rigid straight tubes with axially uniform cross sections, which mimic collapsed vessels. For each tube configuration, the “Navier–Stokes” equations are solved using the finite element method. The numerical tests are performed with the same value of the volume flow-rate whatever the tube configuration for three “Reynolds numbers”. The wall shear stress is computed and determined along the axis of the tube, then the entry length is estimated by introducing two indexes by using: (i) the axial fluid velocity, and (ii) the wall shear stress. The results are analysed in order to exhibit the mechanical environment of cultured endothelial cells in the flow chamber for which the test conditions will be well-defined. For example, in a tube configuration where the opposite walls are in contact for which the inner perimeter and the area of the cross section are respectively given by 45 mm and 37.02 mm2, the computed entry lengths with the criteria defined by (i) and (ii) are equals to about 118 and 126 mm, respectively for Address: INRIA, Action REO, BP 105, 78153 Le Chesnay Cédex, Francemarc.thiriet@inria.fr ribreau@univ-paris12.fr