Nicolas Forestier

Postural sway under muscle vibration and muscle fatigue in humans

Separate studies have demonstrated that vibration and fatigue of ankle muscles alter postural control. The purpose of the present experiment was to investigate the effect of ankle muscle vibration on the regulation of postural sway in bipedal stance following ankle muscle fatigue. Center of foot pressure displacements were recorded using a force platform. Results showed a similar increase in postural sway under muscle fatigue as well as under muscle vibration. Interestingly, under muscle fatigue muscle vibration did not induce a further increase in postural sway. Two hypotheses could, at least, account for this observation: (1). fatigued muscles are less sensitive to muscle vibration and (2). the central nervous system relies less upon proprioceptive information originating from fatigued muscles for regulating postural sway.

Effects of distal and proximal arm muscles fatigue on multi-joint movement organization

To investigate the strategies developed by the central nervous system to compensate for fatigue in muscles, we studied the changes in the relative mechanical contribution of the joint torques in a multi-joint movement following an isometric exhaustion test. Eighteen male subjects performed throws, moving the arm in the horizontal plane, before and after two fatigue protocols. Muscular fatigue was induced either in the distal (extensor digitorum communis) or in the proximal (triceps brachii) agonist muscle of the arm. The kinematic, kinetic and electromyographic parameters of the movement were analysed. The subjects produced two different coordinations following the fatigue protocols. In the distal fatigue condition, the wrist angular velocity was maintained by decreasing elbow active torque. In the proximal fatigue condition, the compensatory strategy involved increasing the contribution of the wrist. In fact, the control of elbow and wrist was modified in order to compensate for the different mechanical effects.

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.

Peroneal reaction time measurement in unipodal stance for two different destabilization axes

BACKGROUND The variability of peroneal reaction time measurements is a major problem when using this parameter to control rehabilitation or proprioceptive training processes. In order to control peroneal reaction time values, some extrinsic factors should be considered. The purpose of this study was to measure peroneal reaction time in unipodal stance for two different destabilization axes. METHODS The peroneal reaction time of 10 healthy subjects was measured from kinematic and electromyograhic data in an experimental study using an ankle destabilization device. FINDINGS In a preliminary analysis, results showed that the destabilization axis orientation did not affect peroneal reaction time values (68.5 ms, standard deviation=9.5 ms and 71.5 ms, standard deviation=8 ms for destabilizations in the frontal plane and around the Henkes axis, respectively). However, the inter-trial variance of inversion velocity peaks explained between 40% and 49% of the peroneal reaction time variance. When trials were selected on the basis of homogeneous inversion velocity peaks, results showed that peroneal reaction time values for the peroneus brevis were shorter during inversion movements performed around the physiological Henkes tilting axis (63 ms, standard deviation=9 ms vs. 71 ms, standard deviation=8 ms). INTERPRETATION Our findings evidenced that tilting axis orientation must be considered as an extrinsic factor that may influence peroneal reaction time. Moreover it also seems necessary to consider inversion speed values to adequately compare peroneal reaction time values.

Effect of terminal accuracy requirements on temporal gaze-hand coordination during fast discrete and reciprocal pointings.

BackgroundRapid discrete goal-directed movements are characterized by a well known coordination pattern between the gaze and the hand displacements. The gaze always starts prior to the hand movement and reaches the target before hand velocity peak. Surprisingly, the effect of the target size on the temporal gaze-hand coordination has not been directly investigated. Moreover, goal-directed movements are often produced in a reciprocal rather than in a discrete manner. The objectives of this work were to assess the effect of the target size on temporal gaze-hand coordination during fast 1) discrete and 2) reciprocal pointings.MethodsSubjects performed fast discrete (experiment 1) and reciprocal (experiment 2) pointings with an amplitude of 50 cm and four target diameters (7.6, 3.8, 1.9 and 0.95 cm) leading to indexes of difficulty (ID = log2[2A/D]) of 3.7, 4.7, 5.7 and 6.7 bits. Gaze and hand displacements were synchronously recorded. Temporal gaze-hand coordination parameters were compared between experiments (discrete and reciprocal pointings) and IDs using analyses of variance (ANOVAs).ResultsData showed that the magnitude of the gaze-hand lead pattern was much higher for discrete than for reciprocal pointings. Moreover, while it was constant for discrete pointings, it decreased systematically with an increasing ID for reciprocal pointings because of the longer duration of gaze anchoring on target.ConclusionOverall, the temporal gaze-hand coordination analysis revealed that even for high IDs, fast reciprocal pointings could not be considered as a concatenation of discrete units. Moreover, our data clearly illustrate the smooth adaptation of temporal gaze-hand coordination to terminal accuracy requirements during fast reciprocal pointings. It will be interesting for further researches to investigate if the methodology used in the experiment 2 allows assessing the effect of sensori-motor deficits on gaze-hand coordination.

Impaired control of weight bearing ankle inversion in subjects with chronic ankle instability.

BACKGROUND Previous studies have proposed that evertor muscle weakness represents an important factor affecting chronic ankle instability. For research purposes, ankle evertor strength is assessed by means of isokinetic evaluations. However, this methodology is constraining for daily clinical use. The present study proposes to assess ankle evertor muscle weakness using a new procedure, one that is easily accessible for rehabilitation specialists. To do so, we compared weight bearing ankle inversion control between patients suffering from chronic ankle instability and healthy subjects. METHODS 12 healthy subjects and 11 patients suffering from chronic ankle instability conducted repetitions of one leg weight bearing ankle inversion on a specific ankle destabilization device equipped with a gyroscope. Ankle inversion control was performed by means of an eccentric recruitment of evertor muscles. Instructions were to perform, as slow as possible, the ankle inversion while resisting against full body weight applied on the tested ankle. RESULTS Data clearly showed higher angular inversion velocity peaks in patients suffering from chronic ankle instability. This illustrates an impaired control of weight bearing ankle inversion and, by extension, an eccentric weakness of evertor muscles. INTERPRETATION The present study supports the hypothesis of a link between the decrease of ankle joint stability and evertor muscle weakness. Moreover, it appears that the new parameter is of use in a clinical setting.

Effects of fatigue of elbow extensor muscles voluntarily induced and induced by electromyostimulation on multi-joint movement organization

To investigate the capacity of the central nervous system to integrate and differentiate two different muscular fatigue states, the present study examines the changes on multi-joint movement organization following muscular fatigue of elbow extensor muscles (triceps brachii) induced by voluntary versus electrically induced contractions. Twenty right-handed male volunteers performed throws in the horizontal plane before and after two fatiguing procedures. First, success rate of throws was not affected by fatigue neither after voluntary contractions, nor after electrically induced contractions. Despite similar reductions of the maximal voluntary isometric force and the median frequency of the electromyographic signal following both fatiguing protocols, voluntary contractions induced greater changes in muscle activation, kinematics and kinetics during throws than electrically induced contractions. The changes observed following voluntary contractions are interpreted as a compensatory strategy involving a greater contribution of the wrist. In contrast, the greater activation of the triceps brachii could compensate the weakness of this muscle induced by fatigue without any modification of the initial multi-joint movement organization.

Ankle Muscular Proprioceptive Signals' Relevance for Balance Control on Various Support Surfaces An Exploratory Study

ObjectiveThe purpose of this study was to test the effect of various unstable support surfaces on the relevance of muscular proprioceptive signals originating from the ankle joint. DesignTen healthy subjects were instructed to stand as still as possible on a force plate during 40 secs on three different surfaces: (1) stable, (2) unstable-unspecific (foam), and (3) unstable-specific (inspired from rearfoot anatomy). Muscular vibration was applied on the paraspinals and fibularis muscles. The effects of vibrations on postural stability as well as fibularis longus and tibialis anterior electrical activities for each support condition were investigated. ResultsThe unstable-specific support surface was associated with higher fibularis muscular activity and greater postural perturbations when fibularis muscles were vibrated than the unspecific-unstable surface. ConclusionBalance control on unstable-specific support surface maintains the relevance on muscular proprioceptive signals originating from ankle and increases ankle evertor muscle activity.

Compensation of lateralized fatigue due to referent static positional signals in an ankle-matching task. A feedforward mechanism.

The main objective of this study was to examine whether static and dynamic signals from a non-fatigued reference ankle can differently improve the movement accuracy of a fatigued ankle. To address this question, subjects performed an ankle-matching task in a control condition and in a condition of local fatigue induced in the right tibialis anterior, in two matching conditions. In a bilateral condition, the matching task was completed with the two ankles simultaneously. In a unilateral condition subjects had to match the exact target position with the left non-fatigued ankle first and then tried to match the target position with the right fatigued ankle. Results showed that the final accuracy of the right fatigued ankle was degraded when matched with both ankles simultaneously while, for the unilateral condition, the final accuracy remained constant whether the ankle was fatigued or not. In addition, the EMG activity of the right tibialis increased significantly in the acceleration portion of the ankle movement for the unilateral condition. These results suggest that the CNS was able to integrate the correct static positional signals originating from the left reference ankle, to improve the movement accuracy of the controlateral fatigued ankle. This compensation of lateralized fatigue is operated in a feedforward manner.

Effets d’une fatigue unilatérale du triceps surae sur la posture non-perturbée

OBJECTIVE Fatigue of lower limb presents various impacts on postural control. The purpose of this study was to explore the effects of a unilateral lower-limb muscle fatigue on undisturbed stance. METHODS Nine healthy men performed until exhaustion a fatiguing exercise of the limb extensor muscles of one leg (exercised leg). Just before and after the fatiguing exercise, postural sway measures (centre of pressure) were recorded simultaneously from a double force platform and an in-shoe pressure system in quiet standing conditions. Maximal plantar pressure was computed from each sensor of the in-shoe system, before being averaged for the medial and lateral heel, the external mid-foot, the external and internal foreparts. RESULTS In undisturbed stance, the fatiguing exercise induced postural destabilisation for the non-exercised leg (P < 0.001). Changes in antero-posterior mean position of the exercised leg were also observed reflecting a mean loading in a more rear foot stance (P < 0.01). The in-shoe system parameters revealed for the exercised foot, an increase of the maximal pressure value for the heel region (P < 0.05) and a decrease for the mid-foot and forepart (P < 0.05). For the non-exercised foot, the maximal pressure decreased only in forepart (P < 0.05). CONCLUSIONS Unilateral triceps surae fatigue revealed an immediate destabilisation of undisturbed stance and the observed postural strategy appears similar to these developed by patients who presented pain pathology and/or sensory deficits in lower limb.