Sébastien Boyas - Academia.edu (original) (raw)

Papers by Sébastien Boyas

Research paper thumbnail of Endurance Time Prediction using Electromyography

The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing ... more The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing muscle contraction before it occurs. Tlim prediction would then allow the evaluation of muscle capacities while limiting fatigue and deleterious effects associated with exhaustive exercises. Fatigue is a progressive phenomenon which manifestations can be observed since the beginning of the exercise using electromyography (EMG). Studies have reported significant relationships between Tlim and changes in EMG signal suggesting that Tlim could be predicted from early EMG changes recorded during the first half of the fatiguing contraction. However some methodological factors can influence the reliability of the relationships between Tlim and EMG changes. The aim of this chapter is to present the methodology used to predict Tlim from early changes in EMG signal and the factors that may influence its feasibility and reliability. It will also present the possible uses and benefits of the Tlim prediction.

Research paper thumbnail of Endurance time prediction using EMG

The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing ... more The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing muscle contraction before it occurs. Tlim prediction would then allow the evaluation of muscle capacities while limiting fatigue and deleterious effects associated with exhaustive exercises. Fatigue is a progressive phenomenon which manifestations can be observed since the beginning of the exercise using electromyography (EMG). Studies have reported significant relationships between Tlim and changes in EMG signal suggesting that Tlim could be predicted from early EMG changes recorded during the first half of the fatiguing contraction. However some methodological factors can influence the reliability of the relationships between Tlim and EMG changes. The aim of this chapter is to present the methodology used to predict Tlim from early changes in EMG signal and the factors that may influence its feasibility and reliability. It will also present the possible uses and benefits of the Tlim prediction.

Research paper thumbnail of Body stance influences the visual vertical perception in patients with vestibular disorders

Computer Methods in Biomechanics and Biomedical Engineering, 2015

a laboratoire 'motricité, interactions, performance' (ea 4334), université du maine, le mans, Fra... more a laboratoire 'motricité, interactions, performance' (ea 4334), université du maine, le mans, France; b Cabinet orl docteur philippe lorin, Vertiges & rééducation des troubles Vestibulaires, le mans, France ABSTRACT To describe the visual vertical perception ability during dynamic optokinetic stimulations, 10 visual dependent patients (VD) and 10 patients with unilateral vestibular hypo-areflexia (HPA), completed the Rolf Jacob (RJ) questionnaire and performed the rod and disk test (RDT: Seated and Erected: S-E). RJ: HPA were significantly less sensitive than VD to phobia and the visual criteria (p<0.05). RDT: in erected stance, when the disk and the rod were orientated in the same direction, VD observed less vertical deviation than HPA (p<0.05). All RDT criteria showed only a significant deviation decrease in erected stance for VD (p<0.05). HPA patients did not compensate visual vertical perception deficit when erected. Rehabilitation with physical activity interventions could lead to improved proprioceptive responses and faster compensation.

Research paper thumbnail of Influence of ankle plantarflexor fatiguing exercise intensity on postural control

INTRODUCTION. Numerous studies have reported an impairment in postural control during quiet stand... more INTRODUCTION. Numerous studies have reported an impairment in postural control during quiet standing after fatigue of the ankle plantarflexor muscles. These studies used many types of fatiguing exercises, including exercises differing in their intensity. The purpose of this study was to investigate the influence of the intensity of ankle plantarflexors fatiguing exercises on the impairment in postural control during unipedal quiet standing with eyes closed. METHODS. Twenty (10 men, 10 women) healthy adults (21.2 ± 3.2 years, 172.4 ± 10.3 cm, 68.8 ± 13.2 kg) with no history of lower limb injury participated in this study. Subjects randomly performed three experimental sessions. All sessions were identical except for the intensity of the fatiguing exercise. The fatiguing exercises consisted in maintaining an isometric contraction of the ankle plantarflexor muscles until the torque produced decreased below i) 25% of the pre-fatigue maximal voluntary isometric torque (MVIT); ii) 50% of ...

Research paper thumbnail of Effects of static and dynamic ankle fatiguing exercises on postural control

INTRODUCTION. The use of different fatiguing exercises to induce alterations in postural control ... more INTRODUCTION. The use of different fatiguing exercises to induce alterations in postural control across studies makes it hard to truly understand the mechanisms that could explain the reduction in postural control with fatigue. This study aimed to compare the effects of static versus dynamic fatiguing plantarflexion exercises on postural control on a firm and a compliant surface. METHODS. Ten healthy young men (29 ± 4 years, 178.6 ± 7.5 cm, 73.6 ± 9.6 kg) were asked to stand as steadily as possible for 30 s, blindfolded with feet together, on a firm and a compliant surface. Three trials of each of the two surface conditions (firm; compliant) were performed before and immediately after a) a static fatiguing exercise where subjects maintained an isometric contraction of the plantarflexors at 50% of the maximal voluntary contraction (MVC) until exhaustion, and b) a dynamic fatiguing exercise where subjects performed repeated isokinetic MVCs of the plantarflexors at 30°/s until force de...

Research paper thumbnail of Effects of instructions and postural task difficulty on dual-task performances in young adults

BACKGROUND. Many activities of daily living require maintaining an upright stance while performin... more BACKGROUND. Many activities of daily living require maintaining an upright stance while performing a cognitive task. Studies that investigated the attentional cost required to control posture during different dual-task contexts have led to equivocal results. Instructions and the difficulty of the postural task have been identified as potential factors influencing dual-task performance. The aim of this study was to examine how the instructed focus of attention influenced dual-task performance during quiet standing tasks of various levels of difficulty. METHODS. Thirteen young adults (7 women; 6 men) participated in two testing sessions consisting of standing as still as possible on a force platform in different postural conditions, while performing a simple reaction time (RT) task (i.e. verbal responses to auditory stimuli). These two sessions were identical, except for the instructions given to the participants relative to the focus of attention during dual-task conditions. Dependin...

Research paper thumbnail of Specific Neuromuscular Responses of High Skilled Laser Sailors during a Multi-Joint Posture Sustained Until Exhaustion

International Journal of Sports Medicine, 2006

The major physical demands of Olympic dinghy sailing are related to the maintenance of hiking pos... more The major physical demands of Olympic dinghy sailing are related to the maintenance of hiking posture during extended periods of several minutes in order to balance the capsizing effect of the force of the wind on the sail . Hiking is mainly achieved by leaning the body out over the windward side of the boat using sustained isometric actions of anterior bilateral muscular groups that cross the hip and knee joints . Previous studies have reported a higher endurance capacity in dinghy sailors during simulated isometric and dynamic hiking tasks compared to nonhiker sailors or physical students . Furthermore, previous studies have suggested that the hiking-induced level of isometric contraction predominantly determined the metabolic

Research paper thumbnail of Fatiguing exercise intensity influences the relationship between parameters reflecting neuromuscular function and postural control variables.

PLoS One, 2013

The purpose of this study was to investigate the influence of fatiguing exercise intensity on the... more The purpose of this study was to investigate the influence of fatiguing exercise intensity on the nature and extent of fatigue-induced changes in neuromuscular function and postural stability in quiet standing. We also explored the contribution of selected neuromuscular mechanisms involved in force production to postural stability impairment observed following fatigue using an approach based on multivariate regressions. Eighteen young subjects performed 30-s postural trials on one leg with their eyes closed. Postural trials were performed before and after fatiguing exercises of different intensities: 25, 50 and 75% of maximal isometric plantarflexor torque. Fatiguing exercises consisted of sustaining a plantarflexor isometric contraction at the target intensity until task failure. Maximal isometric plantarflexor torque, electromyographic activity of plantarflexor and dorsiflexor muscles, activation level (twitch interpolation technique) and twitch contractile properties of plantarflexors were used to characterize neuromuscular function. The 25% exercise was associated with greater central fatigue whereas the 50 and 75% exercises involved mostly peripheral fatigue. However, all fatiguing exercises induced similar alterations in postural stability, which was unexpected considering previous literature. Stepwise multiple regression analyses showed that fatigue-related changes in selected parameters related to neuromuscular function could explain more than half (0.51≤R(2)≤0.82) of the changes in postural variables for the 25% exercise. On the other hand, regression models were less predictive (0.17≤R(2)≤0.73) for the 50 and 75% exercises. This study suggests that fatiguing exercise intensity does not influence the extent of postural stability impairment, but does influence the type of fatigue induced and the neuromuscular function predictors explaining changes in postural variables.

Research paper thumbnail of Power Responses of a Rowing Ergometer: Mechanical Sensors vs. Concept2® Measurement System

International Journal of Sports Medicine, 2006

The aim of this study was to compare the power provided by a recent ergometer with the power deve... more The aim of this study was to compare the power provided by a recent ergometer with the power developed by the rower determined using mechanical sensors set on the same apparatus. Six rowers and six non-rowers performed a power graded test and an all-out start on an instrumented ergometer (Concept2 system, model D, Morrisville, VT, USA). Power values displayed by the ergometer were recorded with a specific software. A strain gauge placed near the handle and a position sensor installed on the chain allowed the calculation of the power developed by the rower. Power values provided by the ergometer were strongly correlated to those determined with a direct measurement and calculation of power. However, power values given by the Concept2 system were lower (- 17.4 to - 72.4 W) than those calculated using mechanical sensors. This difference in power measurements was lower at a steady pace and for rowers. The Concept2 system underestimates the power produced by the rower by approximately 25 W. This difference in power seems to be independent of the level of power developed but increases with variations in intensity and pace. The deletion of the first strokes following changes in power production allows to limit this phenomenon. According to the use of the power parameter in the experimental design, it could be appropriate to correct values provided by the Concept2 ergometer.

Research paper thumbnail of Changes in posture associated with postural control impairment induced by ankle plantarflexor fatigue during unipedal quiet standing

Neuroscience Letters, 2013

h i g h l i g h t s

Research paper thumbnail of Effects of fatiguing isometric and isokinetic ankle exercises on postural control while standing on firm and compliant surfaces

Journal of NeuroEngineering and Rehabilitation, 2012

BACKGROUND: Fatiguing exercises used across studies to induce alterations in postural control are... more BACKGROUND: Fatiguing exercises used across studies to induce alterations in postural control are diverse and may explain the different findings reported. This study aimed to compare the effects of two types of fatiguing plantarflexion exercises on postural control on a firm and a compliant surface. Ten healthy young men (29 ± 4 years) were asked to stand as steadily as possible for 30 s, blindfolded with feet together, on a firm and a compliant surface before and immediately after an isometric and an isokinetic fatiguing exercise. RESULTS: Maximal force reduction due to fatigue was found significant but similar between exercises. No significant difference was found between the fatiguing exercises on all Center of Pressure (CoP) parameters. Both fatiguing exercises induced increases in CoP excursion area, CoP variability and CoP velocity in both planes (antero-posterior, mediolateral) on the compliant surface. On the firm surface, both fatiguing exercises only induced increases in CoP variability and CoP velocity in the fatigued plane (antero-posterior). CONCLUSIONS: Isometric and isokinetic fatiguing exercises, when producing a similar level of force reduction, induce similar decreases in postural control. The effects of fatigue on postural control in healthy young men are more pronounced when standing on a compliant surface, i.e. when proprioceptive information at the ankle is altered.

Research paper thumbnail of Attentional Demands Associated With Postural Control Depend on Task Difficulty and Visual Condition

Journal of Motor Behavior, 2012

The authors aimed to assess the effect of vision on variations in attentional resources allocated... more The authors aimed to assess the effect of vision on variations in attentional resources allocated to postural control during tasks of various levels of difficulty. Test-retest reliability of postural and cognitive parameters was also evaluated. Twenty adults stood quietly on a force platform during 30-s trials (primary task). Twelve conditions involving combinations of three bases of support, two vision conditions, and the presence or absence of a simple reaction time task (secondary task) were tested. Baseline trials of the reaction time task were also performed with the participants seated. Reaction time and postural parameters demonstrated good to excellent test-retest reliability in most conditions. Postural control was altered by the reduction of the base of support and by the absence of vision. Maintaining an upright stance increased reaction time compared with a seated position, indicating that quiet standing tasks required some attention even in young adults. Changes in postural steadiness were correlated with changes in reaction time, showing a significant relationship between the difficulty of the postural task and the attentional resources allocated to postural control. However, reaction time increased with the reduction of the base of support only without vision. This dual task paradigm showed that vision can compensate for the increase in attentional demands during the most difficult postural tasks.

Research paper thumbnail of Changes in sEMG parameters among trunk and thigh muscles during a fatiguing bilateral isometric multi-joint task in trained and untrained subjects

Journal of Electromyography and Kinesiology, 2009

This study aimed to explore changes in the electrical activity distribution among synergist muscl... more This study aimed to explore changes in the electrical activity distribution among synergist muscles involved in the maintenance of a bilateral multi-joint task, the hiking position. It also tested relations between changes in surface electromyographic (sEMG) parameters with endurance time. Eighteen subjects, trained and untrained in hiking, performed a submaximal (50% of maximal contraction) isometric hiking test until exhaustion. The electrical activity of main superficial muscles implicated in this posture was recorded bilaterally. Trained subjects sustained the hiking position for 315 ± 82 s, versus 225 ± 68 s for untrained subjects. Patterns of electrical activity and mean power frequency (MPF) were different between populations. MPF shift in abdominal muscles was higher than in other synergists for both groups. Although typical changes in sEMG parameters were observed, few relations with endurance time were found, and for untrained subjects only. Changes in the relative contribution among synergists were observed, mainly for trained subjects. It is hypothesized that the task (a complex multi-joint posture involving numerous joints and muscles) may allow some variability in the contribution of synergist muscles during fatigue especially for the trained group. This probably explains the absence of relationship between endurance time and sEMG changes for trained subjects.

Research paper thumbnail of Influence of ankle plantarflexor fatigue on postural sway, lower limb articular angles, and postural strategies during unipedal quiet standing

Gait & Posture, 2013

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.

Research paper thumbnail of Attentional focus influences postural control and reaction time performances only during challenging dual-task conditions in healthy young adults.

Experimental Brain Research, 2013

The dual-task paradigm has previously been used to investigate the attentional demands associated... more The dual-task paradigm has previously been used to investigate the attentional demands associated with postural control. Previous studies have identified both the focus of attention and the difficulty of a postural task as potential factors influencing dual-task performance. The aim of this study was to examine how the instructed focus of attention influences dual-task performance during quiet standing tasks of various levels of difficulty. Thirteen young adults participated in two testing sessions consisting of standing as still as possible on a force platform in different postural conditions, while simultaneously performing a simple reaction time (RT) task. Postural task difficulty was manipulated by various combinations of three bases of support (feet together, tandem and single leg) and two visual conditions (eyes opened and closed). Participants were instructed to focus on either their balance or their RT performance, depending on the testing session. When comparing postural control with respect to session focus, anterior-posterior sway velocity decreased with the addition of the simple RT task when the focus was on balance, but only during the more difficult dual-task conditions. In contrast, sway area and medial-lateral sway velocity did not change between the two instructed focus sessions. Participants responded faster in all dual-task conditions when focusing on RT performance than on balance. The modified attention allocation index indicated that participants' ability to modulate their allocation of attentional resources to respond positively to instruction was more pronounced in the most challenging postural condition. The present findings could have important implications for the interpretation of dual-task performance in both clinical and research settings.

Research paper thumbnail of Influence of exercise intensity and joint angle on endurance time prediction of sustained submaximal isometric knee extensions

European Journal of Applied Physiology, 2011

The purpose of endurance time (T (lim)) prediction is to determine the exertion time of a muscle ... more The purpose of endurance time (T (lim)) prediction is to determine the exertion time of a muscle contraction before it occurs. T (lim) prediction would then allow the evaluation of muscle capacities limiting fatigue and deleterious effects associated with exhaustive exercises. The present study aimed to analyze the influence of exercise intensity and joint angle on T (lim) prediction using changes in surface electromyographic (sEMG) signals recorded during the first moments of the exercise. Fifteen male performed four knee extensions sustained until exhaustion that were different in exercise intensity (20% or 50% of maximal voluntary torque-MVT) and in joint angle (40 or 70º, 0° = full extension). T (lim) prediction was explored using some parameters of the sEMG signals from rectus femoris, vastus medialis and vastus lateralis muscles. Changes in sEMG parameters (root mean square, mean power frequency and frequency banding 6-30 Hz) were expressed using the slope of the linear regression and the area ratio index. Results indicated that relationships between changes in sEMG signal and T (lim) (0.51 &amp;amp;amp;amp;lt; r &amp;amp;amp;amp;lt; 0.83) were greater for experimental conditions associated with higher exercise intensity (50% MVT) and so to lower time duration. Knee joint angle had little influence on T (lim) prediction results. Results also showed higher T (lim) prediction considering spectral parameters and area ratio. This could be in relation to differences in relative contribution of central and peripheral fatigue that seems to change according to the exercise intensity, but also to the influence of psychological factors that increases with the duration of the task.

Research paper thumbnail of Neuromuscular fatigue in healthy muscle: Underlying factors and adaptation mechanisms

Annals of Physical and Rehabilitation Medicine, 2011

This review aims to define the concept of neuromuscular fatigue and to present the current knowle... more This review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue. One hundred and eighty-two articles indexed in PubMed (1954-2010) have been considered. Neuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units. Fatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production.

Research paper thumbnail of Impairment in postural control is greater when ankle plantarflexors and dorsiflexors are fatigued simultaneously than when fatigued separately

Gait & Posture, 2011

The aim of the present study was to compare the effects of ankle muscle fatigue on postural contr... more The aim of the present study was to compare the effects of ankle muscle fatigue on postural control when plantarflexors (PFs) and dorsiflexors (DFs) are fatigued simultaneously compared with separately. This study also investigated the recovery of postural control after fatigue. Sixteen adults (eight women and eight men) performed postural trials before and after an isokinetic fatigue task involving either (i) only PFs (30°s(-1)), (ii) only DFs (120°s(-1)), or (iii) both PFs and DFs simultaneously. The fatigue task involved maximal contractions repeated until the torque produced decreased below 50% of the maximal torque. Postural trials lasted 30s and were performed on one leg with eyes open (EO) or eyes closed (EC). Sway area, medio-lateral (ML) and antero-posterior (AP) positions and velocities were calculated from the center of pressure displacements. With EO, no effect of fatigue was found on postural variables. With EC, sway area and AP velocity increased only when both PFs and DFs were fatigued simultaneously. An effect of fatigue present only when both muscle groups are fatigued simultaneously could be due to impairment in the compensatory activity between agonist and antagonist muscles and/or a greater decrease in proprioception due to a greater number of fatigued muscles. In addition, when PFs and DFs were fatigued simultaneously, sway area and AP velocity returned to pre-fatigue values within 2min, whereas a posterior shift in AP position persisted for 10min. This last result may suggest a longer-lasting change in postural strategy needed for optimal postural control.

Research paper thumbnail of Endurance Time Prediction using Electromyography

The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing ... more The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing muscle contraction before it occurs. Tlim prediction would then allow the evaluation of muscle capacities while limiting fatigue and deleterious effects associated with exhaustive exercises. Fatigue is a progressive phenomenon which manifestations can be observed since the beginning of the exercise using electromyography (EMG). Studies have reported significant relationships between Tlim and changes in EMG signal suggesting that Tlim could be predicted from early EMG changes recorded during the first half of the fatiguing contraction. However some methodological factors can influence the reliability of the relationships between Tlim and EMG changes. The aim of this chapter is to present the methodology used to predict Tlim from early changes in EMG signal and the factors that may influence its feasibility and reliability. It will also present the possible uses and benefits of the Tlim prediction.

Research paper thumbnail of Endurance time prediction using EMG

The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing ... more The purpose of endurance time (Tlim) prediction is to determine the exertion time of a fatiguing muscle contraction before it occurs. Tlim prediction would then allow the evaluation of muscle capacities while limiting fatigue and deleterious effects associated with exhaustive exercises. Fatigue is a progressive phenomenon which manifestations can be observed since the beginning of the exercise using electromyography (EMG). Studies have reported significant relationships between Tlim and changes in EMG signal suggesting that Tlim could be predicted from early EMG changes recorded during the first half of the fatiguing contraction. However some methodological factors can influence the reliability of the relationships between Tlim and EMG changes. The aim of this chapter is to present the methodology used to predict Tlim from early changes in EMG signal and the factors that may influence its feasibility and reliability. It will also present the possible uses and benefits of the Tlim prediction.

Research paper thumbnail of Body stance influences the visual vertical perception in patients with vestibular disorders

Computer Methods in Biomechanics and Biomedical Engineering, 2015

a laboratoire 'motricité, interactions, performance' (ea 4334), université du maine, le mans, Fra... more a laboratoire 'motricité, interactions, performance' (ea 4334), université du maine, le mans, France; b Cabinet orl docteur philippe lorin, Vertiges & rééducation des troubles Vestibulaires, le mans, France ABSTRACT To describe the visual vertical perception ability during dynamic optokinetic stimulations, 10 visual dependent patients (VD) and 10 patients with unilateral vestibular hypo-areflexia (HPA), completed the Rolf Jacob (RJ) questionnaire and performed the rod and disk test (RDT: Seated and Erected: S-E). RJ: HPA were significantly less sensitive than VD to phobia and the visual criteria (p<0.05). RDT: in erected stance, when the disk and the rod were orientated in the same direction, VD observed less vertical deviation than HPA (p<0.05). All RDT criteria showed only a significant deviation decrease in erected stance for VD (p<0.05). HPA patients did not compensate visual vertical perception deficit when erected. Rehabilitation with physical activity interventions could lead to improved proprioceptive responses and faster compensation.

Research paper thumbnail of Influence of ankle plantarflexor fatiguing exercise intensity on postural control

INTRODUCTION. Numerous studies have reported an impairment in postural control during quiet stand... more INTRODUCTION. Numerous studies have reported an impairment in postural control during quiet standing after fatigue of the ankle plantarflexor muscles. These studies used many types of fatiguing exercises, including exercises differing in their intensity. The purpose of this study was to investigate the influence of the intensity of ankle plantarflexors fatiguing exercises on the impairment in postural control during unipedal quiet standing with eyes closed. METHODS. Twenty (10 men, 10 women) healthy adults (21.2 ± 3.2 years, 172.4 ± 10.3 cm, 68.8 ± 13.2 kg) with no history of lower limb injury participated in this study. Subjects randomly performed three experimental sessions. All sessions were identical except for the intensity of the fatiguing exercise. The fatiguing exercises consisted in maintaining an isometric contraction of the ankle plantarflexor muscles until the torque produced decreased below i) 25% of the pre-fatigue maximal voluntary isometric torque (MVIT); ii) 50% of ...

Research paper thumbnail of Effects of static and dynamic ankle fatiguing exercises on postural control

INTRODUCTION. The use of different fatiguing exercises to induce alterations in postural control ... more INTRODUCTION. The use of different fatiguing exercises to induce alterations in postural control across studies makes it hard to truly understand the mechanisms that could explain the reduction in postural control with fatigue. This study aimed to compare the effects of static versus dynamic fatiguing plantarflexion exercises on postural control on a firm and a compliant surface. METHODS. Ten healthy young men (29 ± 4 years, 178.6 ± 7.5 cm, 73.6 ± 9.6 kg) were asked to stand as steadily as possible for 30 s, blindfolded with feet together, on a firm and a compliant surface. Three trials of each of the two surface conditions (firm; compliant) were performed before and immediately after a) a static fatiguing exercise where subjects maintained an isometric contraction of the plantarflexors at 50% of the maximal voluntary contraction (MVC) until exhaustion, and b) a dynamic fatiguing exercise where subjects performed repeated isokinetic MVCs of the plantarflexors at 30°/s until force de...

Research paper thumbnail of Effects of instructions and postural task difficulty on dual-task performances in young adults

BACKGROUND. Many activities of daily living require maintaining an upright stance while performin... more BACKGROUND. Many activities of daily living require maintaining an upright stance while performing a cognitive task. Studies that investigated the attentional cost required to control posture during different dual-task contexts have led to equivocal results. Instructions and the difficulty of the postural task have been identified as potential factors influencing dual-task performance. The aim of this study was to examine how the instructed focus of attention influenced dual-task performance during quiet standing tasks of various levels of difficulty. METHODS. Thirteen young adults (7 women; 6 men) participated in two testing sessions consisting of standing as still as possible on a force platform in different postural conditions, while performing a simple reaction time (RT) task (i.e. verbal responses to auditory stimuli). These two sessions were identical, except for the instructions given to the participants relative to the focus of attention during dual-task conditions. Dependin...

Research paper thumbnail of Specific Neuromuscular Responses of High Skilled Laser Sailors during a Multi-Joint Posture Sustained Until Exhaustion

International Journal of Sports Medicine, 2006

The major physical demands of Olympic dinghy sailing are related to the maintenance of hiking pos... more The major physical demands of Olympic dinghy sailing are related to the maintenance of hiking posture during extended periods of several minutes in order to balance the capsizing effect of the force of the wind on the sail . Hiking is mainly achieved by leaning the body out over the windward side of the boat using sustained isometric actions of anterior bilateral muscular groups that cross the hip and knee joints . Previous studies have reported a higher endurance capacity in dinghy sailors during simulated isometric and dynamic hiking tasks compared to nonhiker sailors or physical students . Furthermore, previous studies have suggested that the hiking-induced level of isometric contraction predominantly determined the metabolic

Research paper thumbnail of Fatiguing exercise intensity influences the relationship between parameters reflecting neuromuscular function and postural control variables.

PLoS One, 2013

The purpose of this study was to investigate the influence of fatiguing exercise intensity on the... more The purpose of this study was to investigate the influence of fatiguing exercise intensity on the nature and extent of fatigue-induced changes in neuromuscular function and postural stability in quiet standing. We also explored the contribution of selected neuromuscular mechanisms involved in force production to postural stability impairment observed following fatigue using an approach based on multivariate regressions. Eighteen young subjects performed 30-s postural trials on one leg with their eyes closed. Postural trials were performed before and after fatiguing exercises of different intensities: 25, 50 and 75% of maximal isometric plantarflexor torque. Fatiguing exercises consisted of sustaining a plantarflexor isometric contraction at the target intensity until task failure. Maximal isometric plantarflexor torque, electromyographic activity of plantarflexor and dorsiflexor muscles, activation level (twitch interpolation technique) and twitch contractile properties of plantarflexors were used to characterize neuromuscular function. The 25% exercise was associated with greater central fatigue whereas the 50 and 75% exercises involved mostly peripheral fatigue. However, all fatiguing exercises induced similar alterations in postural stability, which was unexpected considering previous literature. Stepwise multiple regression analyses showed that fatigue-related changes in selected parameters related to neuromuscular function could explain more than half (0.51≤R(2)≤0.82) of the changes in postural variables for the 25% exercise. On the other hand, regression models were less predictive (0.17≤R(2)≤0.73) for the 50 and 75% exercises. This study suggests that fatiguing exercise intensity does not influence the extent of postural stability impairment, but does influence the type of fatigue induced and the neuromuscular function predictors explaining changes in postural variables.

Research paper thumbnail of Power Responses of a Rowing Ergometer: Mechanical Sensors vs. Concept2® Measurement System

International Journal of Sports Medicine, 2006

The aim of this study was to compare the power provided by a recent ergometer with the power deve... more The aim of this study was to compare the power provided by a recent ergometer with the power developed by the rower determined using mechanical sensors set on the same apparatus. Six rowers and six non-rowers performed a power graded test and an all-out start on an instrumented ergometer (Concept2 system, model D, Morrisville, VT, USA). Power values displayed by the ergometer were recorded with a specific software. A strain gauge placed near the handle and a position sensor installed on the chain allowed the calculation of the power developed by the rower. Power values provided by the ergometer were strongly correlated to those determined with a direct measurement and calculation of power. However, power values given by the Concept2 system were lower (- 17.4 to - 72.4 W) than those calculated using mechanical sensors. This difference in power measurements was lower at a steady pace and for rowers. The Concept2 system underestimates the power produced by the rower by approximately 25 W. This difference in power seems to be independent of the level of power developed but increases with variations in intensity and pace. The deletion of the first strokes following changes in power production allows to limit this phenomenon. According to the use of the power parameter in the experimental design, it could be appropriate to correct values provided by the Concept2 ergometer.

Research paper thumbnail of Changes in posture associated with postural control impairment induced by ankle plantarflexor fatigue during unipedal quiet standing

Neuroscience Letters, 2013

h i g h l i g h t s

Research paper thumbnail of Effects of fatiguing isometric and isokinetic ankle exercises on postural control while standing on firm and compliant surfaces

Journal of NeuroEngineering and Rehabilitation, 2012

BACKGROUND: Fatiguing exercises used across studies to induce alterations in postural control are... more BACKGROUND: Fatiguing exercises used across studies to induce alterations in postural control are diverse and may explain the different findings reported. This study aimed to compare the effects of two types of fatiguing plantarflexion exercises on postural control on a firm and a compliant surface. Ten healthy young men (29 ± 4 years) were asked to stand as steadily as possible for 30 s, blindfolded with feet together, on a firm and a compliant surface before and immediately after an isometric and an isokinetic fatiguing exercise. RESULTS: Maximal force reduction due to fatigue was found significant but similar between exercises. No significant difference was found between the fatiguing exercises on all Center of Pressure (CoP) parameters. Both fatiguing exercises induced increases in CoP excursion area, CoP variability and CoP velocity in both planes (antero-posterior, mediolateral) on the compliant surface. On the firm surface, both fatiguing exercises only induced increases in CoP variability and CoP velocity in the fatigued plane (antero-posterior). CONCLUSIONS: Isometric and isokinetic fatiguing exercises, when producing a similar level of force reduction, induce similar decreases in postural control. The effects of fatigue on postural control in healthy young men are more pronounced when standing on a compliant surface, i.e. when proprioceptive information at the ankle is altered.

Research paper thumbnail of Attentional Demands Associated With Postural Control Depend on Task Difficulty and Visual Condition

Journal of Motor Behavior, 2012

The authors aimed to assess the effect of vision on variations in attentional resources allocated... more The authors aimed to assess the effect of vision on variations in attentional resources allocated to postural control during tasks of various levels of difficulty. Test-retest reliability of postural and cognitive parameters was also evaluated. Twenty adults stood quietly on a force platform during 30-s trials (primary task). Twelve conditions involving combinations of three bases of support, two vision conditions, and the presence or absence of a simple reaction time task (secondary task) were tested. Baseline trials of the reaction time task were also performed with the participants seated. Reaction time and postural parameters demonstrated good to excellent test-retest reliability in most conditions. Postural control was altered by the reduction of the base of support and by the absence of vision. Maintaining an upright stance increased reaction time compared with a seated position, indicating that quiet standing tasks required some attention even in young adults. Changes in postural steadiness were correlated with changes in reaction time, showing a significant relationship between the difficulty of the postural task and the attentional resources allocated to postural control. However, reaction time increased with the reduction of the base of support only without vision. This dual task paradigm showed that vision can compensate for the increase in attentional demands during the most difficult postural tasks.

Research paper thumbnail of Changes in sEMG parameters among trunk and thigh muscles during a fatiguing bilateral isometric multi-joint task in trained and untrained subjects

Journal of Electromyography and Kinesiology, 2009

This study aimed to explore changes in the electrical activity distribution among synergist muscl... more This study aimed to explore changes in the electrical activity distribution among synergist muscles involved in the maintenance of a bilateral multi-joint task, the hiking position. It also tested relations between changes in surface electromyographic (sEMG) parameters with endurance time. Eighteen subjects, trained and untrained in hiking, performed a submaximal (50% of maximal contraction) isometric hiking test until exhaustion. The electrical activity of main superficial muscles implicated in this posture was recorded bilaterally. Trained subjects sustained the hiking position for 315 ± 82 s, versus 225 ± 68 s for untrained subjects. Patterns of electrical activity and mean power frequency (MPF) were different between populations. MPF shift in abdominal muscles was higher than in other synergists for both groups. Although typical changes in sEMG parameters were observed, few relations with endurance time were found, and for untrained subjects only. Changes in the relative contribution among synergists were observed, mainly for trained subjects. It is hypothesized that the task (a complex multi-joint posture involving numerous joints and muscles) may allow some variability in the contribution of synergist muscles during fatigue especially for the trained group. This probably explains the absence of relationship between endurance time and sEMG changes for trained subjects.

Research paper thumbnail of Influence of ankle plantarflexor fatigue on postural sway, lower limb articular angles, and postural strategies during unipedal quiet standing

Gait & Posture, 2013

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.

Research paper thumbnail of Attentional focus influences postural control and reaction time performances only during challenging dual-task conditions in healthy young adults.

Experimental Brain Research, 2013

The dual-task paradigm has previously been used to investigate the attentional demands associated... more The dual-task paradigm has previously been used to investigate the attentional demands associated with postural control. Previous studies have identified both the focus of attention and the difficulty of a postural task as potential factors influencing dual-task performance. The aim of this study was to examine how the instructed focus of attention influences dual-task performance during quiet standing tasks of various levels of difficulty. Thirteen young adults participated in two testing sessions consisting of standing as still as possible on a force platform in different postural conditions, while simultaneously performing a simple reaction time (RT) task. Postural task difficulty was manipulated by various combinations of three bases of support (feet together, tandem and single leg) and two visual conditions (eyes opened and closed). Participants were instructed to focus on either their balance or their RT performance, depending on the testing session. When comparing postural control with respect to session focus, anterior-posterior sway velocity decreased with the addition of the simple RT task when the focus was on balance, but only during the more difficult dual-task conditions. In contrast, sway area and medial-lateral sway velocity did not change between the two instructed focus sessions. Participants responded faster in all dual-task conditions when focusing on RT performance than on balance. The modified attention allocation index indicated that participants' ability to modulate their allocation of attentional resources to respond positively to instruction was more pronounced in the most challenging postural condition. The present findings could have important implications for the interpretation of dual-task performance in both clinical and research settings.

Research paper thumbnail of Influence of exercise intensity and joint angle on endurance time prediction of sustained submaximal isometric knee extensions

European Journal of Applied Physiology, 2011

The purpose of endurance time (T (lim)) prediction is to determine the exertion time of a muscle ... more The purpose of endurance time (T (lim)) prediction is to determine the exertion time of a muscle contraction before it occurs. T (lim) prediction would then allow the evaluation of muscle capacities limiting fatigue and deleterious effects associated with exhaustive exercises. The present study aimed to analyze the influence of exercise intensity and joint angle on T (lim) prediction using changes in surface electromyographic (sEMG) signals recorded during the first moments of the exercise. Fifteen male performed four knee extensions sustained until exhaustion that were different in exercise intensity (20% or 50% of maximal voluntary torque-MVT) and in joint angle (40 or 70º, 0° = full extension). T (lim) prediction was explored using some parameters of the sEMG signals from rectus femoris, vastus medialis and vastus lateralis muscles. Changes in sEMG parameters (root mean square, mean power frequency and frequency banding 6-30 Hz) were expressed using the slope of the linear regression and the area ratio index. Results indicated that relationships between changes in sEMG signal and T (lim) (0.51 &amp;amp;amp;amp;lt; r &amp;amp;amp;amp;lt; 0.83) were greater for experimental conditions associated with higher exercise intensity (50% MVT) and so to lower time duration. Knee joint angle had little influence on T (lim) prediction results. Results also showed higher T (lim) prediction considering spectral parameters and area ratio. This could be in relation to differences in relative contribution of central and peripheral fatigue that seems to change according to the exercise intensity, but also to the influence of psychological factors that increases with the duration of the task.

Research paper thumbnail of Neuromuscular fatigue in healthy muscle: Underlying factors and adaptation mechanisms

Annals of Physical and Rehabilitation Medicine, 2011

This review aims to define the concept of neuromuscular fatigue and to present the current knowle... more This review aims to define the concept of neuromuscular fatigue and to present the current knowledge of the central and peripheral factors at the origin of this phenomenon. This review also addresses the literature that focuses on the mechanisms responsible for the adaption to neuromuscular fatigue. One hundred and eighty-two articles indexed in PubMed (1954-2010) have been considered. Neuromuscular fatigue has central and peripheral origins. Central fatigue, preponderant during long-duration, low-intensity exercises, may involve a drop in the central command (motor, cortex, motoneurons) elicited by the activity of cerebral neurotransmitters and muscular afferent fibers. Peripheral fatigue, associated with an impairment of the mechanisms from excitation to muscle contraction, may be induced by a perturbation of the calcium ion movements, an accumulation of phosphate, and/or a decrease of the adenosine triphosphate stores. To compensate for the consequent drop in force production, the organism develops several adaptation mechanisms notably implicating motor units. Fatigue onset is associated with an alteration of the mechanisms involved in force production. Then, the interaction between central and peripheral mechanisms leads to a series of events that ultimately contribute to the observed decrease in force production.

Research paper thumbnail of Impairment in postural control is greater when ankle plantarflexors and dorsiflexors are fatigued simultaneously than when fatigued separately

Gait & Posture, 2011

The aim of the present study was to compare the effects of ankle muscle fatigue on postural contr... more The aim of the present study was to compare the effects of ankle muscle fatigue on postural control when plantarflexors (PFs) and dorsiflexors (DFs) are fatigued simultaneously compared with separately. This study also investigated the recovery of postural control after fatigue. Sixteen adults (eight women and eight men) performed postural trials before and after an isokinetic fatigue task involving either (i) only PFs (30°s(-1)), (ii) only DFs (120°s(-1)), or (iii) both PFs and DFs simultaneously. The fatigue task involved maximal contractions repeated until the torque produced decreased below 50% of the maximal torque. Postural trials lasted 30s and were performed on one leg with eyes open (EO) or eyes closed (EC). Sway area, medio-lateral (ML) and antero-posterior (AP) positions and velocities were calculated from the center of pressure displacements. With EO, no effect of fatigue was found on postural variables. With EC, sway area and AP velocity increased only when both PFs and DFs were fatigued simultaneously. An effect of fatigue present only when both muscle groups are fatigued simultaneously could be due to impairment in the compensatory activity between agonist and antagonist muscles and/or a greater decrease in proprioception due to a greater number of fatigued muscles. In addition, when PFs and DFs were fatigued simultaneously, sway area and AP velocity returned to pre-fatigue values within 2min, whereas a posterior shift in AP position persisted for 10min. This last result may suggest a longer-lasting change in postural strategy needed for optimal postural control.