Frédéric Noé

Electrical Stimulation Superimposed onto Voluntary Muscular Contraction

Electrical stimulation (ES) reverses the order of recruitment of motor units (MU) observed with voluntary muscular contraction (VOL) since under ES, large MU are recruited before small MU. The superimposition of ES onto VOL (superimposed technique: application of an electrical stimulus during a voluntary muscle action) can theoretically activate more motor units than VOL performed alone, which can engender an increase of the contraction force. Two superimposed techniques can be used: (i) the twitch interpolation technique (ITT), which consists of interjecting an electrical stimulus onto the muscle nerve; and (ii) the percutaneous superimposed electrical stimulation technique (PST), where the stimulation is applied to the muscle belly. These two superimposed techniques can be used to evaluate the ability to fully activate a muscle. They can thus be employed to distinguish the central or peripheral nature of fatigue after exhausting exercise. In general, whatever the technique employed, the superimposition of ES onto volitional exercise does not recruit more MU than VOL, except with eccentric actions. Nevertheless, the neuromuscular response associated with the use of the superimposed technique (ITT and PST) depends on the parameter of the superimposed current. The sex and the training level of the subjects can also modify the physiological impact of the superimposed technique. Although the motor control differs drastically between training with ES and VOL, the integration of the superimposed technique in training programmes with healthy subjects does not reveal significant benefits compared with programmes performed only with voluntary exercises. Nevertheless, in a therapeutic context, training programmes using ES superimposition compensate volume and muscle strength deficit with more efficiency than programmes using VOL or ES separately.

Techniques and Methods for Testing the Postural Function in Healthy and Pathological Subjects

The different techniques and methods employed as well as the different quantitative and qualitative variables measured in order to objectify postural control are often chosen without taking into account the population studied, the objective of the postural test, and the environmental conditions. For these reasons, the aim of this review was to present and justify the different testing techniques and methods with their different quantitative and qualitative variables to make it possible to precisely evaluate each sensory, central, and motor component of the postural function according to the experiment protocol under consideration. The main practical and technological methods and techniques used in evaluating postural control were explained and justified according to the experimental protocol defined. The main postural conditions (postural stance, visual condition, balance condition, and test duration) were also analyzed. Moreover, the mechanistic exploration of the postural function often requires implementing disturbing postural conditions by using motor disturbance (mechanical disturbance), sensory stimulation (sensory manipulation), and/or cognitive disturbance (cognitive task associated with maintaining postural balance) protocols. Each type of disturbance was tackled in order to facilitate understanding of subtle postural control mechanisms and the means to explore them.

Effects of two types of neuromuscular electrical stimulation training on vertical jump performance.

Paillard, T, Noe, F, Bernard, N, Dupui, P, and Hazard, C. Effects of Two Types of Neuromuscular Electrical Stimulation Training on Vertical Jump Performance. J Strength Cond Res 22: 1273-1278, 2008-This study examined the effects of different types of neuromuscular electrical stimulation (NMES) programs on vertical jump performance. Twenty seven healthy trained male students in sports-sciences were recruited and randomized into three groups. The control group (C group, n = 8) did not perform NMES training. Two other groups underwent 3 training sessions a week over 5 weeks on the quadriceps femoris muscle [F group (n = 9): stimulation with an 80 Hz current for 15 min for improving muscle strength; E group (n = 10): stimulation with a 25 Hz current for 60 min for improving muscle endurance]. The height of the vertical jump was measured before NMES training (test 1), one week (test 2) and five weeks (test 3) after the end of the programs. The results showed that the height of the vertical jump significantly increased in both the F and E groups between tests 1 and 2 (5 cm and 3 cm respectively). Results of test 3 showed that both groups preserved their gains. A NMES training program destined to improve muscle endurance does not interfere on vertical jump performance. It can even durably enhance it in the same way as a NMES training program destined to improve muscle strength. Thus, to improve muscle endurance without deteriorating muscle power, sportsmen can use electrical stimulation.

The impact of time of day on the gait and balance control of Alzheimer’s patients

ABSTRACT Alzheimer’s patients suffer from circadian dysregulation. The aim of this study was to examine the evolution of balance control and gait at different times of the day (11:00, 14:00, 18:00) in order to identify whether Alzheimer’s patients were more likely to fall at certain periods of the day. Spatio-temporal parameters of centre of foot pressure displacements were measured with a force platform and spatio-temporal parameters of walking were evaluated with a gait analysis device. The results highlighted that balance control was worse in the evening and the afternoon than in the morning. Furthermore, the walking speed was faster and support duration, swing duration and cycle duration were shorter in the evening than in the morning and afternoon. The combined analysis of balance control and gait parameters revealed that balance control and walking are concomitantly altered in the evening which increases the fall risk in the evening, in comparison with the morning, for Alzheimer’s patients.

Fatigue does not conjointly alter postural and cognitive performance when standing in a shooting position under dual-task conditions

ABSTRACT This study investigated the effects of fatigue on balance control and cognitive performance in a standing shooting position. Nineteen soldiers were asked to stand while holding a rifle (single task – ST). They also had to perform this postural task while simultaneously completing a cognitive task (dual task – DT). Both the ST and DT were performed in pre- and post-fatigue conditions. In pre-fatigue, participants achieved better balance control in the DT than in the ST, thus suggesting that the increased cognitive activity associated with the DT improves balance control by shifting the attentional focus away from a highly automatised activity. In post-fatigue, balance control was degraded in both the ST and DT, while reaction time was enhanced in the first minutes following the fatiguing exercise without affecting the accuracy of response in the cognitive task, which highlights the relative independent effects of fatigue on balance control and cognitive performance.

Inter-joint coordination of posture on a seesaw device

Even though specific adjustments of the multi-joint control of posture have been observed when posture is challenged, multi-joint coordination on a seesaw device has never been accurately assessed. The current study was conducted in order to investigate the multi-joint coordination when subjects were standing on either a seesaw device or on a stable surface, with the eyes open or closed. Eighteen healthy active subjects were recruited. A principal component analysis and a Self-Organizing Maps analysis were performed on the joint angles in order to detect and characterize dominant coordination patterns. Intermuscular EMG coherence was analysed in order to assess the neurophysiological mechanisms associated with these coordination patterns. The results illustrated a multi-joint organization of posture on both stable ground and on the seesaw, with a higher variability among the individual postural responses observed when standing on the seesaw. These findings challenge the classical assumption of ankle mechanisms as dominating control on seesaw devices and confirm that inter-joint coordination in postural control is strongly modulated by stance conditions. When standing on the seesaw without vision, a decrease in intermuscular coherence was observed without any impact on the joint coordination patterns, likely due to an increase dependence on proprioceptive information.

Reference Selection Influences the Reliability of Conclusions

We read with great interest the article by Zemkova [1] titled ‘‘Sport-specific balance’’ recently published in Sports Medicine. The main objective of this article was to investigate the effects of different forms of exercise on balance maintenance and to gain insight into the associated physiological mechanisms while also examining how these effects were modulated by sporting expertise. Whereas two recently published reviews [2, 3] focused on the long-term effects of sports exercise while analyzing the relationship between sports expertise and postural control, the originality of Zemkova’s work was to deal with the short-term effects of sports exercise while considering the specificities induced by the performance of different types of exercises. This study also differed from another recent review article about the short-term effects of exercise, which focused on the impact of fatigue [4]. In Zemkova’s work [1], attention was paid to the assessment of balance in sport-specific conditions with the aim of designing specifically oriented, balance-training programs and optimizing the evaluation of the effect of these programs. The author presented a substantial number of experimental studies that analyzed postural sway during and after a wide range of laboratory and/or sport-specific standing conditions. Hence, Zemkova emphasized the importance of adjusting laboratory testing while using exercises that match sport-specific performance as closely as possible to determine accurately their specific effect on balance. Nevertheless, the author’s approach to this interesting topic raises some questions that we discuss below. In a previous issue of Sports Medicine, Zemkova and Hamar published an article titled ‘‘Physiological Mechanisms of Post-Exercise Balance Impairment’’ [5]. Even though this article was mainly concerned with the different physiological mechanisms of post-exercise balance impairment, some redundancies could be found with Zemkova’s subsequent review article in Sports Medicine [1], particularly in considering the second part of the paper, which analyzed the evolution of postural sway after several types of sports exercise. Indeed, among the 76 references used in the first review [5], virtually half of them were included in the subsequent review article [1] (37 common references). All the studies considered by the author that were specifically in line with the topic of this article included experienced sportspeople (mountaineers, skiers, weightlifters, dancers, synchronized swimmers, gymnasts, icehockey players, professional rifle shooters, basketball F. Noe (&) Laboratoire Activite Physique, Performance et Sante (EA 4445), Departement STAPS, Universite de Pau et des Pays de l’Adour, ZA Bastillac Sud, 11 rue Morane Saulnier, 65000 Tarbes, France e-mail: frederic.noe@univ-pau.fr

The influence of wearing ski-boots with different rigidity characteristics on postural control

Abstract External supports that reduce ankle joint mobility such as ski-boots can impair postural control of healthy participants. Although this disruptive effect has been attributed to the rigidity of the external supports, the results remained controversial and no study has been conducted in order to evaluate the influence of ski-boots rigidity. Hence, the question about the influence of ankle support rigidity on postural control remains open. This study was therefore undertaken in order to investigate the effect of ski-boots rigidity on postural control. Ten healthy active participants were recruited. The wearing of soft and rigid ski-boots was compared to barefoot while standing on a seesaw generating mediolateral and anteroposterior instability. Centre of pressure displacements were sampled with a force platform. The surface electromyographic activity of the main muscles from the leg, thigh and trunk was recorded. A motion analysis system was also used to calculate the ankle, knee and hip angles. The results did not reveal any negative influence of ski-boot rigidity on postural control but rather suggest a less active postural control with the rigid ski-boots which offered a higher mechanical contribution.

Ski Boots Do Not Impair Standing Balance by Restricting Ankle-Joint Mobility:

Objective: This study was undertaken in order to provide new insight into sensorimotor control of posture when wearing high-shaft (HS) boots as ski boots. Background: Previous studies into the effects of HS boots on postural control have produced controversial results. Some studies reported postural control impairments with ski boots in bipedal postural tasks due to ankle movement restrictions without quantifying the actual restrictive effect of these boots and specifying the adaptations of the postural control system. Method: Eighteen young healthy subjects took part in the experiment. Bilateral postural control was assessed on stable and unstable surfaces, while standing barefoot or wearing ski boots. Center of pressure (COP) parameters, ankle, knee, and hip joints movements were calculated and EMG activity from main postural muscles was recorded. Results: Ski boots did not restrict the amplitude of ankle angular movements and largely impacted COP parameters and EMG activity on stable ground. In conditions of mediolateral instability, COP data illustrated an enhanced postural control in the frontal plane when wearing ski boots. Conclusions: Ski boots do not affect bipedal postural balance by restricting the ankle angular motions but induce complex adaptations of the postural control system which combine factors of a mechanical, motor, and sensorial nature. They impede postural control mainly when standing on stable ground without producing similar deleterious effects on unstable surfaces. Application: Our results show that HS boots as ski boots can improve lateral balance on unstable surfaces, which can contribute to prevent fall risk and ankle sprain.

Rehabilitation and Improvement of the Postural Function

Posture refers to the position of different body segments at a given time which can be modified through joint mobilization and the action of the neuromuscular system. Maintaining balance during bipedal quiet stance requires complex mechanisms from the postural control system in order to keep the vertical projection of the centre of mass (COM) within the base of support [1]. To achieve this aim, the centre of pressure (COP) plays a crucial role to compensate for any deviations of the COM, which can generate imbalance if they move beyond the limits of the base of support. The ability to control the COM depends on internal body representation in space. Internal representation is acquired by means of a learning process but also depends upon genetic factors [2]. This representation is elaborated by sensory inputs and is based on kinematic (segmental organization, whole body acceleration, and body orientation relative to earth gravity) and kinetic (joint torques and forces efforts between the plantar cutaneous surface and the ground) parameters [3]. Moreover, a postural attitude is never acquired definitively even in quiet stance. The body constantly undergoes changes caused by liquid movements and cardiac and respiratory muscular contractions. This phenomenon modifies its at-rest state and prevents it from maintaining a strict balance [4]. It is characterized by continuous body sway and results from an internal perturbation. In addition, muscle tone constantly varies which both accentuates body sway and complicates the possibility of cushioning it [5]. Postural control is thus a permanent process of balance regulation whose implementation is based on subtle mechanisms. Postural regulation is organized in hierarchical and stereotypic patterns and requires the central integration of afferent inputs from the sensory systems as well as the motor command of antigravity muscles. The proprioceptive (myotendinous and joint sensors), exteroceptive (mainly visual and cutaneous plantar sensors, but also auditory sensors), and vestibular (vestibular sensors) inputs are integrated by the vestibular nuclei located in the brain stem and are controlled by the cerebral cortex and cerebellum [6–10]. The activation of postural muscles is organized in synergies (activation/inhibition of agonists/antagonists muscles) and is based on postural neural networks [3]. Each sensory, central, and motor component of the postural function is either healthy or pathological and will display normal or abnormal functions. In pathological subjects, the dysfunction of certain organs involved in postural control is likely to amplify body sway and/or to affect the ability to cushion it and it may also alter the segmental organization of postural control. Different evaluation methods enable the exploration of each component through protocols of motor perturbation (mechanical disturbance), sensory stimulation (sensory manipulation), and/or cognitive disturbance (e.g., virtual simulation, dual task). Postural behavior of healthy subjects can be characterized in terms of postural performance (i.e., the ability to minimize body sway) and segmental (i.e., the multijoint coordination) and neural strategies (i.e., the preferential involvement of short or long neuronal loops, i.e., myotatic or visuovestibular). A particular postural behavior can be easily considered as normal or abnormal through measures of magnitude, velocity, and acceleration of linear or angular displacements of the COM, COP, and body segments and also through measures of electromyographic activities and evaluations of the contribution of different sensory information. All these measures contribute to describe precisely the compensatory and anticipatory postural adjustments characterizing postural behavior. Compensatory postural adjustments act in a feedback manner to preserve balance in response to the actual balance disturbances whereas anticipatory postural adjustments precede the onset of a postural disturbance while minimizing its feedforward effects. Neurological and muscular pathologies, sensitivity deficits (e.g., vestibular, visual), cerebellar syndrome, and many other diseases severely degrade postural control. The postural performance and strategy of healthy subjects notably differ from those of pathological subjects (e.g., [3, 11]). For a given pathology, the postural behaviour evolves in a specific way [12]. However, many scientific considerations in discovering testing and rehabilitation for each pathology of postural function still remain. When subjects present such pathologies, as mentioned above, they are liable to fall, which can have dramatic consequences for their physical integrity. The development of stimulation techniques of the sensory and motor functions in a rehabilitation context is likely to improve and help restore postural function while the refinement of testing techniques improves descriptions of dysfunction of the postural function. For these reasons, this special issue provides supplemental knowledge related to the evaluation and rehabilitation of the postural function in pathological subjects (from children to aged patients) that could advance their therapeutic management. Moreover, among healthy subjects, the postural function can positively and negatively evolve according to age (e.g., development in children, involution in aged subjects) and the status of subjects in terms of physical activity (e.g., active or inactive). For all these populations, postural control can also be positively influenced by repeated regularly exercise or training. Exercise optimises the sensory, central, and motor outputs of the postural function and can induce motor program acquisitions which include specific postural adaptations [13, 14]. Indeed, in a working, leisure, or sporting context, highly skilled subjects are subjected to having a performant postural control since there is a close relationship between postural and motor skill (or postural and motor performance), specific training developing specific postural skills [15]. Postural strategy can also be modified by the effects of training [16]. The progress in the advancement of scientific knowledge in healthy subjects can help in the understanding of pathological postural mechanisms. Thus, this special issue also integrates work dealing with the effects of domestic and leisure physical activities and sport on the postural function in healthy subjects. Thierry Paillard Massimiliano Pau Frederic Noe Luis-Millan Gonzalez