Is there an intermuscular relationship in voluntary activation capacities and contractile kinetics?

PURPOSE: The force-generating capacities of human skeletal muscles are interrelated, highlighting a common construct of limb strength. This study aimed to further determine whether there is an intermuscular relationship in maximal voluntary activation capacities and contractile kinetics of human muscles. METHODS: Twenty-six young healthy individuals participated in this study. Isometric maximal voluntary contraction (MVC) torque, voluntary activation level (VAL), and doublet twitch contractile kinetics (contraction time and half-relaxation time) evoked by a paired supramaximal peripheral nerve stimulation at 100 Hz were obtained in elbow flexors, knee extensors, plantar flexors and dorsiflexors of the dominant limb. RESULTS: Peak MVC torque had significant positive correlations between all muscle group pairs (all P values < 0.01). A significant positive correlation for VAL was found only between knee extensors and plantar flexors (r = 0.60, P < 0.01). There were no significant correlations between all muscle group pairs for doublet twitch contraction time and doublet twitch half-relaxation time. DISCUSSION: These results show that there is a partial common construct of maximal voluntary activation capacities that only concerns muscle groups that have incomplete activation during MVC (i.e., knee extensors and plantar flexors). This suggests that the common construct of MVC strength between these two muscle groups is partly influenced by neural mechanisms. The lack of intermuscular relationship of contractile kinetics showed that there is no common construct of muscle contractile kinetics, as assessed in vivo by investigating the time-course of evoked doublet twitch contractions.

Changes in elbow joint's musculo-articular mechanical properties do not alter reaching-related action-perception coupling.

PURPOSE: Perception of action capabilities can be altered by changes in sensorimotor processes, as showed in previous works in populations dealing with regular and pathological sensorimotor deficits. Misestimating changes in performance ability could lead to risky behavior, injury, and/or reduced performance. However, the relationship between sensorimotor processes, the action-perception coupling, and the related anatomical structures is still a matter of debate. We investigated whether changes in the muscle-tendon system's mechanical properties experimentally induced by eccentric contractions could alter the action-perception coupling (APC) in a reaching-to-grasp task, in which the participants estimated the maximal distance they predicted that they would able to reach a glass. METHODS: Based on their repartition, volunteers performed a conditioning session the first day: a series of isokinetic elbow extension in passive condition (control group, n = 11) or when performing elbow flexors eccentric contractions (eccentric group, n = 11). Performance estimates and actual performances in a reaching-to-grasp task were completed before, and immediately, 24 hours and 48 hours after the conditioning session. Alterations of musculo-articular mechanical properties were assessed through global joint stiffness (joint passive torque through load/unload cycles) and local stiffness (muscle elastography). RESULTS: The results showed that the APC related to reaching-to-grasp performance was not impacted by post-exercise changes in mechanical properties of the musculo-articular system. CONCLUSION: These findings emphasize the central dimension of sensorimotor processing instead of peripheral structures to investigate the APC for an altered sensorimotor environment.

No Alteration of the Neuromuscular Performance of Plantar-Flexor Muscles After Achilles Tendon Vibration

Context: Prolonged tendon vibration may induce muscle fatigue, as assessed by a decrease in maximal force production. It remains unknown, however, whether the decrease in muscle strength after prolonged Achilles tendon vibration is related to the vibration frequency. Objective: To assess the maximal capacity of plantar-flexor (PF) neuromuscular function before and after prolonged Achilles tendon vibration at low and high frequencies generated using a portable device. Design: Pre- and posttest intervention with control. Setting: University laboratory. Participants: 10 healthy men age 22.6 ± 4.0 y. Intervention: Each subject participated in 3 experimental sessions that were randomly distributed and separated by 1 wk. During each experimental session, 1 of the following vibration protocols was applied for 30 min: 40-Hz vibration, 100-Hz vibration, or no vibration (control protocol). Main Outcome Measures: Maximal-voluntary-contraction torque, voluntary activation level, twitch torque, maximal electromyographic activity, and maximal M-wave of PF muscles (measured before and after each vibration or control protocol). Results: Statistical analysis exhibited no significant effect of vibration protocol on the measured variables. Conclusions: The current study demonstrates that 30 min of Achilles tendon vibration at a low or high frequency using a portable stimulator did not affect the neuromuscular performance of the PF muscles. These results emphasize the limits of tendon vibration, whatever the frequency applied, for inducing neuromuscular fatigue.

An enhanced experimental procedure to rationalize on the impairment of perception of action capabilities.

It is well documented that changes in the physiological states of the perceiver-actor influence the perception of action capabilities. However, because experimental procedures of most studies involved a limitless availability for stimuli visual encoding and perceptual strategies, it remains difficult to adopt a single position among the large range of alternative interpretations for impaired perception. A reaching-to-grasp paradigm under breathing restriction was adapted from Graydon et al. (Cogn Emot 26:1301-1305, 2012) to standardize the time for encoding of stimuli information and narrowed the involvement of perceptual strategies. In the present study, we propose a highly controlled environment where the discrete information is presented during 300 ms, congruently with neurophysiological studies focused on visuomotor transformation. An underestimation of the perception of action capabilities is found under breath restriction, suggesting that 300 ms for stimuli encoding is sufficient to induce altered visuomotor brain transformations when limiting the involvement of perceptual strategies. This result suggests that such behavior could refer to an impaired brain potentiation of the perceptual occurrence, providing strong hypotheses on the brain dynamics of sensorimotor integration that underlie impaired perception of action capabilities in stressful situations.

Sit to stand in elderly fallers vs non-fallers: new insights from force platform and electromyography data.

BACKGROUND: The sit-to-stand movement requires balance control and coordination between the trunk and lower limbs. For these reasons, it is commonly used in clinics for evaluating lower limb muscle function in the elderly. The aim of the present study was to point out re levant biomechanical and neurophysiological sit-to-stand parameters allowing comparison between elderly fallers and non-fallers. METHODS: Ten elderly fallers and thirty non-fallers performed sit-to-stand movements. Sit-to-stand mechanical (maximal and mean force, impulse) and temporal parameters were measured in the vertical and anteroposterior axes using force platforms. Activity of rectus femoris, vastus lateralis, and gastrocnemius lateralis muscles was bilaterally recorded by surface electromyography. RESULTS: Time to realize sit-to-stand movements was significantly longer in elderly fallers compared to non-fallers (p < 0.05). In the same way, maximal vertical force and mean posterior force applied on force platform were significantly lower (p < 0.05) in fallers than in non-fallers individual. At muscular activity level, results showed a main statistical difference in gastrocnemius lateralis muscle activity patterns between faller and non-faller groups. CONCLUSION: Vertical and anteroposterior data from force platform, and gastrocnemius lateralis muscle activity determined during sit-to-stand movement are the most relevant parameters to differentiate fallers and non-fallers. Moreover, these factors highlight different strategies to rise from a chair between faller and non-faller group, suggesting that fallers would constantly adjust their control balance during the sit-to-stand movement.

Muscle and tendon stiffness assessment using the alpha method and ultrafast ultrasound.

PURPOSE: The alpha method enables the dissociation of the passive (i.e., tendinous tissues) and active (i.e., fascicles) part of muscle-tendon stiffness. It is based on two main assumptions (i.e., a constant tendon stiffness and a muscle stiffness proportional to the torque produced), which have not been approved in vivo. The purpose of this study was to validate these two assumptions using ultrafast ultrasound, and to compare fascicle and tendon stiffness as determined by both methods. METHODS: Ten healthy males performed a fast-stretch experiment on the ankle plantar flexors. The mathematical model of the alpha method allowed to estimate the stiffness of muscle and tendinous tissues on the basis of the assumptions associated to the behaviors of muscle fascicles and tendinous tissues. Muscle and tendon stiffness of the gastrocnemius medialis were also calculated from ultrafast ultrasound measurements. RESULTS: Muscle stiffness measured by the ultrasound method increased from 217 ± 83 to 720 ± 265 N/mm with an increasing level of force (from 30 to 90 % MVC). Tendinous stiffness measured by the ultrasound method remained constant across the force level (P < 0.001). However, the stiffness values obtained with both methods were significantly different (P < 0.001). CONCLUSIONS: In conclusion, these findings validate the two main assumptions underlying the alpha method using ultrasound. Differences in stiffness values obtained with alpha and ultrasound methods could be associated with physiological and geometrical differences between the whole plantar flexors muscles characterized by the alpha method and the gastrocnemius medialis assessed with the ultrasound method.

Determination of reliable force platform parameters and number of trial to evaluate sit-to-stand movement.

BACKGROUND: Sit-to-stand (STS) movement is useful for evaluating lower limb muscle function, especially from force platforms. Nevertheless, due to a lack of standardization of the STS movement (e.g., position, subject's instructions, etc.), it is difficult to compare results obtained in previous studies. AIMS: The aim of the present study was to determine the most relevant condition, parameters, and number of trial to perform STS movements. METHODS: In this study, STS mechanical (maximal and mean force, impulse) and temporal parameters were measured in the vertical, medio-lateral and antero-posterior axes using a force platform. Five STS conditions (i.e., with or without armrests, variation of the height of the chair and the movement speed) were analyzed to evaluate repeatability of different standardized procedures. RESULTS: Most of the mechanical and temporal parameters were influenced by the STS condition (p < 0.05). Regarding vertical axis, results showed a strong to perfect repeatability for all parameters (0.72 < ICC < 0.9) for only one condition: STS performed as rapidly as possible with a 90° knee angle when seated, without using armrests. Regardless conditions of performance, our results also showed that the most repeatable parameters were mean and peak force in medio-lateral axis, and the impulse measured in the three directions. Three trials should be performed to reach high repeatability. CONCLUSION: Our results suggest that the fast condition, with a 90° knee joint angle, with arms crossed over the chest, is the most reliable condition to evaluate performance during STS movement.

Effects of total sleep deprivation on the perception of action capabilities.

Changes in a subject's state have been shown to modulate the perceptual update of his or her action capabilities. In parallel, sleep deprivation impairs in cognitive functions. It involves common neural structures that support the perception of successfully achieving a motor task. Thus, the study investigated the effect of 24 h of sleep deprivation on the perception of action capabilities. Twenty-four healthy participants were randomly separated into two groups (control group vs. 24 h sleep deprivation group). Participants in the control group slept at home according to their habitual sleep-wake schedule. The 24-h sleep deprivation group stayed awake in the laboratory. Participants estimated the limit of their maximal height of stepping-over a bar before and after the sleep intervention. These estimations were compared to each participant's actual maximal stepping-over height. Physical performance (measured by maximal voluntary quadriceps contraction and repetitive vertical jumping tests) and perceptual inhibition tests (measured by choice reaction time tasks) were also performed for three sessions at three time points t 0, t +12h, and t +24h with t 0 = 8:00 a.m. for all participants. Participants in the 24-h sleep deprivation group showed impairments in perceived over-stepping performance and impaired cognitive functioning (higher reaction time), while no changes were observed in actual performance in the over-stepping, voluntary quadriceps contraction, or jumping tasks. The cognitive processing of inputs that specify the estimated consequences of motor action is discussed as the main explanation for the inability to successfully update the perception of action capabilities after sleep deprivation.

Reliability of a 2D-Panoramic Ultrasound System for the Determination of Muscle Volume in Older Hospitalized Patients.

ABSTRACT: Reliability of muscle thickness assessed in B-mode ultrasound has been confirmed in adults but remains a less direct assessment of muscle mass than muscle volume (MV). The aim of this prospective monocentric study was to assess the interday reliability of MV determined by the 2D-panoramic ultrasound in older hospitalized patients.Ten participants hospitalized in geriatric ward (mean age = 84) underwent 2 sessions of panoramic ultrasound scanning 1 week apart for MV assessment and DXA for leg lean mass (LLM) assessment when it was possible. The interday reliability of MV assessed using the intraclass correlation coefficient (ICC) was excellent, whatever was the muscle analyzed: ICC = 0.99 for tibialis anterior; ICC = 0.99 for vastus lateralis; ICC = 1 for rectus femoris. The pilot analysis of its association with leg lean mass revealed a good association between the vastus lateralis MV and LLM, but this needs to be confirmed on a larger number of participants (Pearson correlation coefficient = 0.91, P = 0.03).The 2D-panoramic ultrasound system for the determination of MV was reliable in older hospitalized patients with measurements performed on the patient's bed. MV seemed to be more correlated to LLM than muscle thickness. Future research will need to reduce the data processing time maybe with automation of data measurement.

Effects of a six-month intradialytic physical ACTIvity program and adequate NUTritional support on protein-energy wasting, physical functioning and quality of life in chronic hemodialysis patients: ACTINUT study protocol for a randomised controlled trial.

BACKGROUND: Protein-energy wasting (PEW) is common in hemodialysis patients and is a powerful predictor of morbidity and mortality. Although much progress has been made in recent years in identifying the causes and pathogenesis of PEW in hemodialysis patients, actual management by nutritional interventions is not always able to correct PEW. Some investigators suggest that physical exercise may increase the anabolic effects of nutritional interventions, and therefore may have a potential to reverse PEW. The aim of this study is to investigate the effect of intra-dialytic progressive exercise training and adequate nutritional supplementation on markers of PEW, functional capacities and quality of life of adult hemodialysis patients. METHODS AND DESIGN: Fifty end-stage renal disease patients undergoing hemodialysis, who meet the diagnostic criteria for PEW, will be randomly allocated into an exercise or control group for 6 months. The exercise consists of a progressive submaximal individualized cycling exertion using an adapted cycle ergometer, during the three weekly dialysis sessions. Biological markers of nutrition (albumin, prealbumin) will be followed monthly and all patients will be assessed for body composition, walk function, muscle strength, postural stability and quality of life at baseline and during the eighth week (t+2), the sixteenth week (t+4) and the twenty-fourth week (t+6) of the 6-month adapted rehabilitation program. DISCUSSION: The successful completion of this current trial may give precious clues in understanding PEW and encourage nephrologists to extend prescription of exercise programs as well as therapeutic and as preventive interventions in this high-risk population. TRIAL REGISTRATION: The protocol for this study was registered with the France Clinical Trials Registry NCT01813851.

Postural control as a function of time-of-day: influence of a prior strenuous running exercise or demanding sustained-attention task.

BACKGROUND: The current experiment investigated the impact of two potential confounding variables on the postural balance in young participants: the induced-experimental activity prior to the static postural measurements and the well-documented time-of-day effects. We mainly hypothesized that an exhaustive exercise and a high attention-demanding task should result in alterations of postural control. METHODS: Ten participants performed three experimental sessions (differentiated by the activity - none, cognitive or physical - prior of the assessment of postural stability), separated by one day at least. Each session included postural balance assessments around 8 a.m., 12.00 p.m. and 5 p.m. ± 30 min. The physical and cognitive activities were performed only before the 12 o'clock assessment. The postural tests consisted of four conditions of quiet stance: stance on a firm surface with eyes open; stance on a firm surface with eyes closed; stance on a foam surface with eyes open and stance on a foam surface with eyes closed. Postural performance was assessed by various center of pressure (COP) parameters. RESULTS: Overall, the COP findings indicated activity-related postural impairment, with an increase in body sway in the most difficult conditions (with foam surface), especially when postural measurements are recorded just after the running exercise (physical session) or the psychomotor vigilance test (cognitive session). CONCLUSIONS: Even if no specific influence of time-of-day on static postural control is demonstrated, our results clearly suggest that the activities prior to balance tests could be a potential confounding variable to be taken into account and controlled when assessing clinical postural balance.

Effects of plyometric training on passive stiffness of gastrocnemii muscles and Achilles tendon.

Plyometric training is commonly used to improve athletic performance; however, it is unclear how each component of the muscle-tendon complex (MTC) is affected by this intervention. The effects of 14 weeks of plyometric training on the passive stiffness of the gastrocnemii muscles and Achilles tendon was determined simultaneously to assess possible local adaptations of elastic properties. The passive force-length relationship of the gastrocnemii MTC and elongation of the gastrocnemii muscles were determined using ultrasonography during passive cyclic stretching in 19 subjects divided into trained (n = 9) and control (n = 10) groups. An upward trend in stiffness of the gastrocnemii MTC (P = 0.09) and a significant increase in the intrinsic gastrocnemii muscle stiffness were found (P < 0.05). In contrast, no significant change in gastrocnemii tendon stiffness, or in muscle and tendon geometry, was determined (P > 0.05). Considering the lack of change in gastrocnemii muscle geometry, the change in the gastrocnemii muscle stiffness may be mainly due to a change in the intrinsic mechanical properties of the muscular tissues.

Effects of plyometric training on both active and passive parts of the plantarflexors series elastic component stiffness of muscle-tendon complex.

The aims of this study were to determine the effects of plyometric training on both active and passive parts of the series elastic component (SEC) stiffness, and on geometrical parameters [i.e., muscle architecture, muscle and tendon cross-sectional area (CSA)] of the plantarflexors muscle-tendon complex to assess possible specific adaptations of the elastic properties. Nineteen subjects were randomly divided into a trained group and a control group. Active and passive components of the SEC stiffness were determined using a fast stretch during submaximal voluntary isometric plantarflexor activity. Geometrical parameters of the triceps surae muscles and the Achilles tendon were determined using ultrasonography. A significant increase in the passive component of the SEC stiffness was found (p < 0.05). In contrast, a significant decrease in the active part of the SEC stiffness was observed (p < 0.05). No significant changes in plantarflexor muscles CSA, architecture and Achilles tendon CSA were seen (p > 0.05). Thus, plyometric training led to specific adaptations within each part of the SEC. Theses adaptations could increase both the efficiency of the energy storage-recoil process and muscular tension transmission leading to an increase in jump performances.

Muscle architecture and EMG activity changes during isotonic and isokinetic eccentric exercises.

The present study aimed to compare muscle architecture and electromyographic activity during isotonic (IT) and isokinetic (IK) knee extensors eccentric contractions. Seventeen subjects were assigned in test and reproducibility groups. During test session, subjects performed two IT and two IK sets of eccentric contractions of knee extensor muscles. Torque, angular velocity, VL architecture and EMG activity of agonist (vastus lateralis, VL; vastus medialis; rectus femoris) and antagonist (semitendinosus; biceps femoris, BF) muscles were simultaneously recorded and averaged on a 5° window. Torque-angle and angular velocity-angle relationships exhibited differences in mechanical load between the IT and IK modes. Changes in muscle architecture were similar in both modes, since VL fascicles length increased and fascicle angle decreased, resulting in a decrease in muscle thickness during eccentric contraction. Agonist activity and BF co-activity levels were higher in IT mode than in IK mode at short muscle lengths, whereas agonist activity was higher in IK mode than in IT mode at long muscle lengths. Differences in mechanical load between both modes induced specific neuromuscular responses in terms of agonist activity and antagonist co-activity. These results suggest that specific neural adaptations may occur after IT or IK eccentric training. This hypothesis needs to be tested in order to gain new insights concerning the most effective eccentric protocols based on whether the objective is sportive or clinical.

Supporting every school to become a foundation for healthy lives.

As a setting where children and adolescents live and learn, linked to the family and embedded within the wider community, schools have an important influence on every student's health. Many health interventions have used schools as a platform, often for standalone programmatic initiatives to reduce health risks, and sometimes for more comprehensive approaches, but the interventions, uptake, and sustainability are generally disappointing. Evidence shows that, to improve health and to reduce inequality, all students must attend school from a young age and for as long as possible, and their educational success therein must be maximised. Thus, beyond educational benefits, schools are also important for health. Coherence between each school's policies, structures and systems, human resources, and practices is required to advance both academic and health outcomes. Beyond simply implementing ready-made programmes into schools, health professionals can position themselves as catalysts for structural change as they have many opportunities to advocate for, and participate in, the intersectoral implementation of reforms and innovations in school systems to promote the health of all students.

Neuromuscular adaptations to 8-week strength training: isotonic versus isokinetic mode.

Previous studies attempted to compare the effectiveness of isokinetic and isotonic training. However, they have provided conflicting results. The purpose of this study was to compare the effects of isotonic versus isokinetic standardized concentric strength training programs of the knee extensors on the neuromuscular system. The standardization of these two training programs was ensured by the equalization of the total external amount of work performed and the mean angular movement velocity. Thirty healthy male students were randomly assigned to the isotonic (IT; n = 11), the isokinetic (IK; n = 11) or the control (C; n = 8) group. Both IT and IK groups trained their dominant lower leg 3 sessions/week for 8 weeks on a dynamometer. The IT group exercised using a preset torque of 40% of the maximal voluntary isometric torque at 70 degrees (0 degrees = leg in horizontal position). The IK group exercised at a velocity ranging between 150 degrees and 180 degrees s(-1). Isotonic, isokinetic and isometric tests were performed on a dynamometer before and after strength training. Surface electromyographic activity of vastus lateralis, vastus medialis, rectus femoris, semitendinosus and biceps femoris muscles was recorded during the tests. Significant strength increases in both dynamic and static conditions were noticed for IT and IK groups without any significant difference between the two trained groups. Agonist muscle activity also increased with training but no change in antagonist muscle co-activity was observed. The two training methods could be proposed by clinicians and athletic coaches to improve concentric muscle strength in dynamic and static conditions.

A new device to study isoload eccentric exercise.

This study was designed to develop a new device allowing mechanical analysis of eccentric exercise against a constant load, with a view in mind to compare isoload (IL) and isokinetic (IK) eccentric exercises. A plate-loaded resistance training device was integrated to an IK dynamometer, to perform the acquisition of mechanical parameters (i.e., external torque, angular velocity). To determine the muscular torque produced by the subject, load torque was experimentally measured (TLexp) at 11 different loads from 30° to 90° angle (0° = lever arm in horizontal position). TLexp was modeled to take friction effect and torque variations into account. Validity of modeled load torque (TLmod) was tested by determining the root mean square (RMS) error, bias, and 2SD between the descending part of TLexp (from 30° to 90°) and TLmod. Validity of TLexp was tested by a linear regression and a Passing-Bablok regression. A pilot analysis on 10 subjects was performed to determine the contribution of the torque because of the moment of inertia to the amount of external work (W). Results showed the validity of TLmod (bias = 0%; RMS error = 0.51%) and TLexp SEM = 4.1 N·m; Intraclass correlation coefficient (ICC) = 1.00; slope = 0.99; y-intercept = -0.13). External work calculation showed a satisfactory reproducibility (SEM = 38.3 J; ICC = 0.98) and moment of inertia contribution to W showed a low value (3.2 ± 2.0%). Results allow us to validate the new device developed in this study. Such a device could be used in future work to study IL eccentric exercise and to compare the effect of IL and IK eccentric exercises in standardized conditions.

Acute effect of tendon vibration applied during isometric contraction at two knee angles on maximal knee extension force production.

The aim of the current study was to investigate the effect of a single session of prolonged tendon vibration combined with low submaximal isometric contraction on maximal motor performance. Thirty-two young sedentary adults were assigned into two groups that differed based on the knee angle tested: 90° or 150° (180° = full knee extension). Participants performed two fatigue-inducing exercise protocols: one with three 10 min submaximal (10% of maximal voluntary contraction) knee extensor contractions and patellar tendon vibration (80 Hz) another with submaximal knee extensor contractions only. Before and after each fatigue protocol, maximal voluntary isometric contractions (MVC), voluntary activation level (assessed by the twitch interpolation technique), peak-to-peak amplitude of maximum compound action potentials of vastus medialis and vastus lateralis (assessed by electromyography with the use of electrical nerve stimulation), peak twitch amplitude and peak doublet force were measured. The knee extensor fatigue was significantly (P<0.05) greater in the 90° knee angle group (-20.6% MVC force, P<0.05) than the 150° knee angle group (-8.3% MVC force, P = 0.062). Both peripheral and central alterations could explain the reduction in MVC force at 90° knee angle. However, tendon vibration added to isometric contraction did not exacerbate the reduction in MVC force. These results clearly demonstrate that acute infrapatellar tendon vibration using a commercial apparatus operating at optimal conditions (i.e. contracted and stretched muscle) does not appear to induce knee extensor neuromuscular fatigue in young sedentary subjects.

Perceptual Inhibition Is Not a Specific Component of the Sensory Integration Process Necessary for a Rapid Voluntary Step Initiation in Healthy Older Adults.

OBJECTIVES: We investigated whether performing step initiation during a proprioceptive perturbation would require greater perceptual or motor inhibitory control in older adults. METHOD: Fifty-two healthy adults (young: n = 26, mean age 22.5 years vs. older: n = 26, mean age 70.1 years) performed a stepping reaction time task, with different inhibition requirements (i.e., perceptual vs. motor inhibitory conflict), with two proprioceptive configurations: with and without application of Achilles tendon vibrations. RESULTS: Beyond a systematically greater stepping reaction time in older adults (p < .01), no difference was found between the perceptual versus motor inhibitory conflict resolution, regardless of age and proprioceptive configuration. Furthermore, slower reaction time was observed for young participants in the presence of Achilles tendon vibrations unlike older adults, who showed the same reactive stepping performance with or without vibrations (p < .05). DISCUSSION: These findings show that perceptual inhibition cannot be considered as specifically involved in the central processing of proprioceptive signals, at least not in active older adults. Rather than motor system malfunctioning or a reduced amount of proprioceptive afference, we propose that cortical-proprioceptive processing in older adults remains as effective as in young adults, regardless of the high attentional requirements for step responses.

Electromechanical delay revisited using very high frame rate ultrasound.

Electromechanical delay (EMD) represents the time lag between muscle activation and muscle force production and is used to assess muscle function in healthy and pathological subjects. There is no experimental methodology to quantify the actual contribution of each series elastic component structures that together contribute to the EMD. We designed the present study to determine, using very high frame rate ultrasound (4 kHz), the onset of muscle fascicles and tendon motion induced by electrical stimulation. Nine subjects underwent two bouts composed of five electrically evoked contractions with the echographic probe maintained over 1) the gastrocnemius medialis muscle belly (muscle trials) and 2) the myotendinous junction of the gastrocnemius medialis muscle (tendon trials). EMD was 11.63 +/- 1.51 and 11.67 +/- 1.27 ms for muscle trials and tendon trials, respectively. Significant difference (P < 0.001) was found between the onset of muscle fascicles motion (6.05 +/- 0.64 ms) and the onset of myotendinous junction motion (8.42 +/- 1.63 ms). The noninvasive methodology used in the present study enabled us to determine the relative contribution of the passive part of the series elastic component (47.5 +/- 6.0% of EMD) and each of the two main structures of this component (aponeurosis and tendon, representing 20.3 +/- 10.7% and 27.6 +/- 11.4% of EMD, respectively). The relative contributions of the synaptic transmission, the excitation-contraction coupling, and the active part of the series elastic component could not be directly quantified with our results. However, they suggest a minor role of the active part of the series elastic component that needs to be confirmed by further experiments.