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The response of spinal motoneurons to synaptic input greatly depends on the activation of persistent inward currents (PICs), the contribution of which can be estimated through the paired motor unit technique. Yet, the intra-session test-retest reliability of this measurement remains to be fully established. Twenty males performed isometric triangular dorsiflexion contractions to 20 and 50 % of maximal torque at baseline and after a 15-min resting period. High-density electromyographic signals (HD-EMG) of the tibialis anterior were recorded with a 64-electrode matrix. HD-EMG signals were decomposed, and motor units tracked across time points to estimate the contribution of PICs to motoneuron firing through quantification of motor unit recruitment-derecruitment hysteresis (ΔF). A good intraclass correlation coefficient (ICC = 0.75 [0.63, 0.83]) and a large repeated measures correlation coefficient (r rm  = 0.65 [0.49, 0.77]; p < 0.001) were found between ΔF values obtained at both time points for 20 % MVC ramps. For 50 % MVC ramps, a good ICC (0.77 [0.65, 0.85]) and a very large repeated measures correlation coefficient (r rm  = 0.73 [0.63, 0.80]; p < 0.001) were observed. Our data suggest that ΔF scores can be reliably investigated in tibialis anterior motor units during both low- and moderate-intensity contractions within a single experimental session., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Introduction/purpose: Recently, the use of transcutaneous spinal cord stimulation (TSCS) has been proposed as a viable alternative to the H-reflex. The aim of the current study was to investigate to what extent the two modes of spinal cord excitability investigation would be similarly sensitive to the well-known vibration-induced depression., Methods: Fourteen healthy participants (8 men and 6 women; age: 26.7 ± 4.8 years) were engaged in the study. The right soleus H-reflex and TSCS responses were recorded at baseline (PRE), during right Achilles tendon vibration (VIB) and following 20 min of vibration exposure (POST-VIB). Care was taken to match H-reflex and TSCS responses amplitude at PRE and to maintain effective stimulus intensities constant throughout time points., Results: The statistical analysis showed a significant effect of time for the H-reflex, with VIB (13 ± 5% of maximal M-wave (M max ) and POST-VIB (36 ± 4% of M max ) values being lower than PRE-values (48 ± 6% of M max ). Similarly, TSCS responses changed over time, VIB (9 ± 5% of M max ) and POST-VIB (27 ± 5% of M max ) values being lower than PRE-values (46 ± 6% of M max ). Pearson correlation analyses revealed positive correlation between H-reflex and TSCS responses PRE-to-VIB changes, but not for PRE- to POST-VIB changes., Conclusion: While the sensitivity of TSCS seems to be similar to the gold standard H-reflex to highlight the vibratory paradox, both responses showed different sensitivity to the effects of prolonged vibration, suggesting slightly different pathways may actually contribute to evoked responses of both stimulation modalities., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

The etiology of changes in lower-limb neuromuscular function, especially to the central nervous system, may be affected by exercise duration. Direct evidence is lacking as few studies have directly compared different race distances. This study aimed to investigate the etiology of deficits in neuromuscular function following short versus long trail-running races. Thirty-two male trail runners completed one of five trail-running races as LONG (>100 km) or SHORT (<60 km). Pre- and post-race, maximal voluntary contraction (MVC) torque and evoked responses to electrical nerve stimulation during MVCs and at rest were used to assess voluntary activation and muscle contractile properties of knee-extensor (KE) and plantar-flexor (PF) muscles. Transcranial magnetic stimulation (TMS) was used to assess evoked responses and corticospinal excitability in maximal and submaximal KE contractions. Race distance correlated with KE MVC (ρ = -0.556) and twitch (ρ = -0.521) torque decreases (p ≤ .003). KE twitch torque decreased more in LONG (-28 ± 14%) than SHORT (-14 ± 10%, p = .005); however, KE MVC time × distance interaction was not significant (p = .073). No differences between LONG and SHORT for PF MVC or twitch torque were observed. Maximal voluntary activation decreased similarly in LONG and SHORT in both muscle groups (p ≥ .637). TMS-elicited silent period decreased in LONG (p = .021) but not SHORT (p = .912). Greater muscle contractile property impairment in longer races, not central perturbations, contributed to the correlation between KE MVC loss and race distance. Conversely, PF fatigability was unaffected by race distance., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation (NMES) can generate extra torque (ET) via afferent pathways. Superimposing tendon vibration (TV) to NMES can increase the activation of these afferent pathways and favor ET generation. Knowledge of the characteristics of ET is essential to implement these stimulation paradigms in clinical practice. Thus, we aimed to investigate the effects of frequency and TV superimposition on the occurrence and magnitude of ET in response to wide-pulse NMES. NMES-induced isometric plantar flexion torque was recorded in 30 healthy individuals who performed five NMES protocols: wide-pulse low-frequency (1 ms; 20 Hz; WPLF) and wide-pulse high-frequency (1 ms; 100 Hz; WPHF) without and with superimposed TV (1 mm; 100 Hz) and conventional NMES (50 µs; 20 Hz; reference protocol). Each NMES protocol consisted of three 20-s trains interspersed by 90 s of rest, with NMES intensity being adjusted to reach 10% of maximal voluntary contraction. The ET occurrence was similar for WPLF and WPHF ( P = 0.822). In the responders, the ET magnitude was greater for WPHF than WPLF ( P < 0.001). There was no effect of superimposed TV on ET characteristics. This study reported an effect of NMES frequency on ET magnitude, whereas TV superimposition did not affect this parameter. In the context of our experimental design decisions, the present findings question the clinical use of wide-pulse NMES and its combination with superimposed TV. Yet, further research is needed to maximize force production through the occurrence and magnitude of ET. NEW & NOTEWORTHY This study is the first to assess the effect of stimulation frequency and superimposed tendon vibration on extra torque characteristics generated by wide-pulse neuromuscular electrical stimulation. The percentage of subjects showing extra torque (i.e., considered as responders) was similar for low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation. In the responders, the extra torque was greater for high-frequency than for low-frequency wide-pulse neuromuscular electrical stimulation. The superimposition of tendon vibration had no effect on extra torque occurrence or magnitude.

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whereas large TMS-CMEPs are less affected. As small TMS-CMEPs recruit motoneurons most active during low-intensity contractions whereas large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low intensity) and 70% MVC (high intensity). Small and large TMS-CMEPs with amplitudes of ∼15% and ∼50% M max at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1-, 2.5- and 4-min postexercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min ( P ≤ 0.049) by up to -10% M max , whereas large TMS-CMEPs remained unchanged ( P ≥ 0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time points ( P < 0.01) by up to -14% and -33% M max , respectively, and remained below baseline during all recovery measures ( P ≤ 0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial ( P ≥ 0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation. NEW & NOTEWORTHY This study measured motoneuron excitability using cervicomedullary evoked potentials conditioned using transcranial magnetic stimulation (TMS-CMEPs) of both small and large amplitudes during sustained low- and high-intensity contractions of the elbow flexors. During the low-intensity task, only the small TMS-CMEP was reduced. During the high-intensity task, both small and large TMS-CMEPs were substantially reduced. These results indicate that repetitively active motoneurons are specifically reduced in excitability compared with less active motoneurons in the same pool.

Objective: The purpose of this study was to assess whether accelerometry effectively reflects muscle vibrations measured with ultrafast ultrasonography., Methods: Vibration characteristics initiated on the vastus lateralis muscle by an impactor were compared when assessed with accelerometry and ultrasonography. Continuous wavelet transforms and statistical parametric mapping (SPM) were performed to identify discrepancies in vibration power over time and frequency between the two devices., Results: The SPM analysis revealed that the accelerometer underestimated the muscle vibration power above 50 Hz during the first 0.06 seconds post impact. Furthermore, the accelerometer overestimated the muscle vibration power under 20 Hz, from 0.1 seconds after the impact. Linear regression revealed that the thicker the subcutaneous fat localized under the accelerometer, the more the muscle vibration frequency and damping were underestimated by the accelerometer., Conclusion: The skin and the fat tissues acted like a low-pass filter above 50 Hz and oscillated in a less damped manner than the muscle tissue under 20 Hz., Significance: To eliminate some artifacts caused by the superficial tissues and assess the muscle vibration characteristics with accelerometry, it is suggested to 1) high-pass filter the acceleration signal at a frequency of 20 Hz, under certain conditions, and 2) include participants with less fat thickness. Therefore, the subcutaneous thickness must be systematically quantified under each accelerometer location to clarify the differences between subjects and muscles.

Purpose: Whether muscle stiffness is influenced by fatigue remains unclear. Classical methods used to assess muscle stiffness provide a global measure at the joint level. As fatigue may selectively affect specific muscles, a joint-level approach may not be sensitive enough to detect potential changes in muscle stiffness. Taking advantage of ultrasound shear wave elastography, this study aimed to determine the influence of a fatiguing protocol involving intermittent submaximal isometric contractions on muscle shear modulus (an index of stiffness)., Methods: Shear modulus was measured on either the vastus lateralis (n = 9) or the abductor digiti minimi (n = 10) before and after 15 min of intermittent submaximal isometric contractions at 60% of maximal voluntary contraction (MVC) (4 s ON, 4 s OFF). An index of active muscle stiffness was estimated PRE- and POST-fatigue as the slope of the linear regression established between shear modulus and absolute joint force up to 60% MVC., Results: After the fatiguing exercise, MVC was significantly decreased by 22% ± 7% and 32% ± 15% for knee extension and little finger abduction, respectively (P < 0.001). When compared to PRE-fatigue, the index of active muscle stiffness was 12% ± 15% lower for the vastus lateralis (P < 0.031) and 44% ± 19% lower for the abductor digiti minimi (P < 0.001) POST-fatigue., Conclusions: Although the present results cannot clearly determine the involved mechanisms, they demonstrate a decreased active muscle stiffness after a fatiguing task involving intermittent submaximal isometric contractions. Further studies should now determine whether this change in stiffness affects performance and risk of injury.

The age-related decrease in neuromuscular performance is usually accentuated in very old age (>80 yr) as evidenced by a marked reduction in maximal force production. However, little is known about the ability to rapidly produce force, which limits daily activities and increase the risk of falling. We aimed to assess rapid force production characteristics and rate of muscle activation of the knee extensors in 15 very old (82 ± 1 yr) vs 12 young (24 ± 4 yr) men. Maximal force (F max ) and maximal rate of force development (RFD max ) were determined during separated specific isometric contractions. EMG from the vastus lateralis was analyzed to assess the rate of EMG rise (RER). Finally, RFD and RER were examined at time intervals of 0-50, 50-100, 100-200, 0-200 ms and 0-30, 0-50, 0-75 ms, respectively. We reported lower F max (414 ± 91 N vs. 661 ± 139 N) and absolute RFD max (8720 ± 2056 N*s -1 vs. 5700 ± 2474 N*s -1 ) in the very old men compared to young men (P < 0.01). When normalized to F max , RFD max was similar between groups. Normalized RFD at 0-50 and 0-200 ms were lower (-34% and - 46%, P = 0.04) for the very old men, while no difference was observed at 50-100 ms and 100-200 ms. RER values were higher (~346%, P < 0.01) for the young men at every time interval. These data suggest that the decline in RFD max is associated with the decrease in F max . Impairments in RFD were accentuated in the first phase of the contraction, which has been associated in the literature with impairments in neural factors associated with aging., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Vibratory stimuli are thought to have the potential to promote neural and/or muscular (re)conditioning. This has been well described for whole-body vibration (WBV), which is commonly used as a training method to improve strength and/or functional abilities. Yet, this technique may present some limitations, especially in clinical settings where patients are unable to maintain an active position during the vibration exposure. Thus, a local vibration (LV) technique, which consists of applying portable vibrators directly over the tendon or muscle belly without active contribution from the participant, may present an alternative to WBV. The purpose of this narrative review is (1) to provide a comprehensive overview of the literature related to the acute and chronic neuromuscular changes associated with LV, and (2) to show that LV training may be an innovative and efficient alternative method to the 'classic' training programs, including in the context of muscle deconditioning prevention or rehabilitation. An acute LV application (one bout of 20-60 min) may be considered as a significant neuromuscular workload, as demonstrated by an impairment of force generating capacity and LV-induced neural changes. Accordingly, it has been reported that a training period of LV is efficient in improving muscular performance over a wide range of training (duration, number of session) and vibration (frequency, amplitude, site of application) parameters. The functional improvements are principally triggered by adaptations within the central nervous system. A model illustrating the current research on LV-induced adaptations is provided.

The aim of this study was to evaluate the effects of an 8-wk local vibration training (LVT) program on functional and corticospinal properties of dorsiflexor muscles. Forty-four young subjects were allocated to a training (VIB, n = 22) or control (CON, n = 22 ) group. The VIB group performed twenty-four 1-h sessions (3 sessions/wk) of 100-Hz vibration applied to the right tibialis anterior. Both legs were tested in each group before training (PRE), after 4 (MID) and 8 (POST) wk of training, and 2 wk after training (POST 2W ). Maximal voluntary contraction (MVC) torque was assessed, and transcranial magnetic stimulation (TMS) was used to evaluate cortical voluntary activation (VA TMS ), motor evoked potential (MEP), cortical silent period (CSP), and input-output curve parameters. MVC was significantly increased for VIB at MID for right and left legs [+7.4% ( P = 0.001) and +6.2% ( P < 0.01), respectively] and remained significantly greater than PRE at POST [+12.0% ( P < 0.001) and +10.1% ( P < 0.001), respectively]. VA TMS was significantly increased for right and left legs at MID [+4.4% ( P < 0.01) and +4.7% ( P < 0.01), respectively] and at POST [+4.9% ( P = 0.001) and +6.2% ( P = 0.001), respectively]. These parameters remained enhanced in both legs at POST 2W MEP and CSP recorded during MVC and input-output curve parameters did not change at any time point for either leg. Despite no changes in excitability or inhibition being observed, LVT seems to be a promising method to improve strength through an increase of maximal voluntary activation, i.e., neural adaptations. Local vibration may thus be further considered for clinical or aging populations. NEW & NOTEWORTHY The effects of a local vibration training program on cortical voluntary activation measured with transcranial magnetic stimulation were assessed for the first time in dorsiflexors, a functionally important muscle group. We observed that training increased maximal voluntary strength likely because of the strong and repeated activation of Ia spindle afferents during vibration training that led to changes in the cortico-motoneuronal pathway, as demonstrated by the increase in cortical voluntary activation., (Copyright © 2017 the American Physiological Society.)

Introduction: Wearing compression stockings (CS) may improve postural stability through additional cutaneous feedback. The aim of this study was to further determine how wearing CS could influence spinal excitability by investigating ankle muscle H-reflexes., Methods: Fifteen subjects were asked to stand barefoot on a rigid floor with their eyes open. H-reflex amplitude was measured in the soleus (SOL), fibularis longus (FL), and tibialis anterior (TA) muscles, with and without CS. Concomitant M-waves and baseline electromyographic activity (EMG) were monitored., Results: Baseline EMG activity and concomitant M-wave amplitude remained stable across conditions in all tested muscles. Although CS did not affect the H-reflex in the SOL (+0.8 ± 19.2%; P = 0.77) and FL (-10.0± 33.2%; P = 0.28) muscles, the TA H-reflex was significantly depressed (-21.9% ± 24.0%; P = 0.03)., Conclusions: These results suggest decreased spinal motoneuron excitability and/or increased presynaptic inhibition of Ia-afferent terminals through increased cutaneous inputs provided by CS while standing. Muscle Nerve 55: 596-598, 2017., (© 2016 Wiley Periodicals, Inc.)

This study aimed to evaluate the intra- and inter-day reliability of transcranial magnetic stimulation (TMS)-related measurements recorded from the tibialis anterior (TA) muscle. Thirteen healthy young men and women (23±4years) performed 3 testing sessions to assess intra- (i.e., two sessions performed the same day) and inter-day (i.e. two sessions performed one week apart) reliability of (i) dorsiflexion cortical maximal voluntary activation level (VATMS), (ii) TA corticospinal excitability assessed through the amplitude of the motor evoked potentials (MEP) recorded during 100, 75 and 50% maximal voluntary contractions (MVC), and (iii) intracortical inhibition investigated via the cortical silent period (CSP) recorded at the same % MVC. Absolute (i.e., coefficient of variation (CV) and standard error of the mean (SEM)), and relative (i.e., intraclass correlation coefficients (ICC)) reliability parameters were calculated. VATMS demonstrated excellent intra- and inter-day reliabilities (ICC: 0.80 and 0.99; CV: 1.7 and 0.8%, respectively). MEPs and CSPs presented moderate to excellent intra- and inter-day reliabilities, while input-output curves extracted parameters presented highly variable outcomes. These results suggest that most TA corticospinal measurements during voluntary contractions can be used to quantify corticospinal adaptations after acute (e.g. fatigue) or long term (e.g. training) interventions., (Copyright © 2016 Elsevier B.V. All rights reserved.)

This study aimed to evaluate the sex difference in active muscle stiffness of the tibialis anterior muscle (TA) through shear modulus measurements performed using supersonic shear imaging (SSI) technique. Twenty-five women and twenty-one men participated in this study. Joint torque, electromyographic (EMG) activity and shear modulus were measured during two sets of submaximal dorsiflexions performed at 20, 30, 40, 50 and 60% of maximal voluntary contraction (MVC) in a random order. The first set was devoted to the EMG recordings and the second set was devoted to the elastographic measurements. For each set, subjects performed three 5-s trials at each level of submaximal voluntary contraction. Stiffness indexes were calculated as the slopes of the linear regressions established between shear modulus and joint torque (SI TORQUE ) or estimated TA EMG levels (SI EMG ). In the present study, no sex effect was reported for SI TORQUE , SI EMG (p=0.76 and p=0.86, respectively), and shear modulus measured at various contraction levels. The results highlight that men and women presented similar TA active stiffness indexes determined using SSI. Regardless of sex, this result suggests similar intrinsic stiffness for the contracting TA., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Purpose: Prolonged local vibration is known to impair muscle performance. While involved mechanisms were previously evidenced at the spinal level, changes at the cortical level were also hypothesized. The aims of the present study were to investigate the effects of 30 min of 100-Hz tibialis anterior muscle vibration on force production capacities and to further identify the respective changes in spinal loop properties, descending voluntary drive and corticospinal properties., Methods: Thirteen subjects were tested before and after a vibration condition, and before and after a resting control condition. Maximal voluntary contraction (MVC) in dorsiflexion was measured. Transcranial magnetic stimulation was superimposed during MVCs to assess cortical voluntary activation (VATMS), motor-evoked potential amplitude (MEP) and cortical silent period length (CSP). MEP and CSP were also measured during 50 and 75 % MVC contractions. Spinal excitability was investigated by mean of H-reflex., Results: There were no vibration effects on MVC (p = 0.805), maximal EMG activity (p = 0.653), VATMS (p = 1), and CSP (p = 0.877). Vibration tended to decrease MEP amplitude (p = 0.117). H-reflex amplitude was depressed following vibration (p = 0.008)., Conclusions: Dorsiflexion maximal force production capacities were unaffected by 30 min of tibialis anterior muscle vibration, despite spinal loop and corticospinal excitabilities being reduced. These findings suggest that acute prolonged vibration has the potential to modulate corticospinal excitability of lower limb muscles without a concomitant functional consequence.

Soleus (SOL) corticospinal excitability has been reported to increase during Achilles tendon vibration. The aim of the present study was to further investigate SOL corticospinal excitability and elucidate the changes to intracortical mechanisms during Achilles tendon vibration. Motor-evoked potentials (MEPs) were elicited in the SOL by transcranial magnetic stimulation (TMS) of the corresponding motor cortical area of the leg with and without 50-Hz Achilles tendon vibration. SOL input-output curves were determined. Paired-pulse protocols were also performed to investigate short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) by conditioning test TMS pulses with sub-threshold TMS pulses at inter-stimulus intervals of 3 and 13 ms, respectively. During Achilles tendon vibration, motor threshold was lower than in the control condition (43 ± 13 vs. 49 ± 11 % of maximal stimulator output; p = 0.008). Input-output curves were also influenced by vibration, i.e. there was increased maximal MEP amplitude (0.694 ± 0.347 vs. 0.268 ± 0.167 mV; p < 0.001), decreased TMS intensity to elicit a MEP of half the maximal MEP amplitude (100 ± 13 vs. 109 ± 9 % motor threshold; p = 0.009) and a strong tendency for decreased slope constant (0.076 ± 0.04 vs. 0.117 ± 0.04; p = 0.068). Vibration reduced ICF (98 ± 61 vs. 170 ± 105 % of test MEP amplitude; p = 0.05), but had no effect on SICI (53 ± 26 vs. 48 ± 22 % of test MEP amplitude; p = 0.68). The present results further document the increased vibration-induced corticospinal excitability in the soleus muscle and suggest that this increase is not mediated by changes in SICI or ICF.

This study investigated the effects of acute hypoxia on spinal reflexes and soleus muscle function after a sustained contraction of the plantar flexors at 40% of maximal voluntary isometric contraction (MVC). Fifteen males (age 25.3 ± 0.9 year) performed the fatigue task at two different inspired O₂ fractions (FiO₂ = 0.21/0.11) in a randomized and single-blind fashion. Before, at task failure and after 6, 12 and 18 min of passive recovery, the Hoffman-reflex (H max) and M-wave (M max) were recorded at rest and voluntary activation (VA), surface electromyogram (RMSmax), M-wave (M sup) and V-wave (V sup) were recorded during MVC. Normalized H-reflex (H max/M max) was significantly depressed pre-exercise in hypoxia compared with normoxia (0.31 ± 0.08 and 0.36 ± 0.08, respectively, P < 0.05). Hypoxia did not affect time to task failure (mean time of 453.9 ± 32.0 s) and MVC decrease at task failure (-18% in normoxia vs. -16% in hypoxia). At task failure, VA (-8%), RMSmax/M sup (-11%), H max/M max (-27%) and V sup/M sup (-37%) decreased (P < 0.05), but with no FiO2 effect. H max/M max restored significantly throughout recovery in hypoxia but not in normoxia, while V sup/M sup restored significantly during recovery in normoxia but not in hypoxia (P < 0.05). Collectively, these findings indicate that central adaptations resulting from sustained submaximal fatiguing contraction were not different in hypoxia and normoxia at task failure. However, the FiO₂-induced differences in spinal loop properties pre-exercise and throughout recovery suggest possible specific mediation by the hypoxic-sensitive group III and IV muscle afferents, supraspinal regulation mechanisms being mainly involved in hypoxia while spinal ones may be predominant in normoxia.

Purpose: Since experimental techniques classically used to investigate the mechanical behavior of muscle in vivo assess global mechanical properties of the musculo-articular complex, the aim of the present study was to assess the feasibility and reliability of localized contracting biceps brachii elastic properties' measurements using elastography., Methods: Twelve subjects participated in intra-session, inter-session and inter-observer reliability experiments. They were asked to perform a linear torque ramp of 30 s from 0 to 75 % of maximal voluntary contraction. Joint torque, electromyographic (EMG) activity and shear elastic modulus were synchronously measured in the biceps brachii. Elastic properties were determined by stiffness indexes calculated as the slopes of the linear regressions established between shear modulus and joint torque or EMG levels. Compliance indexes were also obtained by plotting logarithmic values of shear modulus versus torque or EMG., Results: Determination coefficients were high for the four relationships (0.87-0.94). In addition, reliability was significantly higher for the compliance index taking into account logarithmic values of EMG activity (mean coefficient of variation of 5.3 ± 4.1%; P = 0.003). Although not statistically different from other methods (P = 0.111), the mean intra-class correlation coefficient was excellent (0.98 ± 0.01) and the standard error in measurement very low (0.04 ± 0.01 for a mean index of -0.73 ± 0.28)., Conclusions: Biceps brachii elastic properties can be reliably investigated during contraction via elastography using the proposed fitting method. Use of this technique could provide new insights into the understanding of localized muscle elastic properties adaptations.

Projections from the somesthetic cortex are believed to be involved in the modulation of motor cortical excitability by muscle vibration. The aim of the present pilot study was to analyse the effects of a vibration intervention on short-latency afferent inhibition (SAI), long-latency afferent inhibition (LAI), and afferent facilitation (AF), three intracortical mechanisms reflecting sensorimotor integration. Abductor pollicis brevis (APB) SAI, AF and LAI were investigated on 10 subjects by conditioning test transcranial magnetic stimulation pulses with median nerve electrical stimulation at inter-stimuli intervals in the range 15-25 ms, 25-60 ms, and 100-200 ms, respectively. Test motor evoked potentials (MEPs) were compared to unconditioned MEPs. Measurements were performed before and just after 15 min of vibration applied to the muscle belly of APB at a frequency of 80 Hz. SAI and LAI responses were significantly reduced compared to unconditioned test MEPs (P=0.039 and P<0.001, respectively). AF MEP amplitude was greater than SAI and LAI one (P=0.009 and P=0.004, respectively), but not different from test MEP (P=0.511). There was no significant main effect of vibration (P=0.905). However, 4 subjects were clearly identified as responders. Their mean vibration-induced increase was 324 ± 195% in APB SAI MEP amplitude, and 158 ± 53% and 319 ± 80% in AF and LAI, respectively. Significant differences in SAI, AF and LAI vibration-induced changes were found for responders when compared to non-responders (P=0.019, P=0.038, and P=0.01, respectively). A single session of APB vibration may increase sensorimotor integration, via decreased inhibition and increased facilitation. However, such results were not observed for all subjects, suggesting that other factors (such as attention to the sensory inputs) may have played a role., (Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.)

The aim of this study was to investigate the relative contribution of human muscle reoxygenation on force recovery following a maximal voluntary contraction (MVC). Ten athletes (22·9 ± 4·0 years) executed a plantar-flexion sequence including two repeated MVCs [i.e. a 30-s MVC (MVC(30)) followed by a 10-s MVC (MVC(10))] separated by 10, 30, 60, 120 or 300 s of passive recovery. A 10-min passive recovery period was allowed between each MVC sequence. This procedure was randomly repeated with two different recovery conditions: without (CON) or with (OCC) arterial occlusion of the medial gastrocnemius. During OCC, the occlusion was maintained from the end of MVC(30) to the end of MVC(10). Muscle oxygenation (Near-infrared spectroscopy, NIRS, [Hb(diff) ]) was continuously measured during all MVC sequences and expressed as a percentage of the maximal changes in optical density observed during MVC(30). Maximal Torque was analysed at the start of each contraction. Torque during each MVC(10) was expressed as a percentage of the Torque during the previous MVC(30). Torque recovery was complete within 300 s after MVC(30) during CON (MVC(10) = 101·8 ± 5·0%); 88·6 ± 8·9% of the Torque was recovered during OCC (P = 0·005). There was also a moderate correlation between absolute level of muscle oxygenation and Torque (r = 0·32 (90% CI, 0·09;0·52), P = 0·02). Present findings confirm the role of human muscle oxygenation in muscular force recovery during repeated-maximal efforts. However, the correlation between absolute muscle oxygenation and force level during recovery is only moderate, suggesting that other mechanisms are likely involved in the force recovery process., (© 2012 The Authors Clinical Physiology and Functional Imaging © 2012 Scandinavian Society of Clinical Physiology and Nuclear Medicine.)

Alteration of Soleus (SOL) H-reflex has been reported after prolonged vibratory exposure and it was hypothesized that presynaptic inhibition, known to depress the H-reflex during vibration, largely contributed to the H-reflex changes. To confirm this hypothesis, the purpose of the present study was to quantify the SOL H-reflex changes between sitting and standing positions (postural modulation) with or without the after-effects of 1h of Achilles tendon vibration. Indeed, postural modulation of the SOL H-reflex has been reported to inform on the level of presynaptic inhibition exerted on Ia afferents. SOL H-reflex and M waves were measured in healthy voluntary subjects in both sitting and standing positions before and after 1h of Achilles vibration (frequency: 50 Hz) applied in sitting position (vibration group, n=11) or before and after 1h of sitting position only (control group, n=6). SOL H(max)/M(max) ratios were calculated. Furthermore, in order to quantify presynaptic inhibition induced by prolonged vibration, an index of SOL H-reflex postural modulation was calculated as the standing H(max)/M(max) ratio relative to the sitting one. After 1h of Achilles tendon vibration, a significant decrease in the SOL H(max)/M(max) ratio was observed both in sitting and standing positions (p<0.05). However, the decrease was more pronounced in the standing position, leading to a significant decrease of the index of SOL H-reflex postural modulation. Those results suggest that presynaptic inhibition could have largely contributed to the H-reflex decrease observed after one bout of vibration., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

Prolonged vibration is known to alter muscle performance. Attenuation of Ia afferent efficacy is the main mechanism suggested. However, changes in motor cortex excitability could also be hypothesized. The purpose of the present study was therefore to analyze the acute and outlasting effects of 1 h of Achilles tendon vibration (frequency, 50 Hz) on the soleus (SOL) and tibialis anterior (TA) neuromuscular excitability. Spinal excitability was investigated by means of H-reflexes and F-waves while cortical excitability was characterized by motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation. Twelve subjects performed the experimental procedures 3 times: at the beginning of the testing session (PRE), immediately after 1 h of Achilles tendon vibration (POST), and 1 h after the end of vibration (POST-1H). Prolonged vibration led to acute reduced H-reflex amplitudes for SOL only (46.9% ± 7.7% vs. 32.8% ± 7%; p = 0.006). Mainly presynaptic inhibition mechanisms were thought to be involved because of unchanged F-wave persistence and amplitude mean values, suggesting unaffected motoneuronal excitability. While no acute effects were reported for SOL and TA cortical excitability, both muscles were characterized by an outlasting increase in their MEP amplitude (0.64 ± 0.2 mV vs. 0.43 ± 0.18 mV and 2.17 ± 0.56 mV vs. 1.26 ± 0.36 mV, respectively; p < 0.05). The high modulation of Ia afferent input by vibration led to changes in motor cortex excitability that could contribute to the enhancement in muscular activation capacities reported after chronic use of tendon vibration.

In a previous study, Achilles tendon vibrations were enough to improve the triceps surae (TS) activation capacities and also to slightly increase TS Hoffmann reflex (H-reflex) obtained by summing up soleus (Sol) and gastrocnemii (GM and GL) EMGs. The purpose of the present study was to analyze separately Sol and GM or GL reflexes to account for different effects of the vibrations on the reflex excitability of the slow soleus and of the gastrocnemii muscles. A control group (n = 13) and a vibration group (n = 16) were tested in pre-test and post-test conditions. The Achilles tendon vibration program consisted of 1 h of daily vibration (frequency: 50 Hz) applied during 14 days. Maximal Sol, GM and GL H-reflexes, and M-waves were recorded, and their H(max)/M(max) ratios gave the index of reflex excitability. After the vibration protocol, only Sol H(max)/M(max) was enhanced (p < 0.001). The enhanced Sol reflex excitability after vibration is in favor of a decrease in the pre-synaptic inhibition due to the repeated vibrations and the high solicitation of the reflex pathway. Those results of a short period of vibration applied at rest may be limited to the soleus because of its high density in muscle spindles and slow motor units, both structures being very sensitive to vibrations.

Clinical studies frequently report an increase in stiffness and a loss of range of motion at joints placed in disuse or immobilization. This is notably the case for subjects maintained in bed for a long period, whilst their joints are not affected. Recently we documented on healthy subjects the benefit in terms of force and activation capacities of the triceps surae offered by vibrations applied to the Achilles tendon. Knowing that stiffness changes may contribute to force changes, the aim of the present study was to investigate the effects of tendon vibration on the triceps surae stiffness of healthy subjects. The vibration program consisted in 14 days of 1h daily Achilles tendon vibration applied at rest. Nineteen healthy students were involved in this study. Before and at the end of the vibration program, musculo-tendinous stiffness in active conditions was determined by use of a quick-release test. Passive stiffness was also analyzed by a flexibility test: passive torque-angle relationships were established from maximal plantar-flexion to maximal dorsiflexion. Passive stiffness indexes at 10°, 15° and 20° dorsiflexion were defined as the slope of the relationships at the corresponding angle. Tendinous reflex, influenced by stiffness values, was also investigated as well as the H reflex to obtain an index of the central reflex excitability. After the program, musculo-tendinous stiffness was significantly decreased (p=.01). At the same time, maximal passive dorsiflexion was increased (p=.005) and passive stiffness indexes at 10°, 15° and 20° dorsiflexion decreased (p<.001; p<.001 and p=.011, respectively). Tendinous reflex also significantly decreased. As the triceps surae parameters are diminished after the vibration program, it could be beneficial to immobilized persons as hypo-activity is known to increase muscular stiffness., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Clinical manual tests refer to increased ankle stiffness in children immobilized due to hip osteochondritis. The aim of the present study was to investigate musculo-articular stiffness via different techniques in immobilized children to confirm or not and quantify these observations. Ankle stiffness was quantified monthly during the long immobilization period in three diseased children and compared to healthy age-matched children. Sinusoidal perturbations were used to evaluate musculo-articular (MA) stiffness of the ankle plantar-flexors. The stiffness index (SI(MA-EMG)) was the slope of the linear relationship between angular stiffness and plantar-flexion torque normalized with electromyographic activity of the triceps surae (TS). The stiffness of the ankle plantar-flexors was also indirectly evaluated using the TS electromechanical delay (EMD). SI(MA-EMG) was greater for diseased children, and this higher stiffness was confirmed by the higher EMD values found in these immobilized children. Furthermore, both parameters indicated that ankle stiffness continues to increase through immobilization period. This study gives a quantitative evaluation of ankle stiffness changes through the immobilization period imposed to children treated for hip osteochondritis. The use of EMD measurement to indirectly evaluate these stiffness changes is also validated. This study shed for the first time some light into the patterns of muscle modifications following immobilization in children., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)