Are females getting more fatigable as they age?

PURPOSE: The aim of this study was to compare performance fatigability between young (n = 13; 18-35 yr.; 23.5 ± 3.3 yr.), old (n = 13; 60-79 yr.; 68.2 ± 4.3 yr.), and very old (n = 11; ≥ 80 yr.; 85.6 ± 1.8 yr.) females during single-limb isometric (ISO) vs. isokinetic concentric (CON) vs. cycling (BIKE) fatiguing tasks. METHODS: Participants randomly performed three incremental fatiguing tasks where increments were set as percentage of body weight to better reflect the daily life: (1) ISO and (2) CON consisted of stages of 75 contractions (120 s, 0.8 s on/0.8 s off) on an isokinetic dynamometer and (3) BIKE consisted of stages of 120 s at 37.5 rpm with similar duty cycle. Knee extensors maximal force, voluntary activation and potentiated twitch amplitude were measured at baseline, after each stage and at exhaustion. RESULTS: Compared to young, exercise performance was 20% and 53% lower in old and very old females in ISO, 46% and 76% lower in CON and 32% and 62% lower in BIKE (all p < 0.01). For a given workload (i.e. common stages), performance fatigability (i.e. force loss) was greater for very old compared to young females in CON only (p = 0.018). At exhaustion, performance fatigability was similar across groups and conditions (~ 30%; all p > 0.05), with similar impairments in both voluntary activation and twitch amplitude. CONCLUSION: These results emphasize the importance of the kind of fatiguing task and modalities of evaluation when investigating the effects of aging on performance fatigability.

Do soleus responses to transcutaneous spinal cord stimulation show similar changes to H-reflex in response to Achilles tendon vibration?

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 (Mmax) and POST-VIB (36 ± 4% of Mmax) values being lower than PRE-values (48 ± 6% of Mmax). Similarly, TSCS responses changed over time, VIB (9 ± 5% of Mmax) and POST-VIB (27 ± 5% of Mmax) values being lower than PRE-values (46 ± 6% of Mmax). 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.

Motor function and gait decline in individuals with cerebral palsy during adulthood: a narrative review of potential physiological determinants.

Cerebral palsy (CP) is the most common childhood-onset disability. The evolution of gait according to severity is well known amongst children and thought to peak between 8 and 12 years of age among those walking without assistive devices. However, among adults, clinical experience as well as scientific studies report, through clinical assessments, questionnaires and interviews, increasing walking difficulties leading to an increased dependency of assistive devices in everyday ambulation. For many individuals with CP, this change will occur around 30-40 years, with the risk of losing mobility increasing with age. This narrative review aims to first provide objective evidence of motor function and gait decline in adults with CP when ageing, and then to offer mechanistic hypotheses to explain those alterations. Many studies have compared individuals with CP to the typically developing population, yet the evolution with ageing has largely been understudied. Comorbid diagnoses comprise one of the potential determinants of motor function and gait decline with ageing in people with CP, with the first manifestations happening at an early age and worsening with ageing. Similarly, ageing appears to cause alterations to the neuromuscular and cardiovascular systems at an earlier age than their typically developing (TD) peers. Future studies should, however, try to better understand how the physiological particularities of CP change with ageing that could pave the way for better strategies for maintaining function and quality of life in people with CP.

Enhancing endurance performance with combined imagined and actual physical practice.

PURPOSE: The perception of effort exerts influence in determining task failure during endurance performance. Training interventions blending physical and cognitive tasks yielded promising results in enhancing performance. Motor imagery can decrease the perception of effort. Whether combining motor imagery and physical training improves endurance remains to be understood, and this was the aim of this study. METHODS: Participants (24 ± 3 year) were assigned to a motor imagery (n = 16) or a control (n = 17) group. Both groups engaged in physical exercises targeting the knee extensors (i.e., wall squat, 12 training sessions, 14-days), with participants from the motor imagery group also performing motor imagery. Each participant visited the laboratory Pre and Post-training, during which we assessed endurance performance through a sustained submaximal isometric knee extension contraction until task failure, at either 20% or 40% of the maximal voluntary contraction peak torque. Perceptions of effort and muscle pain were measured during the exercise. RESULTS: We reported no changes in endurance performance for the control group. Endurance performance in the motor imagery group exhibited significant improvements when the intensity of the sustained isometric exercise closely matched that used in training. These enhancements were less pronounced when considering the higher exercise intensity. No reduction in perception of effort was observed in both groups. There was a noticeable decrease in muscle pain perception within the motor imagery group Post training. CONCLUSION: Combining motor imagery and physical training may offer a promising avenue for enhancing endurance performance and managing pain in various contexts.

Acute effects of conventional versus wide-pulse neuromuscular electrical stimulation on quadriceps evoked torque and neuromuscular function.

PURPOSE: The effectiveness of a neuromuscular electrical stimulation (NMES) program is proportional to the level of evoked torque, which can be achieved with either conventional or wide-pulse stimulations. The aim of this study was to compare evoked torque, objective fatigability, and related peripheral and central alterations, as well as changes in central nervous system (CNS) excitability induced by an acute session of conventional versus wide-pulse NMES. METHODS: Seventeen young men underwent three 20-min NMES sessions: conventional (0.2 ms/50 Hz), wide-pulse at 50 Hz (1 ms/50 Hz), and wide-pulse at 100 Hz (1 ms/100 Hz). Neuromuscular measurements (i.e., maximal voluntary contraction, voluntary activation, evoked responses to femoral nerve stimulation, and CNS excitability) were performed on the right quadriceps femoris muscle before and after each NMES session. CNS excitability was measured using transcranial magnetic, thoracic, and transcutaneous spinal cord stimulations. RESULTS: The level of evoked torque was not significantly different between conventional and wide-pulse protocols applied at the maximal tolerable current intensity. All NMES protocols induced objective fatigability (~14% decrease in maximal voluntary contraction torque, p < 0.001) associated with peripheral (decrease in doublet torque and potentiated M-wave amplitude, p = 0.002 and p < 0.001, respectively) but not central (unchanged voluntary activation, p = 0.79) alterations. However, these acute changes did not differ between NMES protocols and none of the NMES protocols modified markers of CNS excitability. CONCLUSION: These results may allow to conjecture that chronic effects and treatment effectiveness could be comparable between conventional and wide-pulse NMES.

Acute passive stretching has no effect on gastrocnemius medialis stiffness in children with unilateral cerebral palsy.

PURPOSE: The aim of this study was to investigate the effects of an acute high-intensity, long-duration passive stretching session of the plantar flexor muscles, on maximal dorsiflexion (DF) angle and passive stiffness at both ankle joint and gastrocnemius medialis (GM) muscle levels in children with unilateral cerebral palsy (CP). METHODS: 13 children [mean age: 10 years 6 months, gross motor function classification system (GMFCS): I] with unilateral CP underwent a 5 min passive stretching session at 80% of maximal DF angle. Changes in maximal DF angle, slack angle, passive ankle joint and GM muscle stiffness from PRE- to POST-intervention were determined during passive ankle mobilization performed on a dynamometer coupled with shear wave elastography measurements (i.e., ultrasound) of the GM muscle. RESULTS: Maximal DF angle and maximal passive torque were increased by 6.3° (P < 0.001; + 50.4%; 95% CI 59.9, 49.9) and 4.2 Nm (P < 0.01; + 38.9%; 95% CI 47.7, 30.1), respectively. Passive ankle joint stiffness remained unchanged (P = 0.9; 0%; 95% CI 10.6, - 10.6). GM muscle shear modulus was unchanged at maximal DF angle (P = 0.1; + 34.5%; 95% CI 44.7, 24.7) and at maximal common torque (P = 0.5; - 4%; 95% CI - 3.7, - 4.3), while it was decreased at maximal common angle (P = 0.021; - 35%; 95% CI - 11.4, - 58.5). GM slack angle was shifted in a more dorsiflexed position (P = 0.02; + 20.3%; 95% CI 22.6, 18). CONCLUSION: Increased maximal DF angle can be obtained in the paretic leg in children with unilateral CP after an acute bout of stretching using controlled parameters without changes in passive stiffness at joint and GM muscle levels. CLINICAL TRIAL NUMBER: NCT03714269.

Wide-pulse electrical stimulation of the quadriceps allows greater maximal evocable torque than conventional stimulation.

PURPOSE: The effectiveness of a neuromuscular electrical stimulation (NMES) program has been shown to be proportional to the maximal evocable torque (MET), which is potentially influenced by pulse characteristics such as duration and frequency. The aim of this study was to compare MET between conventional and wide-pulse NMES at two different frequencies. METHODS: MET-expressed as a percentage of maximal voluntary contraction (MVC) torque-and maximal tolerable current intensity were quantified on 71 healthy subjects. The right quadriceps was stimulated with three NMES protocols using different pulse duration/frequency combinations: conventional NMES (0.2 ms/50 Hz; CONV), wide-pulse NMES at 50 Hz (1 ms/50 Hz; WP50) and wide-pulse NMES at 100 Hz (1 ms/100 Hz; WP100). The proportion of subjects reaching the maximal stimulator output (100 mA) before attaining maximal tolerable current intensity was also quantified. RESULTS: The proportion of subjects attaining maximal stimulator output was higher for CONV than WP50 and WP100 (p < 0.001). In subjects who did not attain maximal stimulator output in any protocol, MET was higher for both WP50 and WP100 than for CONV (p < 0.001). Maximal tolerable current intensity was lower for both WP50 and WP100 than for CONV and was also lower for WP100 than for WP50 (p < 0.001). CONCLUSION: When compared to conventional NMES, wide-pulse protocols resulted in greater MET and lower maximal tolerable current intensity. Overall, this may lead to better NMES training/rehabilitation effectiveness and less practical issues associated with maximal stimulator output limitations.

Protocols Targeting Afferent Pathways via Neuromuscular Electrical Stimulation for the Plantar Flexors: A Systematic Review.

This systematic review documents the protocol characteristics of studies that used neuromuscular electrical stimulation protocols (NMES) on the plantar flexors [through triceps surae (TS) or tibial nerve (TN) stimulation] to stimulate afferent pathways. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, was registered to PROSPERO (ID: CRD42022345194) and was funded by the Greek General Secretariat for Research and Technology (ERA-NET NEURON JTC 2020). Included were original research articles on healthy adults, with NMES interventions applied on TN or TS or both. Four databases (Cochrane Library, PubMed, Scopus, and Web of Science) were systematically searched, in addition to a manual search using the citations of included studies. Quality assessment was conducted on 32 eligible studies by estimating the risk of bias with the checklist of the Effective Public Health Practice Project Quality Assessment Tool. Eighty-seven protocols were analyzed, with descriptive statistics. Compared to TS, TN stimulation has been reported in a wider range of frequencies (5-100, vs. 20-200 Hz) and normalization methods for the contraction intensity. The pulse duration ranged from 0.2 to 1 ms for both TS and TN protocols. It is concluded that with increasing popularity of NMES protocols in intervention and rehabilitation, future studies may use a wider range of stimulation attributes, to stimulate motor neurons via afferent pathways, but, on the other hand, additional studies may explore new protocols, targeting for more optimal effectiveness. Furthermore, future studies should consider methodological issues, such as stimulation efficacy (e.g., positioning over the motor point) and reporting of level of discomfort during the application of NMES protocols to reduce the inherent variability of the results.

Cardiorespiratory Fitness and Neuromuscular Function of Mechanically Ventilated ICU COVID-19 Patients.

OBJECTIVES: The aim of the current study was to investigate the level of cardiorespiratory fitness and neuromuscular function of ICU survivors after COVID-19 and to examine whether these outcomes are related to ICU stay/mechanical ventilation duration. DESIGN: Prospective nonrandomized study. SETTING: Patients hospitalized in ICU for COVID-19 infection. PATIENTS: Sixty patients hospitalized in ICU (mean duration: 31.9 ± 18.2 d) were recruited 4-8 weeks post discharge from ICU. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Patients visited the laboratory on two separate occasions. The first visit was dedicated to quality of life questionnaire, cardiopulmonary exercise testing, whereas measurements of the knee extensors neuromuscular function were performed in the second visit. Maximal oxygen uptake (V o2 max) was 18.3 ± 4.5 mL·min -1 ·kg -1 , representing 49% ± 12% of predicted value, and was significantly correlated with ICU stay/mechanical ventilation (MV) duration ( R = -0.337 to -0.446; p < 0.01 to 0.001), as were maximal voluntary contraction and electrically evoked peak twitch. V o2 max (either predicted or in mL· min -1 ·kg -1 ) was also significantly correlated with key indices of pulmonary function such as predicted forced vital capacity or predicted forced expiratory volume in 1 second ( R = 0.430-0.465; p ≤ 0.001) and neuromuscular function. Both cardiorespiratory fitness and neuromuscular function were correlated with self-reported physical functioning and general health status. CONCLUSIONS: V o2 max was on average only slightly above the 18 mL·min -1 ·kg -1 , that is, the cut-off value known to induce difficulty in performing daily tasks. Overall, although low physical capacities at admission in ICU COVID-19 patients cannot be ruled out to explain the association between V o2 max or neuromuscular function and ICU stay/MV duration, altered cardiorespiratory fitness and neuromuscular function observed in the present study may not be specific to COVID-19 disease but seem applicable to all ICU/MV patients of similar duration.

Acute effects of quadriceps muscle versus tendon prolonged local vibration on force production capacities and central nervous system excitability.

PURPOSE: The present study aimed to directly compare the effects of 30 min muscle (VIBmuscle) vs. tendon (VIBtendon) local vibration (LV) to the quadriceps on maximal voluntary isometric contraction (MVIC) and rate of torque development (RTD) as well as on central nervous system excitability (i.e. motoneuron and cortical excitability). METHODS: Before (PRE) and immediately after (POST) LV applied to the quadriceps muscle or its tendon, we investigated MVIC and RTD (STUDY #1; n = 20) or vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) electromyography responses to thoracic electrical stimulation (TMEPs; motoneuron excitability) and transcranial magnetic stimulation (MEPs; corticospinal excitability) (STUDY #2; n = 17). MEP/TMEP ratios were further calculated to quantify changes in cortical excitability. RESULTS: MVIC decreased at POST (P = 0.017) without any difference between VIBtendon and VIBmuscle, while RTD decreased for VIBtendon (P = 0.013) but not VIBmuscle. TMEP amplitudes were significantly decreased for all muscles (P = 0.014, P < 0.001 and P = 0.004 for VL, VM and RF, respectively) for both LV sites. While no changes were observed for MEP amplitude, MEP/TMEP ratios increased at POST for VM and RF muscles (P = 0.009 and P = 0.013, respectively) for both VIBtendon and VIBmuscle. CONCLUSION: The present results suggest that prolonged muscle and tendon LV are similarly effective in modulating central nervous system excitability and decreasing maximal force. Yet, altered explosive performance after tendon but not muscle LV suggests greater neural alterations when tendons are vibrated.

Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity.

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% Mmax 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% Mmax, 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% Mmax, 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.

Central fatigue aetiology in prolonged trail running races.

NEW FINDINGS: What is the central question of this study? Are spinal and/or supraspinal perturbations implicated in central fatigue induced in the plantar flexor muscles following prolonged trail running races? What is the main finding and its importance? The study confirmed the presence of central fatigue following various trail running distances from 40 to 170 km. The reduction in the V-wave in conjunction with the lack of change in the H-reflex suggests that a major component of this central fatigue may arise from supraspinal mechanisms in the plantar flexor muscles. ABSTRACT: Trail running races are known to induce considerable impairments in neuromuscular function of which central mechanisms are a substantial component. However, the loci of this central fatigue (i.e. supraspinal and/or spinal) is not well identified. The aim of this study was to better understand central fatigue aetiology induced in the plantar flexor muscles by various trail running distances from 40 to 170 km. Eighteen runners participated in the study and neuromuscular function of their plantar flexors was tested before (PRE) and after (POST) various races during the Ultra-Trail du Mont Blanc. Neuromuscular function was evaluated with voluntary and evoked contractions using electrical tibial nerve stimulation. H-reflex and V-wave responses were also measured during submaximal and maximal voluntary contraction, respectively. Reductions in maximal voluntary contraction torque (-29%; P < 0.001) and voluntary activation level (-12%; P < 0.001) were observed after trail running races. The V-wave was reduced in soleus (-35%; P = 0.003) and gastrocnemius medialis (-28%; P = 0.031), with no changes for the H-reflex in soleus (P = 0.577). The present study confirmed the presence of central fatigue following trail running exercise. The reduction in the V-wave in conjunction with the lack of change in the H-reflex suggests that a major component of this central fatigue may arise from supraspinal mechanisms.

Bedside voluntary and evoked forces evaluation in intensive care unit patients: a narrative review.

Around one third of intensive care unit (ICU) patients will develop severe neuromuscular alterations, known as intensive care unit-acquired weakness (ICUAW), during their stay. The diagnosis of ICUAW is difficult and often delayed as a result of sedation or delirium. Indeed, the clinical evaluation of both Medical Research Council score and maximal voluntary force (e.g., using handgrip and/or handheld dynamometers), two independent predictors of mortality, can be performed only in awake and cooperative patients. Transcutaneous electrical/magnetic stimulation applied over motor nerves combined with the development of dedicated ergometer have recently been introduced in ICU patients in order to propose an early and non-invasive measurement of evoked force. The aim of this narrative review is to summarize the different tools allowing bedside force evaluation in ICU patients and the related experimental protocols. We suggest that non-invasive electrical and/or magnetic evoked force measurements could be a relevant strategy to characterize muscle weakness in the early phase of ICU and diagnose ICUAW.

Effect of race distance on performance fatigability in male trail and ultra-trail runners.

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.

Acute Effects of Whole-Body Vibration Warm-up on Leg and Vertical Stiffness During Running.

ABSTRACT: Paradisis, GP, Pappas, P, Dallas, G, Zacharogiannis, E, Rossi, J, and Lapole, T. Acute effects of whole-body vibration warm-up on leg and vertical stiffness during running. J Strength Cond Res 35(9): 2433-2438, 2021-Although whole-body vibration (WBV) has been suggested as a suitable and efficient alternative to the classic warm-up routines, it is still unknown how this may impact running mechanics. Therefore, the aim of this study was to investigate the effect of a WBV warm-up procedure on lower-limb stiffness and other spatiotemporal variables during running at submaximal speed. Twenty-two males performed 30-second running bouts at 4.44 m·s-1 on a treadmill before and after a WBV and control warm-up protocols. The WBV protocol (vibration frequency: 50 Hz, peak-to-peak displacement: 4 mm) consisted of 10 sets of 30-second dynamic squatting exercises with 30-second rest periods within sets. Leg and vertical stiffness values were calculated using the spring mass model. The results indicated significant increases only after the WBV protocol for leg stiffness (3.4%), maximal ground reaction force (1.9%), and flight time (4.7%). Consequently, the WBV warm-up protocol produced a change in running mechanics, suggesting a shift toward a more aerial pattern. The functional significance of such WBV-induced changes needs further investigation to clearly determine whether it may influence running economy and peak velocity.

Task failure during sustained low-intensity contraction is not associated with a critical amount of central fatigue.

Fatigue-related mechanisms induced by low-intensity prolonged contraction in lower limb muscles are currently unknown. This study investigated central fatigue kinetics in the knee extensors during a low-intensity sustained isometric contraction. Eleven subjects sustained a 10% maximal voluntary contraction (MVC) until task failure (TF) with neuromuscular evaluation every 3 minutes. Testing encompassed transcranial magnetic stimulation to evaluate maximal voluntary activation (VATMS ), motor evoked potential (MEP), and silent period (SP), and peripheral nerve stimulation to assess M-wave. Rating of perceived exertion (RPE) was also recorded. MVC progressively decreased up to 50% of the time to TF (ie, 50%TTF ) and then plateaued, reaching ~50% at TF (P < .001). VATMS progressively decreased up to 90%TTF and then plateaued, the decrease reaching ~20% at TF (P < .001). SP was lengthened early (ie, from 20%TTF ) during the exercise and then plateaued (P < .01). No changes were reported for MEP evoked during MVC (P = .87), while MEP evoked during submaximal contractions decreased early (ie, from 20%TTF ) during the exercise and then plateaued (P < .01). RPE increased linearly during the exercise to be almost maximal at TF. M-waves were not altered (P = .88). These findings confirm that TF is due to the subjects reaching their maximal perceived effort rather than any particular central event or neuromuscular limitations since MVC at TF was far from 10% of its original value. It is suggested that strategies minimizing RPE (eg, motivational self-talk) should be employed to enhance endurance performance.

Vibration-induced depression in spinal loop excitability revisited.

KEY POINTS: While it has been well described that prolonged vibration locally applied to a muscle or its tendon (up to 1 h) decreases spinal loop excitability between homonymous Ia afferents and motoneurons, the involved mechanisms are not fully understood. By combining electrophysiological methods, this study aimed to provide new insights into the mechanisms involved in soleus decreased spinal excitability after prolonged local vibration. We report that prolonged vibration induces a decrease in motoneuron excitability rather than an increase in presynaptic mechanisms (as commonly hypothesized in the current literature). The present results may help to design appropriate clinical intervention and could reinforce the interest in vibration as a treatment for spastic patients who are characterized by spinal hyper-excitability responsible for spasms and long-lasting reflexes. ABSTRACT: The mechanisms that can explain the decreased spinal loop excitability in response to prolonged local vibration (LV), as assessed by the H-reflex, remain to be precisely determined. This study provides new insights into how prolonged Achilles' tendon LV (30 min, 100 Hz) acutely interacts with the spinal circuitry. The roles of presynaptic inhibition exerted on Ia afferents (Experiment A, n = 15), neurotransmitter release at the synapse level (Experiment B, n = 11) and motoneuron excitability (Experiment C, n = 11) were investigated in soleus. Modulation of presynaptic inhibition was assessed by conditioning the soleus H-reflex (tibial nerve electrical stimulation) with fibular nerve (D1 inhibition) and femoral nerve (heteronymous facilitation, HF) electrical stimulations. Potential vibration-induced changes in neurotransmitter depletion at the Ia afferent terminals was assessed through paired stimulations applied over the tibial nerve (HD). Intrinsic motoneuron excitability was assessed with thoracic motor evoked potentials (TMEPs) in response to electrical stimulation over the thoracic spine. Non-conditioned H-reflex was depressed by ∼60% after LV (P < 0.001), while D1 and HF H-reflexes increased by ∼75% after LV (P = 0.03 and 0.06, respectively). In Experiment B, HD remained unchanged after LV (P = 0.80). In Experiment C, TMEPs were reduced by ∼13% after LV (P = 0.01). Overall, presynaptic mechanisms do not seem to be involved in the depression of spinal excitability after LV. It rather seems to rely, at least in part, on a decrease in intrinsic motoneuron excitability. These results may have implications in reducing spinal hyper-excitability in spastic patients.

What causes increased passive stiffness of plantarflexor muscle-tendon unit in children with spastic cerebral palsy?

PURPOSE: The term 'stiffness' is commonly used in the literature to refer to various components of 'hyperresistance' by which spastic muscles oppose to their passive lengthening, especially in children with cerebral palsy (CP). Originally, stiffness consists of mechanical resistance to passive movement in the absence of any muscle activation. Increased muscle stiffness in CP therefore refers to alterations to the mechanical properties of the tissue. It is closely linked to muscle shortening, yet the two phenomena are not equivalent. Both increased stiffness and shortening are present early in childhood in the plantarflexor muscles of children with spastic CP. METHODS: This narrative review provides a comprehensive overview of the literature on passive stiffness of the plantarflexor muscles measured at the joint, muscles, fascicles, and fiber level in children with CP. Articles were searched through the Pub'Med database using the keywords "cerebral palsy" AND "stiffness". RESULT: The ambiguous use of the term 'stiffness' has been supported by discrepancies in available results, influenced by heterogeneity in materials, methodologies and characteristics of the participants among studies. Increased stiffness at the joint and muscle belly level may be explained by altered structural properties at the microscopic level. CONCLUSION: This thorough investigation of the literature suggests that the pathophysiology and the time course of the development of stiffness and contracture remain to be elucidated. A consideration of both morphological and mechanical measurements in children with CP is important when describing the alterations in their plantarflexors.

Correction to: Acute and chronic neuromuscular adaptations to local vibration training.

The author would like to correct the reference in the publication of the original article. The corrected reference is given below for your reading.

Effects of compression stockings on ankle muscle H-reflexes during standing.

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.