Sustained Isometric Contraction Research Articles

Nonlinear Dynamics Analysis of Handgrip Strength Using the Poincaré Plot Method Through Video Processing Techniques

Background/Objectives: The aim of this study was to analyze the nonlinear dynamics of handgrip strength (HGS) in young adults, focusing on hand dominance, by employing the Poincaré plot method to assess short- and long-term variability utilizing dynamometry and video motion capture during sustained isometric contractions. Methods: A cross-sectional exploratory study was conducted on 30 healthy subjects (mean age 21.6 ± 1.3 years, 13 males and 17 females), measuring HGS for both the dominant hand (DH) and nondominant hand (NDH) using a Saehan hydraulic dynamometer during 25-s sustained isometric contractions. A GoPro HERO11 Black camera recorded the dynamometer’s needle movements, and the video data were analyzed using Kinovea software. Angular values were converted to force using a calibration-based formula, and the Poincaré plot computed variability indices (short-term variability—SD1, long-term variability—SD2, ratio SD1/SD2, and area of the fitting ellipse) for each hand in relation to HGS and angular velocity (AV). Data analysis included descriptive and inferential statistics. Results: We demonstrated a strong correlation between mechanical and video measurements (p ≤ 0.001), confirming the reliability of the video method. The findings highlight the importance of nonlinear analysis in understanding neuromuscular function and fatigue, revealing significant correlations among HGS, AV, Poincaré indices, and fatigue levels in both hands (p ≤ 0.001). Increased maximum HGS and AV correlated with higher nonlinear variability in force production. Conclusions: This study confirms the reliability of the proposed video-based HGS assessment and demonstrates the effectiveness of Poincaré plot analysis for capturing nonlinear variability in HGS.

Oscillatory drive is increased and steadiness is impaired when torque but not EMG is matched during a fatiguing contraction.

The increasing descending drive needed to sustain a submaximal isometric torque makes it difficult to isolate fatigue-related changes to neural excitability because evoked electromyographic (EMG) responses are influenced by the relative activation of the motoneuron pool. Hence, it is becoming increasingly common to investigate fatigue using a sustained contraction with maintained output from the motoneuron pool; i.e., matched surface EMG. Although this approach controls motoneuron pool output, it is unknown how cortical contributions to ongoing muscle activity or common modulation between muscles is altered during a matched-EMG contraction. During separate visits, 16 participants performed a sustained 10-min isometric elbow flexion contraction at 20% maximal voluntary contraction (MVC) torque or the level of integrated biceps brachii EMG recorded at 20% MVC torque. Electroencephalographic and surface EMG recordings were obtained from the sensorimotor area and biceps and triceps brachii, respectively. The matched-torque contraction caused increased corticomuscular coherence for biceps brachii (~75%) and intermuscular coherence (~97%), but reduced MVC torque (~33%), voluntary activation (~9%), and torque steadiness (~83%). In contrast, the matched-EMG contraction caused reduced MVC torque (~21%), with no change in coherence, voluntary activation, or EMG steadiness. Further, participants reported higher ratings of perceived effort scores by 6min into the matched-torque compared to matched-EMG contraction. These findings indicate that, during a matched-torque contraction, the nervous system enhances common oscillatory activity to continue the task, but this does not prevent degradation of performance (torque steadiness). In contrast, when motoneuron pool output was clamped, other neural strategies are used to maintain muscle output.

Effect of prior neuromuscular electrical stimulation of vastus lateralis on the fatigue induced by a sustained voluntary knee extension in men

PurposeThe purpose of this study was to investigate the effect of vastus lateralis (VL) selective fatigue induced by neuromuscular electrical stimulation (NMES) on knee extensor electromyographic (EMG) activity during a sustained submaximal isometric contraction. MethodsThirteen healthy men (28 ± 5 years) completed two experimental sessions in which either the VL was pre-fatigued for 17 min (NMES session) or no intervention was performed (control session, CTRL). Subsequently, participants were asked to sustain an isometric knee extension at 20 % of maximal voluntary contraction (MVC) torque until task failure. ResultsVL M−wave amplitude was reduced (−34 ± 26 %, P = 0.008) following the NMES intervention, while MVC torque was reduced by 26 ± 10 %. The time to task failure was 23 ± 10 % shorter (P = 0.002) in NMES (186 ± 75 s) than in CTRL (251 ± 128 s). EMG activity measured during the sustained contraction was higher for vastus medialis and rectus femoris muscles in NMES compared to CTRL (P < 0.001), but was comparable for VL (P > 0.05). The extent and origin of neuromuscular fatigue at task failure measured through MVCs combined with electrically-evoked contractions did not differ between NMES and CTRL. ConclusionCompensatory activity from synergist muscles occurred in response to a pre-fatigue intervention, which reduced the time to task failure of a sustained submaximal contraction but did not affect the extent and origin of neuromuscular fatigue.

Does Cathodal Preconditioning Enhance the Effects of Subsequent Anodal Transcranial Direct Current Stimulation on Corticospinal Excitability and Grip Strength?

Lewis, A, Rattray, B, and Flood, A. Does cathodal preconditioning enhance the effects of subsequent anodal transcranial direct current stimulation on corticospinal excitability and grip strength? J Strength Cond Res XX(X): 000-000, 2024-Inconsistent effects of transcranial direct current stimulation (tDCS) on corticospinal excitability (CSE) and exercise performance are commonly reported. Cathodal preconditioning, involving cathodal tDCS delivered before anodal tDCS over the same region, may enhance changes in CSE and exercise beyond that resulting from anodal tDCS alone. This study aimed to investigate whether the effects of anodal tDCS on CSE and isometric grip strength can be enhanced by cathodal preconditioning. Thirty-five healthy subjects aged 19-37 years completed a familiarization session followed by 4 stimulation conditions presented in a randomized cross-over design across 4 separate sessions. tDCS doses were applied at 2 mA over the primary motor cortex for 10 minutes. Corticospinal excitability was assessed using 120% of resting motor threshold and an input/output curve of motor evoked potentials of the first dorsal interosseous. Grip strength was evaluated as time to exhaustion (TTE) in a sustained isometric contraction. Relative to conventional sham stimulation, TTE was significantly increased by 15% after conventional anodal tDCS. Corticospinal excitability increased in response to tDCS, but this effect did not differ across conditions. Cathodal preconditioning before anodal stimulation did not increase CSE or grip strength beyond that seen in the other stimulation conditions. Our findings did not reveal any significant impact of stimulation type on CSE. Notably, anodal tDCS led to a significant improvement in grip strength endurance. However, cathodal preconditioning did not seem to increase the effect of subsequent anodal stimulation on CSE nor grip strength.

Enhancing endurance performance with combined imagined and actual physical practice.

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. Participants (24 ± 3year) 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. 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. Combining motor imagery and physical training may offer a promising avenue for enhancing endurance performance and managing pain in various contexts.

Ischemic Conditioning Reduces Neuromuscular Fatigability During Dynamic Exercise Post Stroke

INTRODUCTION: In addition to baseline muscle weakness, stroke survivors often demonstrate increased neuromuscular fatigability (an acute exercise-induced reduction in power, hereafter referred to as fatigability), which limits endurance during functional tasks. Ischemic conditioning (IC) is a non-invasive and easy-to-administer intervention that can reduce fatigability in stroke survivors during a sustained isometric contraction of their paretic knee extensors. However, IC’s effects on fatigability during dynamic contractions are unknown, despite the greater relevance to everyday activities. Additionally, fatigability is known to be task specific. Thus, the purpose of this study was to quantify the effects of IC on fatigability (percent reduction in power) during dynamic contractions of the paretic knee extensors in stroke survivors. We hypothesized that fatigability would be less after a single session of IC versus IC-sham treatment in stroke survivors. Methods: Eight stroke survivors (66 ± 6 years of age, 6 male) performed 60 maximal voluntary concentric contractions (1 every 4 seconds) with their paretic knee extensors against a resistance of 30% of their maximal voluntary isometric contraction torque through a 60-degree range of motion after receiving a single session of IC and IC-sham treatment on two separate days. IC consisted of 5 sets of 5-minute upper thigh blood flow occlusion at 225 mmHg (25 mmHg for IC-sham) with 5 minutes of no occlusion between each set. The order of the 2 sessions was randomized. All participants visited the laboratory on a different day for functional measurements (Fugl-Meyer Assessment, 10-meter walk test, and 6-minute walk test). Reduction in power was calculated as the difference between the highest average power output from 5 consecutive contractions within the first 15 contractions and the average power output from the last 5 contractions. A paired t-test was used to compare if fatigability during dynamic exercise would differ between the 2 sessions after IC and IC-sham treatment. Pearson correlation was performed to determine the relationship between the difference in fatigability (after IC versus IC-sham treatment) and functional measurements. Results: After a single session of IC treatment, stroke survivors were less fatigable compared to IC-sham treatment (21.9 ± 19.2% power reduction vs. 29.4 ± 19.0%, p = 0.016). Improvement in fatigability after IC versus IC-sham treatment was positively correlated with Fugl-Meyer Assessment lower extremity motor score ( r = 0.82, p = 0.013). CONCLUSION: These preliminary results indicate that IC could serve as a potential intervention to reduce fatigability during dynamic exercise in stroke survivors, especially those who are mildly affected by stroke. Funding Source: American Heart Association Predoctoral Fellowship (903373, ZZ) and Eunice Kennedy Shriver National Institute of Child Health and Human Development R01 Grant (HD099340, AH &amp; MD). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Differential Modulation of Motor Unit Behavior When a Fatiguing Contraction Is Matched for Torque versus EMG.

When an isometric contraction is sustained at a submaximal torque, activation of the motoneuron pool increases, making it difficult to interpret neural excitability alterations. Thus, more recently, isometric contractions with maintained electromyographic (EMG) activity (matched-EMG) are being used to induce fatigue; however, little is known about the neurophysiological adjustments that occur to satisfy the requirements of the task. For our study, 16 participants performed a 10-min sustained isometric elbow flexion contraction at 20% maximal voluntary contraction (MVC) torque or the level of integrated biceps brachii EMG recorded at 20% MVC torque. Surface EMG was used to assess global median frequency, and four fine-wire electrode pairs were used to obtain motor unit (MU) discharge rate from biceps brachii. Torque or EMG steadiness was also assessed throughout the fatiguing contractions. MU discharge rate increased and torque steadiness decreased during the matched-torque contraction; however, MU discharge rate decreased during the matched-EMG contraction, and no changes occurred for EMG steadiness. Data pooled for the two contractions revealed a decrease in global median frequency. Lastly, a greater loss of MVC torque was observed immediately after the matched-torque compared with matched-EMG contraction. These findings indicate that, during a matched-torque fatiguing contraction, the nervous system increases MU discharge rates at the cost of poorer steadiness to maintain the requisite torque. In contrast, during a matched-EMG fatiguing contraction, a reduction of MU discharge rates allows for maintenance of EMG steadiness.

Skeletal muscle fiber type and TMS-induced muscle relaxation in unfatigued and fatigued knee-extensor muscles.

The force drop after transcranial magnetic stimulation (TMS) delivered to the motor cortex during voluntary muscle contractions could inform about muscle relaxation properties. Because of the physiological relation between skeletal muscle fiber-type distribution and size and muscle relaxation, TMS could be a noninvasive index of muscle relaxation in humans. By combining a noninvasive technique to record muscle relaxation in vivo (TMS) with the gold standard technique for muscle tissue sampling (muscle biopsy), we investigated the relation between TMS-induced muscle relaxation in unfatigued and fatigued states, and muscle fiber-type distribution and size. Sixteen participants (7F/9M) volunteered to participate. Maximal knee-extensor voluntary isometric contractions were performed with TMS before and after a 2-min sustained maximal voluntary isometric contraction. Vastus lateralis muscle tissue was obtained separately from the participants' dominant limb. Fiber type I distribution and relative cross-sectional area of fiber type I correlated with TMS-induced muscle relaxation at baseline (r = 0.67, adjusted P = 0.01; r = 0.74, adjusted P = 0.004, respectively) and normalized TMS-induced muscle relaxation as a percentage of baseline (r = 0.50, adjusted P = 0.049; r = 0.56, adjusted P = 0.031, respectively). The variance in the normalized peak relaxation rate at baseline (59.8%, P < 0.001) and in the fatigue resistance (23.0%, P = 0.035) were explained by the relative cross-sectional area of fiber type I to total fiber area. Fiber type I proportional area influences TMS-induced muscle relaxation, suggesting TMS as an alternative method to noninvasively inform about skeletal muscle relaxation properties.NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. We showed that fiber type I proportional area influences the TMS-induced muscle relaxation, suggesting that TMS could be used for the noninvasive estimation of muscle relaxation in unfatigued and fatigued human muscles when the feasibility of more direct method to study relaxation properties (i.e., muscle biopsy) is restricted.

Hemodynamic and neuromuscular basis of reduced exercise capacity in patients with end-stage renal disease.

The present study aimed to characterize the exercise-induced neuromuscular fatigue and its possible links with cerebral and muscular oxygen supply and utilization to provide mechanistic insights into the reduced exercise capacity characterizing patients with end-stage renal disease (ESRD). Thirteen patients with ESRD and thirteen healthy males (CTR group) performed a constant-force sustained isometric contraction at 50% of their maximal voluntary isometric contraction (MVC) until exhaustion. Quadriceps muscle activation during exercise was estimated from vastus lateralis, vastus medialis, and rectus femoris EMG. Central and peripheral fatigue were quantified via changes in pre- to postexercise quadriceps voluntary activation (ΔVA) and quadriceps twitch force (ΔQtw,pot) evoked by supramaximal electrical stimulation, respectively. To assess cerebral and muscular oxygenation, throughout exercise, near-infrared spectroscopy allowed investigation of changes in oxyhemoglobin (∆O2Hb), deoxyhemoglobin (∆HHb), and total hemoglobin (∆THb) in the prefrontal cortex and in the vastus lateralis muscle. ESRD patients demonstrated lower exercise time to exhaustion than that of CTR (88.8 ± 15.3s and 119.9 ± 14.6s, respectively, P < 0.01). Following the exercise, MVC, Qtw,pot, and VA reduction were similar between the groups (P > 0.05). There was no significant difference in muscle oxygenation (∆O2Hb) between the two groups (P > 0.05). Cerebral and muscular blood volume (∆THb) and oxygen extraction (∆HHb) were significantly blunted in the ESRD group (P < 0.05). A significant positive correlation was observed between time to exhaustion and cerebral blood volume (∆THb) in both groups (r2 = 0.64, P < 0.01). These findings support cerebral hypoperfusion as a factor contributing to the reduction in exercise capacity characterizing ESRD patients.

Sex related differences in neuromuscular fatiguability between prepubertal children and young adults

Introduction&#x0D; Prepubertal children compared to adults manifest neuromuscular differences in a lower voluntary activation and fatigability, as well as a potentially lesser use of larger motor units innervating fast-twitch-fibers (Dotan et al., 2012). While children tend to fatigue more on a central than peripheral level, these trends vary based on exercise modality and muscle group (Ratel et al., 2015; Souron et al., 2022). Furthermore, females incline to fatigue less than males, but research on sex differences in children remains limited (Hunter, 2014). Therefore, the aim of this study was to investigate neuromuscular age and sex differences in a sustained isometric contraction until failure.&#x0D; Methods&#x0D; 16 prepubertal children (9.2 ± 0.9y) and 16 adults (23.4 ± 2.4y; m = f) performed a maximal voluntary isometric contraction (MVCpre), a failure-task at 60%MVCpre, followed by another MVCpost of knee extensors. Surface electromyography for rectus femoris (RF), vastus medialis (VM), and vastus lateralis muscles (VL) was analysed at 0, 25, 50, 75, and 100% of the individual time to failure (TTF). Linear models and Cohen’s d effect sizes were used to quantify age and sex differences of TTF, MVCpre-post, as well as root mean square (RMS) and median frequency (MDF) of each muscle throughout the fatiguing trial.&#x0D; Results&#x0D; Trivial differences were observed in TTF across age, sex, and age*sex-interaction (d ≤ 0.78). Children displayed a larger decline in MVCpre-post (d = 1.24), whereas sex and age*sex interactions were minimal (d ≤ 0.63).&#x0D; Initially (0% TTF), children had a lower activation in RF, VL and VL (d &gt; 1.32), whereas age-differences across the failure-task were found for RMS VL, MDF RF, VL and VM (d ≥ 0.15), with no prominent sex-differences (d ≤ 0.14). Sex*age-interaction yielded detectable results only in RMS VM (d = -0.36).&#x0D; Conclusion&#x0D; The lower ability to generate an MVCpost combined with a smaller change in EMG parameters in children could be due to more central rather than peripheral fatigue. This is also supported by the lower initial activation in adults, suggesting that larger, more fatigable motor units are initially preserved, which may not be available in children.&#x0D; The small to large effects in neuromuscular fatigability between age and sex groups indicate the need for more in-depth research (tracking of motor units, change of voluntary activation and twitch properties) particularly with regard to sex-related disparities.&#x0D; References&#x0D; Dotan, R., Mitchell, C., Cohen, R., Klentrou, P., Gabriel, D., &amp; Falk, B. (2012). Child-adult differences in muscle activation—A review. Pediatric Exercise Science, 24(1), 2–21. https://doi.org/10.1123/pes.24.1.2&#x0D; Hunter, S. K. (2014). Sex differences in human fatigability: Mechanisms and insight to physiological responses. Acta Physiologica, 210(4), 768–789. https://doi.org/10.1111/apha.12234&#x0D; Ratel, S., Kluka, V., Vicencio, S. G., Jegu, A.-G., Cardenoux, C., Morio, C., Coudeyre, E., &amp; Martin, V. (2015). Insights into the mechanisms of neuromuscular fatigue in boys and men. Medicine &amp; Science in Sports &amp; Exercise, 47(11), 2319–2328. https://doi.org/10.1249/MSS.0000000000000697&#x0D; Souron, R., Carayol, M., Martin, V., Enzo, P., Duché, P., &amp; Gruet, M. (2022). Differences in time to task failure and fatigability between children and young adults: A systematic review and meta-analysis. Frontiers in Physiology, 13, Article 1026012. https://doi.org/10.3389/fphys.2022.1026012

BiLSTM-Based Joint Torque Prediction From Mechanomyogram During Isometric Contractions: A Proof of Concept Study.

Quantifying muscle strength is an important measure in clinical settings; however, there is a lack of practical tools that can be deployed for routine assessment. The purpose of this study is to propose a deep learning model for ankle plantar flexion torque prediction from time-series mechanomyogram (MMG) signals recorded during isometric contractions (i.e., a similar form to manual muscle testing procedure in clinical practice) and to evaluate its performance. Four different deep learning models in terms of model architecture (based on a stacked bidirectional long short-term memory and dense layers) were designed with different combinations of the number of units (from 32 to 512) and dropout ratio (from 0.0 to 0.8), and then evaluated for prediction performance by conducting the leave-one-subject-out cross-validation method from the 10-subject dataset. As a result, the models explained more variance in the untrained test dataset as the error metrics (e.g., root-mean-square error) decreased and as the slope of the relationship between the measured and predicted joint torques became closer to 1.0. Although the slope estimates appear to be sensitive to an individual dataset, >70% of the variance in nine out of 10 datasets was explained by the optimal model. These results demonstrated the feasibility of the proposed model as a potential tool to quantify average joint torque during a sustained isometric contraction.

Novel Metrics for High-Density sEMG Analysis in the Time-Space Domain During Sustained Isometric Contractions.

Goal: This study introduces a novel approach to examine the temporal-spatial information derived from High-Density surface Electromyography (HD-sEMG). By integrating and adapting postural control parameters into a framework for the analysis of myoelectrical activity, new metrics to evaluate muscle fatigue progression were proposed, investigating their ability to predict endurance time. Methods: Nine subjects performed a fatiguing isometric contraction of the lumbar erector spinae. Topographical amplitude maps were generated from two HD-sEMG grids. Once identified the coordinates of the muscle activity, novel metrics for quantifying the muscle spatial distribution over time were calculated. Results: Spatial metrics showed significant differences from beginning to end of the contraction, highlighting their ability of characterizing the neuromuscular adaptations in presence of fatigue. Additionally, linear regression models revealed strong correlations between these spatial metrics and endurance time. Conclusions: These innovative metrics can characterize the spatial distribution of muscle activity and predict the time of task failure.

Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions.

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (31P-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (kPCr) following contractions. In the quadricep muscles, several studies have quantified kPCr following 24-30s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120°·s-1, 1 every 2s) contractions. Muscle kPCr was ∼20% lower following the MVIC protocol compared with the MVDC protocol (p≤0.001), though this was less evident in older adults (p=0.073). Changes in skeletal muscle pH (p≤0.001) and PME accumulation (p≤0.001) were greater following the sustained MVIC protocol, and pH (p≤0.001) and PME (p≤0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.

Low-load blood flow restriction reduces time-to-minimum single motor unit discharge rate.

Resistance training with low loads in combination with blood flow restriction (BFR) facilitates increases in muscle size and strength comparable with high-intensity exercise. We investigated the effects of BFR on single motor unit discharge behavior throughout a sustained low-intensity isometric contraction. Ten healthy individuals attended two experimental sessions: one with, the other without, BFR. Motor unit discharge rates from the tibialis anterior (TA) were recorded with intramuscular fine-wire electrodes throughout the duration of a sustained fatigue task. Three 5-s dorsiflexion maximal voluntary contractions (MVC) were performed before and after the fatigue task. Each participant held a target force of 20% MVC until endurance limit. A significant decrease in motor unit discharge rate was observed in both the non-BFR condition (from 13.13 ± 0.87Hz to 11.95 ± 0.43Hz, P = 0.03) and the BFR condition (from 12.95 ± 0.71Hz to 10.9 ± 0.75Hz, P = 0.03). BFR resulted in significantly shorter endurance time and time-to-minimum discharge rates and greater end-stage motor unit variability. Thus, low-load BFR causes an immediate steep decline in motor unit discharge rate that is greater than during contractions performed without BFR. This shortened neuromuscular response of time-to-minimum discharge rate likely contributes to the rapid rate of neuromuscular fatigue observed during BFR.

Hypercapnia Increases Muscle Activation Without Affecting Force During Sustained Isometric Muscle Contraction

Hypercapnia Increases Muscle Activation Without Affecting Force During Sustained Isometric Muscle Contraction

Comparison between conventional and neuronavigated strategies to assess corticospinal responsiveness in unfatigued and fatigued knee-extensor muscles

In studying neuromuscular fatigability, researchers commonly use functional criteria to position and hold the transcranial magnetic stimulation (TMS) coil during testing sessions. This could influence the magnitude of corticospinal excitability and inhibition responses due to imprecise and unsteady positions of the coil. To reduce coil position and orientation variability, neuronavigated TMS (nTMS) could be used. We evaluated the accuracy of nTMS and a standardized function-guided procedure for maintaining TMS coil position both in unfatigued and fatigued knee extensors. Eighteen participants (10F/8M) volunteered in two identical and randomized sessions. Maximal and submaximal neuromuscular evaluations were performed with TMS three times before (PRE_1) and three times after (PRE_2) a 2 min resting session and one time immediately after (POST) a 2-min sustained maximal voluntary isometric contraction (MVIC). The located “hotspot” [the location that evoked the largest motor-evoked potential (MEP) responses in the rectus femoris] was maintained either with or without nTMS. MEP, silent period (SP) and the distance between the “hotspot” and the actual coil position were recorded. A time × contraction intensity × testing session × muscle interaction was not observed for MEP, SP, and distance. Bland-Altman plots presented adequate agreements for MEP and SP. Spatial accuracy of TMS coil position over the motor cortex did not influence corticospinal excitability and inhibition in unfatigued and fatigued knee extensors. The variability in MEP and SP responses may be due to spontaneous fluctuations in corticospinal excitability and inhibition, and it is not altered by the spatial stability of the stimulation point.

Acute adaptation of central and peripheral motor unit features to exercise-induced fatigue differs with concentric and eccentric loading.

What is the central question of this study? Conflicting evidence exists on motor unit (MU) firing rate in response to exercise-induced fatigue, possibly due to the contraction modality used: Do MU properties adapt similarly following concentric and eccentric loading? What is the main finding and its importance? MU firing rate increased following eccentric loading only despite a decline in absolute force. Force steadiness deteriorated following both loading methods. Central and peripheral MU features are altered in a contraction type-dependant manner, which is an important consideration for training interventions. Force output of muscle is partly mediated by the adjustment of motor unit (MU) firing rate (FR). Disparities in MU features in response to fatigue may be influenced by contraction type, as concentric (CON) and eccentric (ECC) contractions demand variable amounts of neural input, which alters the response to fatigue. This study aimed to determine the effects of fatigue following CON and ECC loading on MU features of the vastus lateralis (VL). High-density surface (HD-sEMG) and intramuscular (iEMG) electromyography were used to record MU potentials (MUPs) from bilateral VLs of 12 young volunteers (six females) during sustained isometric contractions at 25% and 40% of the maximum voluntary contraction (MVC), before and after completing CON and ECC weighted stepping exercise. Multi-level mixed effects linear regression models were performed with significance assumed as P<0.05. MVC decreased in both CON and ECC legs post-exercise (P<0.0001), as did force steadiness at both 25% and 40% MVC (P<0.004). MU FR increased in ECC at both contraction levels (P<0.001) but did not change in CON. FR variability increased in both legs at 25% and 40% MVC following fatigue (P<0.01). From iEMG measures at 25% MVC, MUP shape did not change (P>0.1) but neuromuscular junction transmission instability increased in both legs (P<0.04), and markers of fibre membrane excitability increased following CON only (P=0.018). These data demonstrate that central and peripheral MU features are altered following exercise-induced fatigue and differ according to exercise modality. This is important when considering interventional strategies targeting MU function.

The Clinical Characterization of Blocking Tics in Patients With Tourette Syndrome

Objective Tourette syndrome (TS) is a neurodevelopmental disorder characterized by the presence of motor and phonic tics. Blocking phenomena, characterized by arrests in motor activity causing interruptions in movements or speech, have also been described in patients with TS. In this study, we aimed to characterize the frequency and features of blocking tics in patients with TS. Methods We studied a cohort of 201 patients with TS evaluated at our movement disorders clinic. Results We identified 12 (6%) patients with blocking phenomena. Phonic tic intrusion causing speech arrest was the most common (n = 8, 4%), followed by sustained isometric muscle contractions arresting body movements (n = 4, 2%). The following variables were statistically related to blocking phenomena: shoulder tics, leg tics, copropraxia, dystonic tics, simple phonic tics, and number of phonic tics per patient (all p < 0.050). In the multivariate regression, the presence of dystonic tics (p = 0.014) and a higher number of phonic tics (p = 0.022) were associated with blocking phenomena. Conclusion Blocking phenomena are present in approximately 6% of patients with TS, and the presence of dystonic tics and a higher frequency and number of phonic tics increase the risk for these phenomena.

Neuromuscular fatigue in autoimmune myasthenia gravis: A cross-sectional study

ObjectivesTo investigate the presence of increased neuromuscular fatigue (NMF) in individuals with myasthenia gravis (IwMG), compared to healthy controls. A secondary aim was to assess associations between NMF, strength and perceived health-related quality of life (HRQoL) and symptom severity in IwMG. MethodsIn this cross-sectional study, we assessed NMF using classical myoelectrical indicators (root mean square: RMS, mean power frequency: MPF) obtained from surface electromyography (sEMG) during a sustained submaximal isometric contraction of the right Biceps Brachii and the right Vastus Lateralis and by evaluating the post-effort decline in peak torque following a fatiguing task consisting of a 40-second sustained isometric contraction. Relationships with MG-specific clinical scores (Myasthenia Muscle Score for symptom severity, MGQOL-15-F for HRQoL) were investigated. ResultsForty-one females with MG were compared to 18 control females of similar age. IwMG demonstrated reduced strength in both muscle groups, compared to control subjects. In both populations and both limbs, NMF was demonstrated by an increase in RMS and a decrease in MPF. However, IwMG did not demonstrate greater NMF based on these myoelectrical indicators nor based on post-effort peak torque decline. DiscussionDespite a decrease in baseline strength, IwMG did not display greater NMF in this specific experimental paradigm. This cohort consisted of individuals with mild-to-moderately severe MG which was well-controlled and stable. Further studies are warranted to identify simple and reliable methods to measure NMF in MG and to understand the relationship between NMF and perceived fatigue in activities of daily living for IwMG.

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions

BackgroundFemales are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions. MethodsWe investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18–30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively. ResultsMales were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males. ConclusionThis increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.