Motor Unit Action Potential Amplitude Research Articles

The effects of resistance training to near volitional failure on motor unit recruitment during neuromuscular fatigue.

It is unclear whether chronically training close to volitional failure influences motor unit recruitment strategies during fatigue. We compared resistance training to near volitional failure vs. non-failure on individual motor unit action potential amplitude (MUAP) and surface electromyographic excitation (sEMG) during fatiguing contractions. Nineteen resistance-trained adults (11 males, 8 females) underwent 5 weeks (3×/week) of either low repetitions-in-reserve (RIR; 0-1 RIR) or high RIR training (4-6 RIR). Before and after the intervention, participants performed isometric contractions of the knee extensors at 30% of maximal peak torque until exhaustion while vastus lateralis sEMG signals were recorded and later decomposed. MUAP and sEMG excitation for the vastus lateralis were quantified at the beginning, middle, and end of the fatigue assessment. Both training groups improved time-to-task failure (mean change = 43.3 s, 24.0%), with no significant differences between low and high RIR training groups (low RIR = 28.7%, high RIR = 19.4%). Our fatigue assessment revealed reduced isometric torque steadiness and increased MUAP amplitude and sEMG excitation during the fatiguing task, but these changes were consistent between groups. Both low and high RIR training improved time-to-task failure, but resulted in comparable motor unit recruitment during fatiguing contractions. Our findings indicate that both low and high RIR training can be used to enhance fatiguability among previously resistance-trained adults.

Motor unit firing rates during slow and fast contractions in boys and men.

Motor unit (MU) activation during maximal contractions is lower in children compared with adults. Among adults, discrete MU activation differs, depending on the rate of contraction. We investigated the effect of contraction rate on discrete MU activation in boys and men. Following a habituation session, 14 boys and 20 men completed two experimental sessions for knee extension and wrist flexion, in random order. Maximal voluntary isometric torque (MVIC) was determined before completing trapezoidal isometric contractions (70%MVIC) at low (10%MVIC/s) and high (35%MVIC/s) contraction rates. Surface electromyography was captured from the vastus lateralis (VL) and flexor carpi radialis (FCR) and decomposed into individual MU action potential (MUAP) trains. In both groups and muscles, the initial MU firing rate (MUFR) was greater (p < 0.05) at high compared with low contraction rates. The increase in initial MUFR at the fast contraction in the VL was greater in men than boys (p < 0.05). Mean MUFR was significantly lower during fast contractions only in the FCR (p < 0.05). In both groups and muscles, the rate of decay of MUFR with increasing MUAP amplitude was less steep (p < 0.05) during fast compared with slow contractions. In both groups and muscles, initial MUFRs, as well as MUFRs of large MUs were higher during fast compared with slow contractions. However, in the VL, the increase in initial MUFR was greater in men compared with boys. This suggests that in large muscles, men may rely more on increasing MUFR to generate torque at faster rates compared with boys.

Influence of Blood Flow Restriction on Neuromuscular Function and Fatigue During Forearm Flexion in Men.

Montgomery, TR Jr, Olmos, A, Sears, KN, Succi, PJ, Hammer, SM, Bergstrom, HC, Hill, EC, Trevino, MA, and Dinyer-McNeely, TK. Influence of blood flow restriction on neuromuscular function and fatigue during forearm flexion in men. J Strength Cond Res 38(7): e349-e358, 2024-To determine the effects of blood flow restriction (BFR) on the mean firing rate (MFR) and motor unit action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationships during fatiguing isometric elbow flexions. Ten men (24.5 ± 4.0 years) performed isometric trapezoidal contractions at 50% maximum voluntary contraction to task failure with or without BFR, on 2 separate days. For BFR, a cuff was inflated to 60% of the pressure required for full brachial artery occlusion at rest. During both visits, surface electromyography was recorded from the biceps brachii of the dominant limb and the signal was decomposed. A paired-samples t test was used to determine the number of repetitions completed between BFR and CON. ANOVAs (repetition [first, last] × condition [BFR, CON]) were used to determine differences in MFR vs. RT and MUAPAMP vs. RT relationships. Subjects completed more repetitions during CON (12 ± 4) than BFR (9 ± 2; p = 0.012). There was no significant interaction (p > 0.05) between the slopes and y-intercepts during the repetition × condition interaction for MUAPAMP vs. MFR. However, there was a main effect of repetition for the slopes of the MUAPAMP vs. RT (p = 0.041) but not the y-intercept (p = 0.964). Post hoc analysis (collapsed across condition) indicated that the slopes of the MUAPAMP vs. RT during the first repetition was less than the last repetition (first: 0.022 ± 0.003 mv/%MVC; last: 0.028 ± 0.004 mv/%MVC; p = 0.041). Blood flow restriction resulted in the same amount of higher threshold MU recruitment in approximately 75% of the repetitions. Furthermore, there was no change in MFR for either condition, even when taken to task failure. Thus, BFR training may create similar MU responses with less total work completed than training without BFR.

Motor unit firing rates increase in prepubescent youth following linear periodization resistance exercise training.

The purpose was to examine the effects of 8-weeks (3days/week) of linear periodization resistance exercise training (RET) on neuromuscular function in prepubescent youth. Twenty-five healthy prepubescent youth (11 males, 14 females, age = 9.1 ± 0.8years) completed the RET (n = 17) or served as controls (CON, n = 8). Isometric maximal voluntary contractions (MVCs) and trapezoidal submaximal contractions at 35 and 60% MVC of the right leg extensors were performed with surface electromyography (EMG) recorded from the leg extensors [vastus lateralis (VL), rectus femoris, and vastus medialis] and flexors (biceps femoris and semitendinosus). EMG amplitude of the leg extensors and flexors were calculated during the MVCs. Motor unit (MU) action potential trains were decomposed from the surface EMG of the VL for the 35 and 60% MVCs. MU firing rates and action potential amplitudes were regressed against recruitment threshold with the y-intercepts and slopes calculated for each contraction. Total leg extensor muscle cross-sectional area (CSA) was collected using ultrasound images. ANOVA models were used to examine potential differences. Isometric strength increased post-RET (P = 0.006) with no changes in leg extensor and flexor EMG amplitude. Furthermore, there were no changes in total CSA or the MU action potential amplitude vs. recruitment threshold relationships. However, there were increases in the firing rates of the higher-threshold MUs post-RET as indicated with greater y-intercepts (P = 0.003) from the 60% MVC and less negative slope (P = 0.004) of the firing rates vs. recruitment threshold relationships at 35% MVC. MU adaptations contribute to strength increases following RET in prepubescent youth.

Design and Test of an Intraoral Electrode Grid for Tongue High-Density Electromyography.

Tongue motor function is crucial in a wide range of basic activities and its impairment affects quality of life. The electrophysiological assessment of the tongue relies primarily on needle electromyography, which is limited by its invasiveness and inability to capture the concurrent activity of the different tongue muscles. This work aimed at developing an intraoral grid for high-density surface electromyography (HDsEMG) to non-invasively map the electrical excitation of tongue muscles. We developed a grid of 4×8 electrodes deposited over an adhesive 8- μ m thick polyurethane membrane. The testing protocol was conducted on 7 healthy participants and included functional tasks (vowels articulation and tongue movements) aimed at activating different regions of the tongue. The electrical stability of contact was assessed by measuring electrode-tongue impedances before and after the tasks. The spatial amplitude distribution of global EMG and single motor unit action potentials (MUAPs) was characterized. Electrode-tongue impedance magnitude showed no significant changes in the pre-post comparison ( 58±46 k Ω vs. 67±58 k Ω at 50Hz). Contact stability was confirmed by the quality of the signals that allowed to quantify spatiotemporal characteristics of muscle activation during the different tasks. The analysis of the spatial distribution of individual MUAPs amplitude showed that they were confined to relatively small areas on the tongue surface (range: 0.5cm2 -3.9cm [Formula: see text]. A variety of different spatiotemporal MUAP patterns, likely due to the presence of different muscle compartments with different fiber orientations, were observed. Our results demonstrate that the developed electrode grid enables HDsEMG acquisition from the tongue during functional tasks, thus opening new possibilities in tongue muscle assessment both at global and single motor unit level.

Post-activation potentiation and potentiated motor unit firing patterns in boys and men.

Post-activation potentiation (PAP) describes the enhancement of twitch torque following a conditioning contraction (CC) in skeletal muscle. In adults, PAP may be related to muscle fibre composition and is accompanied by a decrease in motor unit (MU) firing rates (MUFRs). Muscle fibre composition and/or activation is different between children and adults. This study examined PAP and MU firing patterns of the potentiated knee extensors in boys and men. Twenty-three boys (10.5 ± 1.3years) and 20 men (23.1 ± 3.3years) completed familiarization and experimental sessions. Maximal isometric evoked-twitch torque and MU firing patterns during submaximal contractions (20% and 70% maximal voluntary isometric contraction, MVIC) were recorded before and after a CC (5s MVIC). PAP was calculated as the percent-increase in evoked-twitch torque after the CC. MU firing patterns were examined during submaximal contractions before and after the CC using Trigno Galileo surface electrodes (Delsys Inc) and decomposition algorithms (NeuroMap, Delsys Inc). MU action potential amplitudes (MUAPamp) and MUFRs were calculated for each MU and exponential MUFR-MUAPamp relationships were calculated for each participant and trial. PAP was higher in men than in boys (98.3 ± 37.1% vs. 68.8 ± 18.3%, respectively; p = 0.002). Following potentiation, the rate of decay of the MUFR-MUAPamps relationship decreased in both contractions, with a greater decrease among boys during the high-intensity contractions. Lower PAP in the boys did not coincide with smaller changes in potentiated MU firing patterns, as boys had greater reductions in MUFRs with potentiation compared with men in high-intensity contractions.

Differential impact on motor unit characteristics across severities of adult spinal muscular atrophy.

To test the hypotheses that decomposition electromyography (dEMG) motor unit action potential (MUAP) amplitude and firing rate are altered in SMA; dEMG parameters are associated with strength and function; dEMG parameters are correlated with traditional electrophysiological assessments. Ambulatory and non-ambulatory adults with SMA on nusinersen and healthy controls were enrolled. MUAPs were decomposed from multielectrode surface recordings during 30-s maximum contraction of the abductor digiti minimi (ADM). Isometric strength, upper limb function, patient-reported function, and standard electrophysiologic measures of the ADM (compound muscle action potential [CMAP], single motor unit potential [SMUP], motor unit number estimation [MUNE]) were collected. dEMG MUAP amplitudes were higher in ambulatory versus control and non-ambulatory groups and were higher in controls versus non-ambulatory SMA. In contrast, dEMG firing rates were higher in ambulatory versus non-ambulatory and control groups but similar between non-ambulatory and control. dEMG parameters showed moderate to strong positive correlation with strength and function whereas CMAP and MUNE better correlated with function than strength. SMUP did not correlate with strength, function, or dEMG MUAP amplitude. dEMG parameters show overall good test-retest reliability. dEMG provided reliable, noninvasive measure of MUAP amplitude size and firing rate and revealed divergent patterns across disease severity in adults with SMA. Firing rate enhancement, as seen in milder SMA, may provide a therapeutic avenue for improving function in more severe SMA, where firing rates appear preserved. MUAP amplitude size and firing rate, quantified with dEMG, may be promising monitoring biomarker candidates for noninvasive assessment of SMA.

Differences in motor unit firing properties of the vastus lateralis muscle during postural and voluntary tasks.

The firing properties of the motor units are usually affected by the motor task. However, it has not been clarified whether the firing properties of the motor units of a specific muscle are different between postural and voluntary tasks. Therefore, this study investigated whether the recruitment and rate coding of the motor units differ between these two motor tasks. Thirteen healthy volunteers performed trapezoidal muscle contraction with a target value of 15% maximum electromyography (EMG) activity by voluntary left knee extension in the sitting position (voluntary task) and postural maintenance in the semi-squatting position (postural task) with a knee flexion angle of 30°. We obtained four channels of surface EMG activity during each task from left vastus lateralis muscle. We extracted the firing properties of individual motor units using the EMG decomposition algorithm. The recruitment threshold and motor unit action potential amplitude were significantly lower in the postural task than in the voluntary task, and conversely, the mean firing rate was significantly higher. These results were explained by the preferential recruitment of motor units with higher recruitment threshold and amplitude in the voluntary task, while motor units with lower recruitment threshold and higher firing rate were preferentially recruited in the postural task. Preferential activation of fatigue-resistant motor units in the postural task is a reasonable strategy as it allows for sustained postural maintenance. We provide the first evidence that motor unit firing properties are clearly different between postural and voluntary tasks, even at the same muscle activity level.

Early Activation of Quadriceps With Pressure Biofeedback for the Prevention of Arthrogenic Muscle Inhibition Following Lower Limb Orthopedic Surgeries: A Proof of Concept Clinical Trial

ObjectiveThe purpose of this study was to explore the possible role of pressure biofeedback in the early activation of quadriceps following lower limb orthopedic surgeries. MethodsThis was a proof of concept clinical trial. A single-blinded randomized controlled study was conducted on 24 patients who underwent lower limb surgeries. The experimental group received standardized physiotherapy treatment in addition to pressure biofeedback for quadriceps retraining. The control group received a standardized physiotherapy treatment developed for an inpatient orthopedic setting. The motor unit action potential (MUAP) duration and amplitude were measured using electromyography on the second and sixth postoperative days. ResultsTwenty-four participants, with a mean age for the control group of 58.67 ± 17.21 and the experimental group of 40.1 ± 6.96, were enrolled. No statistical significance in MUAP amplitude for within the groups (F[5,85] = 1.735, P = .135) was found. However, there was a statistical significance between the control and experimental groups in amplitude measured by electromyography (F[1,17] = 49.09, P < .01). There was no statistical significance in MUAP duration for within the groups (F[5,85] = 1.303, P = .270). However, there was a statistical significance in duration (F[1,17] = 71.84, P < .01). ConclusionPressure biofeedback was more effective in the early activation of quadriceps muscle when coupled with conventional exercises compared with conventional exercises alone following lower limb orthopedic surgeries. Early activation of quadriceps could be a contributing factor to preventing arthrogenic muscle inhibition.

Sex differences in the detection of motor unit action potentials identified using high-density surface electromyography

Sex-related disparities in force production of humans have been widely observed. Previous literature has attributed differences in peripheral traits, such as muscle size, to explain these disparities. However, less is known about potential sex-related differences in central neuromuscular traits and many comparable studies, not exploring sex-related differences, exhibit a selection-bias in the recruitment of subjects making the generalization of their findings difficult. Utilizing high-density electromyography arrays and motor unit (MU) decomposition, the aim of the current study is to compare MU yield and discharge properties of the tibialis anterior between male and female humans. Twenty-four subjects (10 females) performed two submaximal (20%) isometric dorsiflexion contractions. On average, males yielded nearly twice the amount of MUs as females. Further, females had significantly higher MU discharge rate, lower MU action potential amplitude, and lower MU action potential frequency content than males despite similar levels of torque and MU discharge variability. These findings suggest differences in central neuromuscular control of force production between sexes; however, it is unclear how lower yield counts affect the accuracy of these results.

Neuromotor control associates with muscle weakness observed with McArdle sign of multiple sclerosis

ObjectiveMultiple Sclerosis (MS) is often accompanied by myelopathy, which may be associated with progressive worsening. A specific finding of MS‐associated myelopathy is McArdle sign, wherein neck flexion is associated with prominent increased limb weakness relative to that detected with neck extension. In this study, we characterized neuromotor control properties of finger extensors in association with the McArdle sign.MethodsA custom‐built device was utilized to monitor torque production of the wrist extensors with simultaneous recording of surface electromyography of the extensor digitorum. The electromyography was decomposed and analyzed via both linear and nominal regressions.ResultsLinear regressions demonstrated a strong difference between groups for MS from healthy controls and other myelopathies for motor unit action potential amplitude and average firing rate (p < 0.001). Further, linear regression demonstrated good correlations of neuromotor variables to mechanical torque output (0.24 ≤ R 2 ≤ 0.76). Nominal regression distinguished MS from healthy controls with an AUC of 0.87, specificity of 0.97, and sensitivity of 0.64. Nominal regression of MS from other myelopathies demonstrated an AUC of 0.88, specificity of 0.85, and sensitivity of 0.79.InterpretationThese data demonstrate the neuromotor control factors that largely determine muscle force production change with the observation of McArdle sign; these neuromotor control factors can differentiate MS from both healthy controls and other myelopathy conditions.

Abstract TP69: Change Over Time Feature Of A Range Of The Recruited Motor Unit In The Recovery Period Of Hemispheric Stroke

The paralysis after the stroke makes loss adjustment of the force in the voluntary movement and disturbs the physical activity. The voluntary force is regulated by the motor unit (MU) recruitment and firing rate. Therefore, we should evaluate MU firing patterns to understand a part of the mechanism of the force adjustment disorders of the paralysis side. It might be possible to evaluate the change of the force adjustment function on the paralysis side in the convalescence if we considered a range of the recruited MU over time. This pilot study hypothesizes that a range of the recruited MU spreads depending on a recovery stage. Two-stroke patients participated in this study. We measured the muscle activity of the biceps using surface EMG with three-time points during the recovery period after stroke. The raw EMG data were decomposed into motor unit action potential (MUAP). Then mean firing rate (MFR) and peak amplitude of MUAP were calculated by decomposed MUAP. Additionally, coefficients of the regression line were calculated by MFR and MUAP's peak amplitude. The coefficient of the linear regression line was adopted as the outcome to evaluate a range of the recruited MU size. We also analyzed the change over time of a range of the recruited MU size. The range of the recruited MU of the recovery stage changed over time but became wide during the recovery stage. These data showed that it varied from a restrictive style to a comprehensive recruitment style over time. Therefore, a range of the recruited MU with the stroke of the recovery stage was suggested to be a tendency to enlarge over time. Expanding the diachronic range of the recruited MU may let you enrich a variation of the force in the recovery process.

Adaptive Real-Time Decomposition of Electromyogram During Sustained Muscle Activation: A Simulation Study.

Real-time decomposition of electromyogram (EMG) into constituent motor unit (MU) activity has shown promising applications in neurophysiology and human-machine interactions. Existing decomposition methods could not accommodate stochastic variations in EMG signals such as drifts of action potential amplitudes and MU recruitment-derecruitment (rotation) patterns during long-term recordings. The objective of this study was to develop an adaptive real-time decomposition approach suitable for prolonged muscle activation. We developed a parallel-double-thread computation algorithm. The backend thread initiated and periodically refined and updated the MU information (separation matrix) using independent component analysis and convolution kernel compensation. The frontend thread performed the real-time decomposition. We evaluated our algorithm on synthesized high-density EMG signals, in which MUs were recruited-derecruited sporadically and MU action potentials amplitude drifted over time. Different signal-to-noise levels were also simulated. Compared with the decomposition without the adaptive processes, periodically fine-tuned and updated separation matrix increased identifiable MU number by 3-4 fold over 30-minute of signals. The increased MU number was more prominent at higher signal-to-noise ratios. The decomposition accuracy also increased by up to 10% with greater improvement observed at higher muscle contraction levels. The adaptive algorithm can maintain the decomposition performance over time, allows us to continuously track the same MUs during sustained activation, and, at the same time, can add newly recruited MU information to existing separation matrix. Our approach showed robust performance over time, which has the potential to longitudinally evaluate MU firing and recruitment properties and improve neural decoding performance for neural-machine interactions.

Decoding muscle force from individual motor unit activities using a twitch force model and hybrid neural networks

A deep learning method involving hybrid neural networks incorporated with a physiological twitch force model is presented for decoding muscle force from individual motor unit (MU) activities. A series of MU action potential (MUAP) trains were obtained using a progressive FastICA peel-off algorithm through high-density surface electromyogram (HD-sEMG) decomposition. The twitch force model was adopted to transform MUAP trains into twitch force sequences. The hybrid convolutional neural networks (CNNs) consisting of a spatial attention model (SA-CNN) and a bidirectional gated recurrent unit (Bi-GRU) were fed by the twitch force sequences to predict the muscle force. The performance of the proposed method was evaluated in predicting the thumb abduction force using both simulated data and experimental HD-sEMG recordings from abductor pollicis brevis muscles of ten subjects. The simulation approach assisted our method design in confirming a simplified relationship between MUAP amplitude and force contribution of each MU. When processing experimental data, the root mean square deviation (RMSD) values between the predicted and the actual forces were 6.57% ± 1.26% and 7.18% ± 0.96% under the user-specific testing scheme and the user-independent testing scheme, respectively. The proposed method outperformed other common sEMG amplitude-based or MU-based methods with significantly decreased RMSD values (p < 0.001). These findings indicate the advance of mining spatial information from individual MU activities using deep neural networks in decoding the muscle force. This study provides a useful tool for muscle force estimation, with wide applications in biomechanics and sports rehabilitation.

Relevant muscle selection for needle electromyography examination in polio survivors

Objectives: Needle electromyography (EMG) has an important role in the diagnosis of poliomyelitis. Since needle EMG is a painful and time consuming procedure, selecting the most effective muscles for needle EMG is important. We aimed to determine the clinically or subclinically affected regions in polio survivors by examining the minimum number of muscles with EMG. Methods: Polio survivors with weakness in at least one limb were included in this retrospective cohort study. The extremities were divided into two groups: 1) Group 1: The extremities with Medical Research Council score of ≤ 1 in at least one muscle, 2) Group 2: Clinically unaffected or mildly to moderately affected extremities that do not meet the criteria of Group 1. The needle EMG findings of the muscles were analyzed. Results: Twenty-nine polio survivors were included in the study. Needle EMG findings of 352 muscles were reviewed. Needle EMG findings in 57 lower extremities and thirty-two upper extremities were analyzed. There was no upper extremity meeting the criteria of Group 1. Thirty-eight lower extremities were included in Group 1. The amplitudes and durations of motor unit action potentials (MUAPs) were not different between the muscles of the Group 1 lower extremities (p &amp;gt; 0.05). Among the muscles of Group 2 upper and lower extremities, the amplitude and duration of MUAPs were higher in the deltoid and the vastus lateralis muscles compared to the other muscles, respectively (p = 0.002 and p = 0.003 for upper extremity muscles; p = 0.005 and p &amp;lt; 0.001 for lower extremity muscles). Conclusions: Using the needle EMG findings, an algorithm was made to determine the affected regions. Thus, the affected regions can be identified by applying needle EMG to a minimum number of muscles.

Motor unit action potential amplitude during low torque fatiguing contractions versus high torque non-fatiguing contractions: a multilevel analysis.

The ability to maintain an absolute, submaximal torque level during fatiguing contractions is controlled, in part, by the recruitment of larger motor units. These motor units are commonly identified based on greater action potential peak-to-peak amplitude values. It is unclear, however, if motor unit action potential (MUAP) amplitude values during low torque, fatiguing contractions reach similar levels as those observed during non-fatigued, high torque contractions. To establish a clearer understanding of motor unit control during fatigue, we compared MUAP amplitude during 50 and 80% maximum voluntary contraction (MVC) torque contractions and at the beginning, middle, and end of a 30% MVC fatigue protocol. Eleven untrained men (mean age = 24years) performed isometric contractions at 50 and 80% MVC, followed by repeated contractions at 30% MVC. Surface electromyographic (EMG) signals were detected from the vastus lateralis and decomposed to quantify the peak-to-peak amplitude of individual MUAPs. A two-level multilevel model was estimated, allowing examination of simultaneous measures of MUAP amplitude within participants and controlling for the dependence between measures within participants. Results from the multilevel analyses suggested that there were not statistically significant differences in MUAP amplitude between 80% MVC and end fatigue. Separate repeated-measures analyses of variance indicated that there were not statistically significant mean differences in greatest MUAP or surface EMG amplitude between 80% MVC and end fatigue. MUAP and surface EMG amplitude values during a 30% MVC fatiguing protocol appear to be comparable to those observed during a non-fatigued 80% MVC condition.

The time course of neuromuscular impairment during short-term disuse in young women.

Skeletal muscle disuse results in rapid functional declines. Previous studies have typically been at least 1 week in duration and focused on the responsiveness of men. Herein, we report the timeline of initial impairments in strength, voluntary activation (VA), and motor unit control during 2 weeks of knee joint immobilization. Thirteen women (mean age =21 years) underwent 2 weeks of left knee joint immobilization via ambulation on crutches and use of a brace. Participants visited the laboratory for testing on seven occasions (two familiarization visits, pretest, 48 and 72 h, 1 and 2 weeks). Knee extensor isometric and concentric isokinetic strength at two velocities (180 and 360 degrees⋅s−1), VA, and submaximal vastus lateralis motor unit activity were evaluated. Moderate‐to‐large decreases in isometric and concentric strength at 180 degrees⋅s−1 and VA were observed within 48 hours. Isometric strength continued to decline beyond 72 h, whereas other variables plateaued. The B‐term of the motor unit mean firing rate versus action potential amplitude relationship demonstrated a moderate increase 1 week into immobilization, suggesting that greater firing rates were necessary to maintain pretest torque levels. Concentric strength at a velocity of 360 degrees s−1 was not affected. Decreases in knee extensor strength occur within a matter of days after immobilization, although the time course and magnitude vary among assessment methods. These changes are mediated by the nervous system's capacity to activate skeletal muscle. Clinically appropriate interventions which target nervous system plasticity should be implemented early to minimize the rapid functional impairments associated with disuse.

Increases in motor unit action potential amplitudes are related to muscle hypertrophy following eight weeks of high-intensity exercise training in females

We examined the motor unit action potential amplitude versus recruitment threshold relationship (MUAPAMP-RT) as an indicator of MU-specific hypertrophy following high-intensity exercise training in females. Participants were assigned to either a high-intensity exercise (EX, n = 9) or control (CON, n = 18) condition and completed pre- (PRE) and post-testing (POST) during which maximal voluntary isometric leg extension strength (MVIT), vastus lateralis (VL) muscle cross sectional area (mCSA), whole leg skeletal muscle mass (SMMRL), and high-density surface EMG (HD-sEMG) signals were recorded from the VL during an isometric ramp contraction at 70% MVIT. The HD-sEMG signals were decomposed and yielded a MUAPAMP and an absolute (ABS; Nm) and normalized (NORM; %MVIC) RT for each MU. Individual MUAPAMP-RT slopes and intercepts were calculated for each subject. Changes in the pooled MUAPAMP-RT relationships for each group were also examined. Finally, relationships among individual changes in slopes of MUAPAMP-RT and individual changes in mCSA and SMMRL were examined. Training elicited increases in MVIT (+18%), mCSA (+12%), and mean and pooled slopes of MUAPAMP-RTNORM. The individual changes in slopes of both the MUAPAMP-RT relationships were moderately to strongly (r = 0.48–0.68) related to changes in mCSA and SMMRL. Eight-weeks of high-intensity exercise elicited increases in MUAPAMP-RT slope in females. Further, the observed change in slope was related to both VL mCSA and SMM of the tested leg. However, changes in slope for the MUAPAMP-RT relationship were more subdued when MUAPAMP was expressed relative to the absolute versus relative RT.

Neural Drive is Greater for a High-Intensity Contraction Than for Moderate-Intensity Contractions Performed to Fatigue.

Miller, JD, Lippman, JD, Trevino, MA, and Herda, TJ. Larger motor units are recruited for high-intensity contractions than for fatiguing moderate-intensity contractions. J Strength Cond Res 34(11): 3013-3021, 2020-The purpose of this study was to investigate whether moderate-intensity contractions performed to fatigue activate the motor unit (MU) pool to the same extent as a higher-intensity contraction. Subjects (7 men, 2 women, age = 22.78 ± 4.15 years, height = 173.78 ± 14.19 cm, mass = 87.39 ± 21.19 kg) performed 3 isometric maximum voluntary contractions (MVCs), an isometric trapezoidal contraction at 90% MVC (REP90), and repetitive isometric trapezoidal contractions at 50% MVC performed to failure with the first (REP1) and final repetition (REPL) used for analysis. Surface EMG was recorded from the vastus lateralis. Action potentials were extracted into firing events of single MUs with recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP), and mean firing rates (MFRs) recorded. Linear MFR and MUAPAMP vs. RT and exponential MFR vs. MUAPAMP relationships were calculated for each subject. The level of significance was set at p ≤ 0.05. B terms for the MFR vs. MUAPAMP relationships (p = 0.001, REPL = -4.77 ± 1.82 pps·mV, REP90 = -2.63 ± 1.00 pps·mV) and predicted MFRs for MUs recruited at 40% MVC (p < 0.001, REPL = 11.14 ± 3.48 pps, REP90 = 18.38 ± 2.60 pps) were greater for REP90 than REPL indicating firing rates were greater during REP90. In addition, larger mean (p = 0.038, REPL = 0.178 ± 0.0668 mV, REP90 = 0.263 ± 0.128 mV) and maximum (p = 0.008, REPL = 0.320 ± 0.127 mV, Rep90 = 0.520 ± 0.234 mV) MUAPAMPS were recorded during REP90 than REPL. Larger MUs were recruited and similar sized MUs maintained greater firing rates during a high-intensity contraction in comparison to a moderate-intensity contraction performed at fatigue. Individuals seeking maximized activation of the MU pool should use high-intensity resistance training paradigms rather than moderate-intensity to fatigue.

Sex-related differences in motor unit firing rates and action potential amplitudes of the first dorsal interosseous during high-, but not low-intensity contractions.

Despite ample evidence that females are weaker and possess smaller muscle cross-sectional areas (CSAs) compared to males, it remains unclear if there are sex-related differences in the properties of motor units (MU). Eleven males (age 22 ± 3years) and 12 females (age 21 ± 1years) performed isometric trapezoid muscle actions at 10% and 70% of maximal voluntary contraction (MVC). Surface electromyography signals were recorded and decomposed into MU action potential (AP) waveforms and firing instances. Average MUAP amplitudes (MUAPAMPS), mean firing rates (MFRs), initial firing rates (IFRs), and recruitment thresholds (RT) were calculated for the 10% MVC, while MUAPAMPS, IFRs, and MFRs were regressed against RT for the 70% MVC. Ultrasonography was used to measure CSA of the first dorsal interosseous (FDI). Males had greater CSAs (p < 0.001; males 2.34 ± 0.28cm2, females 1.82 ± 0.18cm2) and MVC strength (p < 0.001; males 25.9 ± 5.5N, females 16.44 ± 2.5N). No differences existed for MUAPAMPS, IFRs, MFRs, or RTs (p > 0.05) during the 10% MVC. For the 70% MVC, the y-intercepts from the MUAPAMPS vs. RT relationships were greater (p < 0.05) for the males (males - 0.19 ± 0.53mV; females - 0.78 ± 0.75mV), while the inverse was true for the MFR vs. RT relationships (males 31.55 ± 6.92pps, females 38.65 ± 6.71pps) with no differences (p > 0.05) in the slopes. Therefore, smaller CSAs and weaker MVCs are likely the result of smaller higher-threshold MUs for females.