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Introduction: When performing an exercise or a functional test, pain that is evoked by movement or muscle contraction could be a stronger stimulus for changing how individuals move compared to tonic pain. We investigated whether the decrease in muscle force production is larger when experimentally-induced knee pain is directly associated to the torque produced (movement-evoked) compared to a constant painful stimulation (tonic)., Methods: Twenty-one participants performed three isometric knee extension maximal voluntary contractions without pain (baseline), during pain, and after pain. Knee pain was induced using sinusoidal electrical stimuli at 10 Hz over the infrapatellar fat pad, applied continuously or modulated proportionally to the knee extension torque. Peak torque and contraction duration were averaged across repetitions and normalized to baseline., Results: During tonic pain, participants reported lower pain intensity during the contraction than at rest (p < 0.001), whereas pain intensity increased with contraction during movement-evoked pain (p < 0.001). Knee extension torque decreased during both pain conditions (p < 0.001), but a larger reduction was observed during movement-evoked compared to tonic pain (p < 0.001). Participants produced torque for longer during tonic compared to movement-evoked pain (p = 0.005)., Conclusion: Our results indicate that movement-evoked pain was a more potent stimulus to reduce knee extension torque than tonic pain. The longer contraction time observed during tonic pain may be a result of a lower perceived pain intensity during muscle contraction. Overall, our results suggest different motor adaptation to tonic and movement-evoked pain and support the notion that motor adaptation to pain is a purposeful strategy to limit pain. This mechanistic evidence suggests that individuals experiencing prevalently tonic or movement-evoked pain may exhibit different motor adaptations, which may be important for exercise prescription., (© 2023 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

Purpose: To establish intra- and inter-session reliability of high-density surface electromyography (HDEMG)-derived parameters from the thoracic erector spinae (ES) during static and dynamic goal-directed voluntary movements of the trunk, and during functional reaching tasks., Methods: Twenty participants performed: 1) static trunk extension, 2) dynamic trunk forward and lateral flexion, and 3) multidirectional functional reaching tasks on two occasions separated by 7.5 ± 1.2 days. Muscle activity was recorded bilaterally from the thoracic ES. Root mean square (RMS), coordinates of the barycentre, mean frequency (MNF), and entropy were derived from the HDEMG signals. Reliability was determined with intraclass correlation coefficient (ICC), coefficient of variation, and standard error of measurement., Results: Good-to-excellent intra-session reliability was found for all parameters and tasks (ICC: 0.79-0.99), whereas inter-session reliability varied across tasks. Static tasks demonstrated higher reliability in most parameters compared to functional and dynamic tasks. Absolute RMS and MNF showed the highest overall reliability across tasks (ICC: 0.66-0.98), while reliability of the barycentre was influenced by the direction of the movements., Conclusion: RMS and MNF derived from HDEMG show consistent inter-session reliability in goal-directed voluntary movements of the trunk and reaching tasks, whereas the measures of the barycentre and entropy demonstrate task-dependent reliability., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The repeated bout effect (RBE) confers protection following exercise-induced muscle damage. Typical signs of this protective effect are significantly less muscle soreness and faster recovery of strength after the second bout. The aim of this study was to compare regional changes in medial gastrocnemius (MG) muscle activity and mechanical hyperalgesia after repeated bouts of eccentric exercise. Twelve healthy male participants performed two bouts of eccentric heel drop exercise (separated by 7 days) while wearing a vest equivalent to 20% of their body weight. High-density MG electromyographic amplitude maps and topographical pressure pain sensitivity maps were created before, two hours (2H), and two days (2D) after both exercise bouts. Statistical parametric mapping was used to identify RBE effects on muscle activity and mechanical hyperalgesia, using pixel-level statistics when comparing maps. The results revealed a RBE, as a lower strength loss (17% less; P < .01) and less soreness (50% less; P < .01) were found after the second bout. However, different muscle regions were activated 2H and 2D after the initial bout but not following the repeated bout. Further, no overall changes in EMG distribution or mechanical hyperalgesia were found between bouts. These results indicate that muscle activation is unevenly distributed during the initial bout, possibly to maintain muscle function during localized mechanical fatigue. However, this does not reflect a strategy to confer protection during the repeated bout by activating undamaged/non-fatigued muscle areas., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

We investigated changes in motor unit (MU) behavior and vasti-muscle contractile properties during sustained submaximal fatiguing contractions with a new time-domain tracking technique to understand the mechanisms responsible for task failure. Sixteen participants performed a nonfatiguing 15-s isometric knee extension at 50% of the maximum voluntary (MVC) torque, followed by a 30% MVC sustained contraction until exhaustion. Two grids of 64 surface electromyography electrodes were placed over vastus medialis and lateralis. Signals were decomposed into MU discharge times and the MUs from the 30% MVC sustained contraction were followed until task failure by overlapping decomposition intervals. These MUs were then tracked between 50% and 30% MVC. During the sustained fatiguing contraction, MUs of the two muscles decreased their discharge rate until ∼40% of the endurance time, referred to as the reversal time, and then increased their discharge rate until task failure. This reversal in firing behavior predicted total endurance time and was matched by opposite changes in twitch force (increase followed by a decrease). Despite the later increase in MU firing rates, peak discharge rates at task failure did not reach the frequency attained during a nonfatiguing 50% MVC contraction. These results show that changes in MU firing properties are influenced by adjustments in contractile properties during the course of the contraction, allowing the identification of two phases. Nevertheless, the contraction cannot be sustained, possibly because of progressive motoneuron inhibition/decreased excitability, as the later increase in firing rate saturates at a much lower frequency compared with a higher-force nonfatiguing contraction. NEW & NOTEWORTHY Motor unit firing and contractile properties during a submaximal contraction until failure were assessed with a new tracking technique. Two distinct phases in firing behavior were observed, which compensated for changes in twitch area and predicted time to failure. However, the late increase in firing rate was below the rates attained in absence of fatigue, which points to an inability of the central nervous system to sufficiently increase the neural drive to muscle with fatigue.

High-density surface electromyography (HDsEMG) provides a detailed analysis of a muscle's spatial distribution of activity. We applied a Bayesian spatio-temporal statistical method to quantify how acute nociception and task repetition alters the upper trapezius instantaneous spatial distribution of activity during dynamic muscular contractions. Ten male adults performed repeated lifting of a 1 kg box between shelves positioned at hip and shoulder heights with a cycle time of 3 s for 50 cycles under four conditions: baseline, isotonic and hypertonic saline injections (nociception) to the right upper trapezius, and 15 min post injection. Activity of the right upper trapezius was measured using a 64-channel surface electrode grid. Statistical inference was performed using Integrated Nested Laplace Approximations (INLA), and significance was determined by a non-zero crossing of the Bayesian 95% credible intervals (CrI). The maximal decrease in activity after nociception was -38.1 µV [95% CrI -40.9 to -35.3] at 30% of the lift cycle when compared to baseline. The maximal reduction in muscle activity between the early and later phases of lifting in the presence of nociception was by 10.4 µV [95% CrI 8.2-12.6]. A more holistic understanding of muscle behaviour is achieved using spatio-temporal inference than traditional reductionist methods., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Isometric bite force control, via measures of force accuracy, force steadiness and force proprioception, was assessed in patients with myogenic temporomandibular disorders (TMDs) compared to healthy controls. Twelve people with myogenic TMDs and twelve age- and gender-matched asymptomatic controls performed maximal voluntary contractions (MVC) of unilateral jaw clenching followed by submaximal isometric contractions, with and without visual feedback of force, at 10, 30 50% and 70% MVC. Force performance was assessed with indices of accuracy (mean distance, MD) and precision (standard deviation, SD) and reported as a percentage of the MVC. A mixed-effect model was used to evaluate differences in MVC, MD and SD. The MVC was lower in the TMD group when clenching either ipsilateral or contralateral to the side of greatest pain (p < 0.05). No difference in MD was observed between groups. The SD depended on the interaction between group and painful side (p = 0.04) with the TMD group displaying higher SD when executing the task with the most painful side when compared to the ipsilateral or contralateral sides of the control group. The reduced maximal bite force and force steadiness observed in people with myogenic pain may interfere with masticatory function and should be considered when planning therapeutic interventions for TMDs., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Study Design: Quasi-Experimental., Objective: To assess the activation of semispinalis cervicis (d-SSC) and splenius capitis (s-SC) muscles, and the activation between the two during neck pulley and free weight exercises., Background: Altered activation of cervical extensors may occur with neck pain, suggesting that exercises should be designed to target these muscles., Methods: d-SSC and s-SC activity was recorded unilaterally with intramuscular electromyography from healthy volunteers during cervical isometric exercises: 1) extension with a pulley rope angled from incline to vertical, 2) extension with right, left and central forehead hanging weight, and 3) rotation with pulley rope angled from incline to decline., Results: Extension against a vertical force led to greater activation of d-SSC (P < 0.001) and s-SC (P < 0.001) compared to the inclined, declined and horizontal pulley. With each of these conditions, amplitude of muscle activity was higher for the d-SSC compared to the s-SC muscle (P < 0.0001). Extension with free weight hanging on right, left or central forehead, showed no differences across conditions, although in each condition, the d-SSC amplitude was higher than the s-SC. For cervical rotation, the declined pulley led to the greatest activation of both muscles (P < 0.05). Higher levels of activity were observed for the s-SC compared to the d-SSC (P < 0.01) for all rotation conditions., Conclusion: A vertical resistance during an extension exercise or a declined resistance during cervical rotation, increased neck extensor activation. The results from this preliminary study provide guidance for future work on the exploration and development of low-load exercise design for patients with neck pain disorders., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Objective: Previous studies have demonstrated the presence of active trigger points (TrPs) in women with migraine reproducing their headache attacks. No study has investigated whether these TrPs can alter cervical muscle function in migraine. Our objective was to analyze differences in the activation of superficial neck flexor and extensor muscles in women with migraine considering the presence of active TrPs in the splenius capitis (SC), the upper trapezius (UT), and the sternocleidomastoid (SCM) muscles., Methods: Surface electromyography (EMG) was recorded from the superficial flexors (SCM and anterior scalene) and the extensor (SC, UT) muscles bilaterally as participants performed a staged task of cranio-cervical flexion (CCF; 5 contractions representing a progressive increase in CCF range of motion) in 70 women with migraine. They were stratified according to the presence or the absence of active TrPs in the SCM, the SC, or the UT musculature. A comparison of EMG normalized root mean square (RMS) values was conducted with a 2×5 analysis of covariance with the task level as the within-subject variable, group stratified by active TrPs as the between-subjects variable and the presence of neck pain as a covariable., Results: All patients exhibited active TrPs in their cervical muscles, which reproduced their migraine. Women with migraine exhibiting active TrPs in the SCM (P<0.01), the UT (P<0.05), or the SC (P<0.05) muscles had lower normalized RMS values of their superficial neck flexors than those without active TrPs in the same muscles. In addition, individuals exhibiting active TrPs in the SC and the UT (both, P<0.05) muscles had higher normalized RMS values in the SC muscle than those without active TrPs in the same muscles., Conclusions: The presence of active TrPs in the cervical musculature determines an altered activation of superficial neck and extensor muscles during low-load, isometric CCF contractions in women with migraine.

Delayed onset of muscle activation can be a descriptor of impaired motor control. Activation onset can be estimated from electromyography (EMG)-registered muscle excitation and from ultrasound-registered muscle motion, which enables non-invasive measurements in deep muscles. However, in voluntary activation, EMG- and ultrasound-detected activation onsets may not correspond. To evaluate this, ten healthy men performed isometric elbow flexion at 20% to 70% of their maximal force. Utilising a multi-channel electrode transparent to ultrasound, EMG and M(otion)-mode ultrasound were recorded simultaneously over the biceps brachii muscle. The time intervals between automated and visually estimated activation onsets were correlated with the regional variation of EMG and muscle motion onset, contraction level and speed. Automated and visual onsets indicated variable time intervals between EMG- and motion onset, median (interquartile range) 96 (121) ms and 48 (72) ms, respectively. In 17% (computed analysis) or 23% (visual analysis) of trials, motion onset was detected before local EMG onset. Multi-channel EMG and M-mode ultrasound revealed regional differences in activation onset, which decreased with higher contraction speed (Spearman ρ ≥ 0.45, P < 0.001). In voluntary activation the heterogeneous motor unit recruitment together with immediate motion transmission may explain the high variation of the time intervals between local EMG- and ultrasound-detected activation onset., Competing Interests: The authors declare no competing financial interests.

Objectives: To characterize the distribution of masseter muscle activity and force control during bilateral jaw clenching tasks in people with chronic nonspecific neck pain, without an associated temporomandibular disorder., Methods: Twelve volunteers with nonspecific neck pain and 12 age-matched and sex-matched healthy individuals participated. Submaximal symmetrical and asymmetrical bilateral jaw clenching was performed with and without visual feedback of force. Force performance was assessed with indices of accuracy (mean distance, offset error) and precision (standard deviation, coefficient of variation of force). High-density, 2-dimensional, surface electromyography (EMG) was recorded to characterize bilateral masseter muscle activity. The EMG root mean square was computed for each location of the electrode grid to form a map of the EMG amplitude distribution, and the location of the center of activity was measured., Results: The patient group showed a different distribution of masseter muscle activity compared with pain-free individuals during both symmetrical and asymmetrical bilateral jaw clenching. The position of the center of activity was positioned more cranial (P<0.001; right masseter only) and more anteriorly in the patient group (P<0.0001). In addition, the patients with chronic neck pain displayed higher levels of masseter muscle activation compared with the control participants regardless of the specific task performed (P<0.0001)., Discussion: People with chronic neck pain display increased activation and altered distribution of masseter muscle activity during a jaw-clenching coordination task. These results provide a greater appreciation of how secondary orofacial pain or temporomandibular disorders may develop in people with neck pain.

Convex hull area (CHA) and mean angle (MA) have been proposed to describe the behaviour of the helical axis during joint kinematics. This study investigates the intra- and inter-session reliability of CHA and MA during active movements of the cervical spine. Twenty-seven healthy volunteers (19 women) aged 23 ± 2.8 years participated. Each volunteer was tested on two sessions. All participants were instructed to perform the following active movements of the cervical spine: rotation, flexion/extension and lateral bending, each performed to full range and repeated ten consecutive times. Cervical movements were registered with an electromagnetic tracking system. For each participant, each movement and each session, range of motion (ROM), CHA and MA were extracted. ROM showed high intra- and inter-session reliability during all cervical spine movements using this method. Overall, the intra- and inter-session reliability of the helical axis parameters varied depending on the movement direction and ranged from fair to almost perfect. The intra- and inter-session reliability of CHA and MA were almost perfect during rotation whereas the intra- and inter-session reliability of CHA was substantial during lateral bending and intra- and inter-session reliability of MA ranged from fair to substantial during flexion/extension and lateral bending. This is the first study to evaluate the reliability of helical axis measures during active movements of the cervical spine. The results show that CHA and MA are promising descriptors of cervical kinematics which could be applied in future studies to evaluate neck function in patients with cervical spine disorders., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Pain is associated with changes in the neural drive to muscles. For the upper trapezius muscle, surface electromyography (EMG) recordings have indicated that acute noxious stimulation in either the cranial or the caudal region of the muscle leads to a relative decrease in muscle activity in the cranial region. It is, however, not known if this adaption reflects different recruitment thresholds of the upper trapezius motor units in the cranial and caudal region or a nonuniform nociceptive input to the motor units of both regions. This study investigated these potential mechanisms by direct motor unit identification. Motor unit activity was investigated with high-density surface EMG signals recorded from the upper trapezius muscle of 12 healthy volunteers during baseline, control (intramuscular injection of isotonic saline), and painful (hypertonic saline) conditions. The EMG was decomposed into individual motor unit spike trains. Motor unit discharge rates decreased significantly from control to pain conditions by 4.0 ± 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 ± 2.8 pps; not significant). These changes were compatible with variations in the synaptic input to the motoneurons of the two regions. These adjustments were observed, irrespective of the location of noxious stimulation. These results strongly indicate that the nociceptive synaptic input is distributed in a nonuniform way across regions of the upper trapezius muscle., (Copyright © 2016 the American Physiological Society.)

A motor task can be performed via different patterns of muscle activation that show regularities that can be factorized in combinations of a reduced number of muscle groupings (also referred to as motor modules, or muscle synergies). In this study we evaluate whether an acute noxious stimulus induces a change in the way motor modules are combined to generate movement by neck muscles. The neck region was selected as it is a region with potentially high muscular redundancy. We used the motor modules framework to assess the redistribution of muscular activity of 12 muscles (6 per side) in the neck region of 8 healthy individuals engaged in a head and neck aiming task, in non-painful conditions (baseline, isotonic saline injection, post pain) and after the injection of hypertonic saline into the right splenius capitis muscle. The kinematics of the task was similar in the painful and control conditions. A general decrease of activity was noted for the injected muscle during the painful condition together with an increase or decrease of the activity of the other muscles. Subjects did not adopt shared control strategies (motor modules inter subject similarity at baseline 0.73±0.14); the motor modules recorded during the painful condition could not be used to reconstruct the activation patterns of the control conditions, and the painful stimulus triggered a subject-specific redistribution of muscular activation (i.e., in some subjects the activity of a given muscle increased, whereas in other subjects it decreased with pain). Alterations of afferent input (i.e., painful stimulus) influenced motor control at a multi muscular level, but not kinematic output. These findings provide new insights into the motor adaptation to pain.

Aim: To assess whether patients with persistent neck pain display evidence of altered masticatory muscle behavior during a jaw-clenching task, despite the absence of orofacial pain or temporomandibular disorders., Methods: Ten subjects with persistent, nonspecific neck pain and 10 age- and sex-matched healthy controls participated. Maximal voluntary contractions (MVCs) of unilateral jaw clenching followed by 5-second submaximal contractions at 10%, 30%, 50%, and 70% MVC were recorded by two flexible force transducers positioned between the first molar teeth. Task performance was quantified by mean distance and offset error from the reference target force as error indices, and standard deviation of force was used as an index of force steadiness. Electromyographic (EMG) activity was recorded bilaterally from the masseter muscle with 13 X 5 grids of electrodes and from the anterior temporalis with bipolar electrodes. Normalized EMG root mean square (RMS) was computed for each location of the grid to form a map of the EMG amplitude distribution, and the average normalized RMS was determined for the bipolar acquisition. Between-group differences were analyzed with the Kruskal Wallis analysis of variance., Results: Task performance was similar in patients and controls. However, patients displayed greater masseter EMG activity bilaterally at higher force levels (P<.05)., Conclusion: This study has provided novel evidence of altered motor control of the jaw in people with neck pain despite the absence of orofacial pain or temporomandibular disorders.

Contracting the knee flexor muscles immediately before a maximum voluntary contraction (MVC) of knee extension increases the maximal force that the extensor muscles can exert. It is hypothesized that this phenomenon can be impaired by muscle fiber damage following eccentric exercise [delayed onset muscle soreness (DOMS)]. This study investigates the effect of eccentric exercise and DOMS on knee extension MVC immediately following a reciprocal-resisted knee flexion contraction. Electromyography (EMG) was recorded from the knee extensors and flexors of 12 healthy men during knee extension MVCs performed in a reciprocal (maximal knee extension preceded by resisted knee flexion), and nonreciprocal condition (preceded by relaxation of the knee flexors). At baseline, knee extension MVC force was greater during the reciprocal condition (P < 0.001), whereas immediately after, 24 and 48 h after eccentric exercise, the MVC force was not different between conditions. Similarly, at baseline, the EMG amplitude of the quadriceps during the MVC was larger for the reciprocal condition (P < 0.001). However, immediately after, 24 and 48 h postexercise the EMG amplitude was similar between conditions. In conclusion, eccentric exercise abolished the facilitation of force production for the knee extensors, which normally occurs when maximum knee extension is preceded by activation of the knee flexors., (© 2012 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: Appropriate timing of activity of the vastus medialis obliqus (VMO) and vastus lateralis (VL) muscles is a key factor for proper tracking of the patella in the trochlear groove during knee extension. This study investigates the relative timing of activation of the VMO and VL muscles during unexpected perturbations performed before and after eccentric exercise., Methods: Surface electromyography signals were recorded from the VMO and VL muscles of the right leg in 11 healthy men during rapid postural perturbations performed at baseline, immediately after eccentric exercise of the quadriceps, and at 24 and 48 h after exercise. Participants stood on a moveable platform during which eight randomised postural perturbations were performed (4 repetitions of 2 perturbation types: 8 cm forward slides, 8 cm backward slides)., Results: Before the eccentric exercise, the onset of VMO activity was significantly earlier than the VL muscle (average for both forward and backward perturbations: VMO 39.0±7.1 ms; VL 43.7±7.9 ms). However, the onset of VMO activity was significantly later compared with VL muscle immediately after eccentric exercise and this remained 24 and 48 h after eccentric exercise (average across all postexercise sessions and perturbation directions: VMO 72.3±11.1 ms; VL 56.0±8.2 ms; p<0.05)., Conclusions: The onset of VMO-VL activity in response to rapid destabilising perturbations is altered immediately after eccentric exercise and during eccentric exercise-induced muscle soreness up to 48 h later. These observations may help explain the high prevalence of knee disorders after high intensity eccentric exercise.

Muscle fatigue is associated with reduced power output and work capacity of the skeletal muscle. Fatigue-induced impairments in muscle function are believed to be a potential cause of increased injury rates during the latter stages of athletic competition and often occur during unexpected perturbations. However the effect of fatigue on functionally relevant, full body destabilizing perturbations has not been investigated. This study examines the effect of muscle fatigue on the activation of the quadriceps and hamstrings to fast, full body perturbations evoked by a moveable platform. Surface electromyographic (EMG) signals were recorded from the knee extensor (vastus medialis, rectus femoris, and vastus lateralis) and flexor muscles (biceps femoris and semitendinosus) of the right leg in nine healthy men during full body perturbations performed at baseline and immediately following high intensity exercise performed on a bicycle ergometer. In each condition, participants stood on a moveable platform during which 16 randomized postural perturbations (eight repetitions of two perturbation types: 8 cm forward slides, 8 cm backward slides) with varying inter-perturbation time intervals were performed over a period of 2-3 min. Maximal voluntary knee extension force was measured before and after the high intensity exercise protocol to confirm the presence of fatigue. Immediately after exercise, the maximal force decreased by 63% and 66% for knee extensors and flexors, respectively (P<0.0001). During the post-exercise postural perturbations, the EMG average rectified value (ARV) was significantly lower than the baseline condition for both the knee extensors (average across all muscles; baseline: 19.7±25.4μV, post exercise: 16.2±19.4 μV) and flexors (baseline: 24.3±20.9 μV, post exercise: 13.8±11.0 μV) (both P<0.05). Moreover the EMG onset was significantly delayed for both the knee extensors (baseline: 132.7±32.9 ms, post exercise: 170.8±22.9 ms) and flexors (baseline: 139.1±38.8 ms, post exercise: 179.3±50.9 ms) (both P<0.05). A significant correlation (R(2)=0.53; P<0.05) was identified between the percent reduction of knee extension MVC and the percent change in onset time of the knee extensors post exercise. This study shows that muscle fatigue induces a reduction and delay in the activation of both the quadriceps and hamstring muscles in response to rapid destabilizing perturbations potentially reducing the stability around the knee., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Due to the variations in morphological and architectural characteristics of fibers within a skeletal muscle, regions of a muscle may be differently affected by eccentric exercise. Although eccentric exercise may be beneficial for increasing muscle mass and can be beneficial for the treatment of tendinopathies, the non-uniform effect of eccentric exercise results in regional muscle damage and as a consequence, non-uniform changes in muscle activation. This regional muscle weakness can contribute to muscle strength imbalances and may potentially alter the load distribution on joint structures, increasing the risk of injury. In this brief review, the non-uniform effects of eccentric exercise are reviewed and their implications for training and sport are considered., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Objective: The objective of the study was to investigate the effect of strength and endurance training on muscle fiber membrane properties and discharge rates of low-threshold motor units of the vasti muscles during fatiguing contractions., Methods: Twenty-five sedentary healthy men (age (mean ± SD) = 26.3 ± 3.9 yr) were randomly assigned to one of three groups: strength training, endurance training, or a control group. Conventional endurance and strength training was performed 3 d·wk⁻¹, during a period of 6 wk. Motor unit conduction velocity and EMG amplitude of the vastus medialis obliquus and lateralis muscles and biceps femoris were measured during sustained isometric knee extensions at 10% and 30% of the maximum voluntary contraction before and immediately after training., Results: After 6 wk of training, the reduction in motor unit conduction velocity during the sustained contractions at 30% of the maximum voluntary force occurred at slower rates compared with baseline (P < 0.05). However, the rate of decrease was lower after endurance training compared with strength training (P < 0.01). For all groups, motor unit discharge rates declined during the sustained contraction (P < 0.001), and their trend was not altered by training. In addition, the biceps femoris-vasti coactivation ratio declined after the endurance training., Conclusions: Short-term strength and endurance training induces alterations of the electrophysiological membrane properties of the muscle fiber. In particular, endurance training lowers the rate of decline of motor unit conduction velocity during sustained contractions more than strength training.

This study investigates the effects of eccentric exercise and delayed onset muscle soreness (DOMS) of the quadriceps on agonist-antagonist activity during a range of motor tasks. Ten healthy volunteers (age, mean ± SD, 24.9 ± 3.2 years) performed maximum voluntary contractions (MVC) and explosive isometric contractions of the knee extensors followed by isometric contractions at 2.5, 5, 10, 15, 20, and 30% MVC at baseline, immediately after and 24 h after eccentric exercise of the quadriceps. During each task, force of the knee extensors and surface EMG of the vasti and hamstrings muscles were recorded concurrently. Rate of force development (RFD) was computed from the explosive isometric contraction, and the coefficient of variation of the force (CoV) signal was estimated from the submaximal contractions. Twenty-four hours after exercise, the subjects rated their perceived pain intensity as 4.1 ± 1.2 (score out of 10). The maximum RFD and MVC of the knee extensors was reduced immediately post- and 24 h after eccentric exercise compared to baseline (average across both time points: 19.1 ± 17.1% and 11.9 ± 9.8% lower, respectively, P < 0.05). The CoV for force during the submaximal contractions was greater immediately after eccentric exercise (up to 66% higher than baseline, P < 0.001) and remained higher 24 h post-exercise during the presence of DOMS (P < 0.01). For the explosive and MVC tasks, the EMG amplitude of the vasti muscles decreased immediately after exercise and was accompanied by increased antagonist EMG for the explosive contraction only. On the contrary, reduced force steadiness was accompanied by a general increase in EMG amplitude of the vasti muscles and was accompanied by increased antagonist activity, but only at higher force levels (>15% MVC). This study shows that eccentric exercise and subsequent DOMS of the quadriceps reduce the maximal force, rate of force development and force steadiness of the knee extensors, and is accompanied by different adjustments of agonist and antagonist muscle activities.

This study examined the effects of 3 wk of either endurance or strength training on plasticity of the neural mechanisms involved in the soleus H reflex and V wave. Twenty-five sedentary healthy subjects were randomized into an endurance group (n = 13) or strength group (n = 12). Evoked V-wave, H-reflex, and M-wave recruitment curves, maximal voluntary contraction (MVC), and time-to-task-failure (isometric contraction at 40% MVC) of the plantar flexors were recorded before and after training. Following strength training, MVC of the plantar flexors increased by 14.4 ± 5.2% in the strength group (P < 0.001), whereas time-to-task-failure was prolonged in the endurance group (22.7 ± 17.1%; P < 0.05). The V wave-to-maximal M wave (V/M(max)) ratio increased significantly (55.1 ± 28.3%; P < 0.001) following strength training, but the maximal H wave-to-maximal M wave (H(max)/M(max)) ratio remained unchanged. Conversely, in the endurance group the V/M(max) ratio was not altered, whereas the H(max)/M(max) ratio increased by 30.8 ± 21.7% (P < 0.05). The endurance training group also displayed a reduction in the H-reflex excitability threshold while the H-reflex amplitude on the ascending limb of the recruitment curve increased. Strength training only elicited a significant decrease in H-reflex excitability threshold, while H-reflex amplitudes over the ascending limb remained unchanged. These observations indicate that the H-reflex pathway is strongly involved in the enhanced endurance resistance that occurs following endurance training. On the contrary, the improvements in MVC following strength training are likely attributed to increased descending drive and/or modulation in afferents other than Ia afferents.

Changes in motor behavior are a known feature of chronic mechanical neck pain disorders. This study examined the strength of the association between reported levels of pain and disability from 84 individuals (63 women, 21 men) with chronic mechanical neck pain and levels of electromyographic activity recorded from superficial cervical flexor (sternocleidomastoid; SCM and anterior scalene; AS) muscles during progressive stages of the cranio-cervical flexion muscle test. A significant positive association was observed between superficial muscle activity and pain intensity (P < 0.003), but not pain duration (P > 0.5) or perceived disability (P > 0.21). The strongest relationship between pain intensity and superficial muscle activity occurred at the final increment of the cranio-cervical flexion test (inner-range test position) for both the SCM and AS muscles (R(2) = 0.16). Although a positive and significant relationship between pain intensity and superficial muscle activity was shown, the relationship was only modest (16% explained variance), indicating that multiple factors contribute to the altered motor function observed in individuals with chronic mechanical neck pain., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

The purpose of this study was to assess the activation of the erector spinae (ES) and external oblique (EO) in response to unanticipated, bi-directional postural perturbations before and after the induction of acute low back pain (LBP) in healthy individuals. An experimental session consisted of a baseline, control, and an acute LBP condition. For the control and acute LBP condition, isotonic or hypertonic saline (HS), respectively, was injected into the right ES muscle. In each condition, participants stood on a moveable platform during which 32 randomized postural perturbations (8 repetitions of 4 perturbation types: 8 cm anterior slides, 8 cm posterior slides, 10° anterior tilts, and 10° posterior tilts) with varying inter-perturbation time intervals were performed over a period of 4-5 min. Bilateral surface electromyography (EMG) was recorded from the ES and EO in addition to subjective pain records. During the acute LBP condition: (1) the onset time of the ES and EO was delayed for the forward and backward sliding perturbations (P < 0.05); (2) EMG amplitude was reduced bilaterally for all perturbations (P < 0.05); (3) the order of activation and interval between the onset times of the ES and EO were unaltered and (4) ES, but not EO, activity was adjusted to account for the directional differences between the perturbations. This study revealed that re-establishment of posture and balance was a result of the individuals' ability to rapidly modulate ES with respect to EO activity and that the bi-directional postural responses, although shifted in time and amplitude, retained temporal features in the presence of acute LBP.

This study compares neck force steadiness in women with neck pain and controls and the way this is influenced by short term vibration of the neck. In the first experiment, 9 women with chronic neck pain and 9 controls performed 10-s isometric cervical flexion at 15N. Intramuscular EMG was recorded from the sternocleidomastoid muscle. In the second experiment, 10 women with neck pain and 10 controls performed 10-s isometric cervical flexion at 25% of their maximal force before and after vibration to the neck (bursts of 50Hz with duration 20, 40, 60 and 120s). Surface EMG was acquired from the sternocleidomastoid and splenius capitis. In both experiments, force steadiness was characterized by the coefficient of variation (CoV) and the relative power in three frequency subbands (low: 0-3Hz; middle: 4-6Hz; high: 8-12Hz) of the force signal. Women with neck pain exhibited decreased force steadiness (Exp 1: patients 3.9±1.3%, controls 2.7±0.9%, P<0.05; Exp 2: patients 3.4±1.2%, controls 1.7±0.6%, P<0.01) which was associated with higher power in the low-frequency band (patients 71.2±9.6%, controls 56.7±9.2%, P<0.01). Following vibration, CoV (2.6±1.1%, P<0.05) and the power in the low-frequency band of the force signal decreased (63.1±13.9%, P<0.05) in the patient group. These effects were not present in controls. Motor unit behavior and surface EMG amplitude were similar between groups. In conclusion, women with neck pain have reduced force steadiness, likely due to alterations in Ia afferent input. Vibration, which modulates Ia afferent input, increases force steadiness in patients with neck pain., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

This study investigates the relationship between neck muscle coactivation, neck strength and perceived pain and disability in women with neck pain. Surface electromyography (EMG) was acquired from the sternocleidomastoid (SCM) and splenius capitis (SC) muscles of 13 women with chronic neck pain and 10 controls as they performed 1) maximal voluntary contractions (MVC) in flexion, extension and left and right lateral flexion, 2) ramped contractions from 0% to 50% MVC in flexion and extension and 3) circular contractions in the horizontal plane at 15N and 30N force. Higher values of EMG amplitude were observed for the SC (antagonist) during ramped neck flexion and for the SCM during ramped extension in the patient group (P<0.05). The patients displayed reduced values of directional specificity in the surface EMG of the SCM and SC for the circular contractions (P<0.05). The EMG amplitude of SC during cervical flexion was positively correlated with the patients' pain (R² =0.35, P<0.05) and perceived disability (R² 0.53, P<0.01). An inverse correlation was evident between the amount of activation of SC during cervical flexion and strength (R² =0.54, P<0.01). These observations indicate a relationship between alterations in neuromuscular control in patients with neck pain and functional consequences, including impaired motor performance and increased levels of perceived disability., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

This study compares a non-linear (neural network) and a linear (linear regression) power mapping using a set of features of the surface electromyogram recorded from the vastus medialis and lateralis muscles. Fifteen healthy participants performed 5 sets of 10 repetitions leg press using the individual maximum load corresponding what they could perform 10 times (10RM) with 120s of rest between them. The following sEMG variables were computed from each extension contraction and used as inputs to both approaches: mean average value (MAV), median frequency (Fmed), the spectral parameter proposed by Dimitrov (FInsm5), average (over the observation interval) of the instantaneous mean frequency obtained from a Choi-Williams distribution (MFM), and wavelet indices ratio between moments at different scales (WIRM1551, WIRM1M51, WIRM1522, WIRE51, and WIRW51). The non-linear mapping (neural network) provided higher correlation coefficients and signal-to-noise ratios values (although not significantly different) between the actual and the estimated changes of power compared to linear mapping (linear regression) using the sEMG variables alone and a combination of WIRW51 and MFM (obtained by a stepwise multiple linear regression). In conclusion, non-linear mapping of force loss during dynamic knee extension exercise showed higher signal-to-noise ratio and correlation coefficients between the actual and estimated power output compared to linear mapping. However, since no significant differences were observed between linear and non-linear approaches, both were equally valid to estimate changes in peak power during fatiguing repetitive leg extension exercise.

The study compared the distribution of electromyographic (EMG) signal amplitude in the upper trapezius muscle in 10 women with fibromyalgia and in 10 healthy women before and after experimentally-induced muscle pain. Surface EMG signals were recorded over the right upper trapezius muscle with a 10 x 5 grid of electrodes during 90 degrees shoulder abduction sustained for 60s. The control subjects repeated the abduction task following injections of isotonic and hypertonic (painful) saline into the upper trapezius muscle. The EMG amplitude was computed for each electrode pair and provided a topographical map of the distribution of muscle activity. The pain level rated by the patients at the beginning of the sustained contraction was 5.9+/-1.5. The peak pain intensity for the control group following the injection of hypertonic saline was 6.0+/-1.6. During the sustained contractions, the EMG amplitude increased relatively more in the cranial than caudal region of the upper trapezius muscle for the control subjects (shift in the distribution of EMG amplitude: 2.3+/-1.3mm; P<0.01). The patient group showed lower average EMG amplitude than the controls during the contraction (P<0.05) and did not show different changes in EMG amplitude between different regions of the upper trapezius. A similar behavior was observed for the control group following injection of hypertonic saline. The results indicate that muscle pain prevents the adaptation of upper trapezius activity during sustained contractions as observed in non-painful conditions, which may induce overuse of similar muscle compartments with fatigue., ((c) 2009 Elsevier Ltd. All rights reserved.)

The purpose of the study was to explore changes in the spatial distribution of erector spinae electromyography amplitude during static, sustained contractions and during contractions of increasing load. Surface electromyographic (EMG) signals were detected from nine healthy subjects using a grid of 13 x 5 electrodes placed unilaterally over the lumbar erector spinae musculature. Subjects stood in a 20 degrees forward flexed position and performed: (1) six 20-s long contractions with loads ranging from 2.5 kg to 12.5 kg (2.5 kg increments) and (2) a 6 min sustained contraction with 7.5 kg load. Root mean square (RMS) and mean power spectral frequency (MNF) were computed from the recorded EMG signals. EMG RMS increased (P<0.0001) and MNF remained constant during contractions of increased load. During the sustained contraction, MNF decreased (P<0.0001) and RMS did not change over time. The centroid (center of activity) of the RMS map shifted caudally (P<0.0001) with time during the sustained contraction but did not change with varying load. These results suggest a change in the distribution of erector spinae muscle activity with fatigue and a uniform distribution of muscle activation across loads.

We analyzed individual motor units of the sternohyoid muscle with the aim of characterizing their minimum and maximum discharge rates and their variability in discharge during voluntary contractions. Surface EMG signals were recorded with an array of eight electrodes from the sternohyoid muscle of seven healthy men (age: 30.2+/-3.5 yr). The multichannel surface EMG signals were displayed as feedback for the subjects who identified and modulated the activity of one target motor unit in 30-s contractions during which the discharge rate was increased from minimum to maximum (ramp contraction), sustained at maximum level (sustained), or increased in brief bursts (burst). During the ramp contractions, the minimum average discharge rate over epochs of 1 s was 11.6+/-1.5 pulses per second (pps) and the maximum 57.0+/-5.7 pps (P<0.001). During the sustained contractions, the motor unit discharge rate decreased from 65.5+/-8.4 pps at the beginning to 52.9+/-7.6 pps at the end of the contraction (P<0.05). The coefficient of variation for the interspike interval during the sustained contractions was 40.2+/-9.8% and a large percentage of discharges had instantaneous rates>50 pps (52.2+/-12.5%) and>100 pps (8.0+/-1.2%), with peak values>150 pps. During the burst contractions, the instantaneous discharge rate reached average maximum values of 97.6+/-36.8 pps. The observed discharge rates and their variability are higher than those reported for limb muscles, which may be due to large synaptic input and noise received by these motor neurons.

This study examined gender differences in the effect of experimental muscle pain on changes in the relative activation of regions of the upper trapezius muscle during a sustained contraction. Surface electromyographic (EMG) signals were recorded from multiple locations over the upper trapezius muscle with a 10 x 5 grid of electrodes from nine women and nine men during 90 degrees shoulder abduction sustained for 60s. Measurements were performed before and after the injection of 0.4 ml hypertonic (painful) and isotonic (control) saline into the cranial region of the upper trapezius muscle. The EMG root mean square (RMS) was computed for each location of the grid to form a map of the EMG amplitude distribution. The peak pain intensity following the injection of hypertonic saline was greater for women (numerical rating scale 0-10: women 6.0+/-2.1, men 4.2+/-0.9; P<0.01). For both genders, upper trapezius RMS averaged across the grid decreased following the injection of hypertonic saline (P<0.0001). Moreover, there was a relatively larger pain-induced decrease in RMS in the cranial region compared to the caudal region of the muscle for both genders. During the non-painful sustained contractions, the EMG RMS progressively increased more in the cranial than the caudal region, for both men and women, due to fatigue. This mechanism was maintained in men but not in women during the painful condition. The results demonstrate that muscle pain alters the normal adaptation of upper trapezius muscle activity to fatigue in women but not in men.

Measurements of muscle-fiber conduction velocity during voluntary contractions have been used in the diagnosis of neuromuscular diseases. However, the velocity of propagation of action potentials depends on the interspike interval of activation due to the velocity recovery function (VRF) of muscle fibers. The comparison of muscle-fiber conduction velocity estimates between individuals may thus be influenced by differences in motor unit discharge rate. This study investigates action potential properties of motor units of the sternocleidomastoid muscle during voluntary modulation of discharge rate with the purpose of assessing the effect of the VRF on motor unit properties in voluntary contractions. Nineteen healthy men trained to control a target motor unit with feedback of surface multichannel electromyographic (EMG) signals. The subjects performed three 30-s contractions of cervical flexion/rotation modulating the discharge rate of the target motor unit from 6.6 +/- 1.6 pps to 28.0 +/- 6.4 pps. Action potential conduction velocity was correlated to instantaneous discharge rate (R = 0.38 +/- 0.21). Action potential conduction velocity, peak-to-peak amplitude, and duration varied between minimum and maximum discharge rate (P < 0.01; percent change 12.3 +/- 5.0, -11.8 +/- 9.9, and -12.9 +/- 7.3). Thus, the properties of surface motor unit action potentials vary with modulation of discharge rate. This has implications for the use of conduction velocity values measured during voluntary contractions to differentiate patient populations from healthy individuals and for the development of normative data.

Aim: To compare the discharge patterns of motor unit populations from different locations within the upper trapezius muscle during brief submaximal constant-force contractions., Methods: Intramuscular and surface electromyographic (EMG) signals were collected from three sites of the right upper trapezius muscle distributed along the cranial-caudal direction in 11 volunteers during 10 s shoulder abduction at 25% of the maximum voluntary force., Results: A total of 38 motor units were identified at the cranial location, 36 from the middle location and 17 from the caudal location. Initial discharge rate was greatest at the caudal location (P < 0.05; mean +/- SD, cranial: 16.7 +/- 3.6 pps, middle: 16.9 +/- 4.0 pps, caudal: 19.2 +/- 3.3 pps). Discharge rate decreased during the contraction for the most caudal location only (P < 0.05). Initial estimates of surface EMG root mean square values were highest at the most caudal location (P < 0.05; cranial: 32.3 +/- 20.9 microV, middle: 41.3 +/- 21.0 microV, caudal: 51.6 +/- 23.6 microV)., Conclusion: This study demonstrates non-uniformity of motor unit discharge within the upper trapezius muscle during a brief submaximal constant-force contraction. Location-dependent modulation of discharge rate may reflect spatial dependency in the control of motor units necessary for the development and maintenance of force output.

The aim of the study was to examine the effect of experimental unilateral upper trapezius muscle pain on the relative activation of trapezius muscle subdivisions bilaterally during repetitive movement of the upper limb. Surface EMG signals were detected from nine healthy subjects from the upper, middle and lower divisions of trapezius during a repetitive bilateral shoulder flexion task. Measurements were performed before and after injection of 0.5 ml hypertonic (pain condition) and isotonic (control) saline into the upper division of the right trapezius muscle in two experimental sessions. On the painful side, upper trapezius showed decreased EMG amplitude (average rectified value, ARV) and lower trapezius increased ARV throughout the entire task following the injection of hypertonic saline (40.0 +/- 22.2 vs. 26.0 +/- 17.4 microV, and 12.5 +/- 7.6 vs. 25.6 +/- 14.8 microV, respectively, at the beginning of the contraction). On the side contralateral to pain, greater estimates of ARV were identified for the upper division of trapezius as the task progressed (37.4 +/- 20.2 vs. 52.7 +/- 28.4 microV, at the end of the contraction). Muscle fiber conduction velocity did not change with pain in all three divisions of the right trapezius muscle. The results suggest that local elicitation of nociceptive afferents in the upper division of the trapezius induces reorganization in the coordinated activity of the three subdivisions of the trapezius in repetitive dynamic tasks.

The purpose of this study was to compare average muscle fiber conduction velocity (CV) and its changes over time in the upper trapezius muscle during a repetitive upper limb task in people with chronic neck pain and in healthy controls. Surface EMG signals were detected bilaterally from the upper trapezius muscle of 19 patients and nine healthy controls using linear adhesive arrays of four electrodes. Subjects were asked to tap their hands in a cyclic manner between targets positioned mid-thigh and 120 degrees of shoulder flexion, to the beat of a metronome set at 88 beats/min for up to 5 min. Muscle fiber CV and instantaneous mean power spectral frequency were estimated for each cycle at the time instant corresponding to 90 degrees of shoulder flexion. Average muscle fiber CV of the upper trapezius muscle was higher in people with chronic neck pain (mean+/-SE, 4.8+/-0.1m/s) than in control subjects (4.4+/-0.1 m/s; P<0.05). Furthermore, the exercise-induced decrease in CV over time was enhanced in the patient group (P<0.05). It was concluded that membrane muscle fiber properties of the upper trapezius and their changes over time during dynamic contraction of the upper limb are different in a sample of people with chronic neck pain with respect to controls. This may be associated with the histological and morphological changes, which have previously been identified in people with pain over the trapezius muscle.

The objective of this study was to compare onset of deep and superficial cervical flexor muscle activity during rapid, unilateral arm movements between ten patients with chronic neck pain and 12 control subjects. Deep cervical flexor (DCF) electromyographic activity (EMG) was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid (SCM) and anterior scalene (AS) muscles. While standing, subjects flexed and extended the right arm in response to a visual stimulus. For the control group, activation of DCF, SCM and AS muscles occurred less than 50 ms after the onset of deltoid activity, which is consistent with feedforward control of the neck during arm flexion and extension. When subjects with a history of neck pain flexed the arm, the onsets of DCF and contralateral SCM and AS muscles were significantly delayed ( p<0.05). It is concluded that the delay in neck muscle activity associated with movement of the arm in patients with neck pain indicates a significant deficit in the automatic feedforward control of the cervical spine. As the deep cervical muscles are fundamentally important for support of the cervical lordosis and the cervical joints, change in the feedforward response may leave the cervical spine vulnerable to reactive forces from arm movement.

Introduction: Tonic experimentally induced shoulder pain alters the activation of periscapular muscles1. However, tonic pain may not entirely reproduce the symptoms experienced by people with painful shoulder conditions2 since they commonly report an increase of symptoms during movement. In this study we adopted a novel experimental pain model3 to investigate motor control adaptations of the scapular muscles during movement-evoked pain. Methods: Fifteen healthy participants performed a repeated box-lifting task for twelve sets. Pain was induced over the posterior acromion using electrical stimulation, modulated to induce a pain intensity of 4/10 when the activity of the anterior deltoid was equal or higher than the peak activity measured at baseline, and of 1/10 when lower than 80%. Muscle activity from the Upper (UT), Middle (MT) and Lower (LT) divisions of the Trapezius, and elevation of the scapula, were compared between baseline (sets 1-2), early-and late adaptation (respectively sets 3-4 and 11-12) using a Friedman test and post-hoc comparisons. Results: Early adaptation was characterised by lower scapular elevation (p = 0.002) and decreased activation of the UT (p = 0.006). In the late adaptation phase, participants exhibited lower scapular elevation (p = 0.032), decreased MT activation (p = 0.032) and increased LT activation (p = 0.006). Discussion: Movement-evoked pain results in lower scapular elevation, and different muscle strategies may be responsible for the changes in scapular kinematics in the early-and late-adaptation phase. Those results may provide new insights about motor adaptations and motor control in clinical condition. Process evaluation: Stimulation artifacts in the EMG were removed by filtering. [ABSTRACT FROM AUTHOR]

The aim of the study was to examine whether six weeks of endurance training minimizes the effects of fatigue on postural control during dynamic postural perturbations. Eighteen healthy volunteers were assigned to either a 6-week progressive endurance training program on a cycle ergometer or a control group. At week 0 and 7, dynamic exercise was performed on an ergometer until exhaustion and immediately after, the anterior–posterior centre of pressure (COP) sway was analyzed during full body perturbations. Maximal voluntary contractions (MVC) of the knee flexors and extensors, muscle fiber conduction velocity (MFCV) of the vastus lateralis and medialis during sustained isometric knee extension contractions, and power output were measured. Following the training protocol, maximum knee extensor and flexor force and power output increased significantly for the training group with no changes observed for the control group. Moreover, the reduction of MFCV due to fatigue changed for the training group only (from 8.6% to 3.4%). At baseline, the fatiguing exercise induced an increase in the centre of pressure sway during the perturbations in both groups (>10%). The fatiguing protocol also impaired postural control in the control group when measured at week 7. However, for the training group, sway was not altered after the fatiguing exercise when assessed at week 7. In summary, six weeks of endurance training delayed the onset of muscle fatigue and improved the ability to control balance in response to postural perturbations in the presence of muscle fatigue. Results implicate that endurance training should be included in any injury prevention program. [ABSTRACT FROM AUTHOR]

An increase to above-baseline levels of electromyography (EMG) mean power spectral frequency (MPF) has been observed previously during muscle recovery following fatiguing contractions and has been explained by membrane hyperpolarization due to increased activation of the Na+–K+ pump. It is hypothesized that this membrane mechanism is impaired by muscle fiber damage following eccentric exercise. Thus, the aim of the study was to investigate surface EMG signal characteristics during recovery from fatigue after eccentric exercise. Ten healthy subjects performed sustained isometric knee extensions at 40% of the maximal torque (MVC) until task failure before, immediately after and 24 and 48 h after eccentric exercise. Bipolar surface EMG signals were recorded from six locations over the quadriceps during the sustained isometric contraction and during 3-s long contractions at 40% MVC separated by 1-min intervals for 15 min (recovery). Before the eccentric exercise, MPF of EMG signals increased to values above baseline during recovery from the fatiguing isometric contraction ( P<0.001), whereas immediately after and 24 and 48 h after the eccentric task, MPF was lower than baseline during the entire recovery period ( P<0.01). In conclusion, delayed-onset muscle soreness abolished the supranormal increase in EMG MPF following recovery from fatigue. [ABSTRACT FROM AUTHOR]

This study examined the effect of experimental neck muscle pain on the EMG-force relationship of cervical agonist and antagonist muscles. Surface EMG signals were detected from the sternomastoid, splenius capitis, and upper trapezius muscles bilaterally from 14 healthy subjects during cervical flexion and extension contractions of linearly increasing force from 0 to 60% of the maximum voluntary contraction (MVC). Measurements were performed before and after injection of 0.5 ml hypertonic and isotonic saline into either the sternomastoid or splenius capitis in two experimental sessions. EMG average rectified value (ARV) of the sternomastoid, splenius capitis, and upper trapezius muscles and the muscle fiber conduction velocity (CV) of the sternomastoid muscle were estimated at 5% MVC force increments. During cervical flexion with injection of hypertonic saline in sternomastoid, ARV of sternomastoid was lower on the side of pain in the force range 25-60% MVC (P < 0.05) and was associated with a bilateral reduction of splenius capitis and upper trapezius ARV (P < 0.01). During cervical extension, injection of hypertonic saline in splenius capitis resulted in lower estimates of splenius capitis ARV on the painful side from 45 to 60% MVC (P < 0.05), which was associated with a bilateral increase in upper trapezius ARV estimates from 50 to 60% MVC (P < 0.001). However, no significant change was identified for estimates of sternomastoid ARV. Experimentally induced neck muscle pain resulted in task-dependent changes in cervical agonist/antagonist activity without modifications in muscle fiber CV. [ABSTRACT FROM AUTHOR]