Motor units in cranial and caudal regions of the upper trapezius muscle have different discharge rates during brief static contractions

Excitation distribution of the trapezius changes in response to increasing contraction intensity, but not repeated contractions

Upper trapezius (UT) excitation redistributes with experimentally-induced muscle pain, fatigue, and repeated contractions. Excitation distribution variability is proposed to reduce the likelihood of shoulder pain and pathology by reducing cumulative stress on musculoskeletal structures. While the middle (MT) and lower (LT) trapezius are pivotal in scapular stabilization, it remains unclear whether they display similar excitation distribution variability with repeated or increasing contraction intensity. We determined if excitation distribution of the UT, MT, and LT differ: 1) during isometric contractions at different intensities (30 % and 60 % of maximum voluntary isometric contraction (MVIC)); and 2) with repeated contractions at 60 % MVIC. Nineteen individuals completed MVICs and submaximal contractions for the UT, MT, and LT while high-density electromyography was collected. Statistical parametric mapping t-tests were performed between intensities and the 1st and 5th repetition at 60 % MVIC. UT, MT, and LT excitation distribution changed with increasing contraction intensity in 358 (∼92 % of the map), 54 (∼14 %), and 270 pixels (∼70 %), respectively. No pixels exceeded significance with repeated contractions for any muscle. Barycentre analyses revealed no significant results. These results suggest that regions of the trapezius muscle use different neuromuscular strategies in response to changes in contraction intensity and repeated contractions.

Quantitative response of healthy muscle following the induction of capsaicin: an exploratory randomized controlled trial

Background

Myofascial pain syndrome (MPS) is a prevalent chronic pain disorder primarily characterized by myofascial trigger points (MTrPs). There is limited knowledge on the pathophysiology and mechanisms underlying MTrP and its development. Research has previously demonstrated the identification of MTrPs using ultrasound and vibration sonoelastography, although there is some contradictory evidence regarding if MTrPs present as hyper or hypoechoic regions. Electromyography (EMG) investigations of MTrP have demonstrated that MTrPs are usually located proximal to innervation zones where the peak surface EMG signals are obtained from. Central sensitization has been proposed as the primary mechanism underlying MTrP development. Central sensitization is associated with hyperexcitability of neuronal responses to normal or noxious stimuli. There is a need for a study that measures ultrasound image textural changes and motor unit activity responses in the muscle following sensitization. The purpose of this study is to determine whether sensitizing healthy muscle using capsaicin induces a regional change in image texture variables within the specific and surrounding muscles, as well as the motor unit frequency and amplitude changes that accompany them. This is an exploratory trial that aims to provide preliminary evidence on whether central sensitization is a direct cause of taut band and MTrP development.

Methods

Ethical approval was obtained from the University Health Network (UHN) Research Ethics Board. This proposed study is a single centered, factorial, randomized placebo-controlled trial with two independent variables, depth of capsaicin application and dose of capsaicin, for a total of six treatment arms and three control treatment groups.

Discussion

This will be the first study that assesses the B-mode ultrasound image texture of induced sensitized muscles and will provide more evidence on muscle motor unit activity and regional changes of central sensitization. Findings from this study may support one of few hypotheses proposed delineating the involvement of central sensitization in the development of trigger points.

Trial registration

National Institutes of Health ClinicalTrials.gov NCT03944889. Registered on May 07, 2019

Differential Modulation of Motor Unit Properties from the Separate Components of the Triceps Surae in Humans

The triceps surae is comprised of the soleus, and medial (MG) and lateral (LG) gastrocnemii. Modulation of triceps surae motor units (MUs) is context- and muscle-dependent, yet it is unknown how the disparate components of the triceps surae work together to achieve the common goal of high-intensity voluntary isometric plantar flexion torque gradation. Thus, the purpose was to assess the interrelationships between MU recruitment thresholds (MURTs) and MU discharge rates (MUDRs) among these three muscles during contractions from low to high intensities. We sampled 157 MU action potential trains from the MG (68), LG (38) and soleus (51) using fine-wire intramuscular electromyography (EMG) during voluntary ramp isometric contractions up to 100% maximal voluntary contraction (MVC). The soleus exhibited 41% and 54% lower MURTs compared to the MG (p < 0.0001) and LG (p < 0.0001), respectively, whereas MG MURTs were 22% lower than the LG (p < 0.0001). Initial MUDRs were 35% and 26% greater for the LG compared with the MG (p < 0.0001) and soleus (p < 0.0001), but no difference was detected between the MG and soleus (p = 0.28). Finally, initial MUDRs displayed a positive relationship with MURTs for each independent triceps surae component (p ≤ 0.002). The relative differences in MU properties of each muscle in this synergistic group illustrate that MU control strategies are likely optimized with respect to the relative contribution of each muscle to plantar flexion torque or functional roles.

Human motor unit characteristics of the superior trapezius muscle with age-related comparisons

Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22-33 yr) and 10 old (77-88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, P < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, P = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation.NEW & NOTEWORTHY The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.

Differences in the activation and co-activation ratios of the four subdivisions of trapezius between genders following a computer typing task

The aim of this study was to provide a description of gender differences of the activation patterns of the four subdivisions of the trapezius (clavicular, upper, middle, lower) following a 60min computer work task. Surface EMG was collected from these subdivisions from 21 healthy subjects during bilateral arm elevation pre-/post- task. Subjects completed a standardized 60min computer work task at a standard, ergonomic workstation. Normalized activation and activation ratios of each trapezius subdivision were compared between genders and condition with repeated measures ANOVAs. The interaction effect of Gender×Condition for upper trapezius% activation approached significance at p=0.051with males demonstrating greater activation post-task. The main effect of Condition was statistically significant for% activation of middle and lower trapezius (p<0.05), with both muscles demonstrating increase activation post-task. There was a statistically significant interaction effect of Gender×Condition for the Middle Trapezius/Upper Trapezius ratio and main effect of Condition for the Clavicular Trapezius/Upper Trapezius ratio, with a decreased ratio post-typing. Gender differences exist following 60min of a low force computer typing task. Imbalances in muscle activation and activation ratios following computer work may affect total shoulder kinematics and should be further explored.

Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle

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.

Changes of HD-sEMG maps of the upper limb during isometric endurance contractions

Recent research in the field of surface EMG recorded with 2D electrode arrays have shown muscle adaptations as reflected on the spatial activation of motor units in response to pain, direction of movement or fatigue. The purpose of this study was to evaluate time- changes in the activation maps of upper limb muscles during endurance tasks associated with the degrees of freedom at the elbow joint. Preliminary results show time-variations in the distribution of intensity, that is, in the spatial recruitment of motor units, and that such changes may be dependent on the type of task.

Increase in heterogeneity of biceps brachii activation during isometric submaximal fatiguing contractions: a multichannel surface EMG study

The effects of fatigue emerge from the beginning of sustained submaximal contractions, as shown by an increase in the amplitude of the surface electromyogram (EMG). The increase in EMG amplitude is attributed to an augmentation of the excitatory drive to the motor neuron pool that, more importantly than increasing discharge rates, recruits additional motor units for the contraction. The aim of this study was to determine whether the spatiotemporal distribution of biceps brachii (BB) activity becomes more or less heterogeneous during a fatiguing isometric contraction sustained at a submaximal target force. Multiple electrodes were attached over the entire BB muscle, and principal component analysis (PCA) was used to extract the representative information from multiple monopolar EMG channels. The development of heterogeneity during the fatiguing contraction was quantified by applying a cluster algorithm on the PCA-processed EMG amplitudes. As shown previously, the overall EMG amplitude increased during the sustained contraction, whereas there was no change in coactivation of triceps brachii. However, EMG amplitude did not increase in all channels and even decreased in some. The change in spatial distribution of muscle activity varied across subjects. As found in other studies, the spatial distribution of EMG activity changed during the sustained contraction, but the grouping and size of the clusters did not change. This study showed for the first time that muscle activation became more heterogeneous during a sustained contraction, presumably due to a decrease in the strength of common inputs with the recruitment of additional motor units.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Uneven spatial distribution of surface EMG: what does it mean?

The aim of this work is to show how changes in surface electromyographic activity (sEMG) during a repetitive, non-constant force contraction can be detected and interpreted on the basis of the amplitude distribution provided by high-density sEMG techniques. Twelve healthy male subjects performed isometric shoulder elevations, repeating five times a force ramp profile up to 25 % of the maximal voluntary contraction (MVC). A 64-electrode matrix was used to detect sEMG from the trapezius muscle. The sEMG amplitude distribution was obtained for the force levels in the range 5-25 % MVC with steps of 5 % MVC. The effect of force level, subject, electrode position and ramp repetition on the sEMG amplitude distribution was tested. The sEMG amplitude was significantly smaller in the columns of the electrode grid over the tendons (repeated measures ANOVA, p < 0.01). The barycentre of the distribution of sEMG amplitude was subject-specific (Kruskal-Wallis test, p < 0.01), and shifted caudally with the increase of force levels and cranially with the repetition of the motor task (both p < 0.01, repeated measures ANOVA). The results are discussed in terms of motor unit recruitment in different muscle sub-portions. It is concluded that the sEMG amplitude distribution obtained by multichannel techniques provides useful information in the study of muscle activity, and that changes in the spatial distribution of the recruited motor units during a force varying isometric contraction might partially explain the variability observed in the activation pattern of the upper trapezius muscle.

Recruitment of motor units in two fascicles of the semispinalis cervicis muscle

This study investigated the behavior of motor units in the semispinalis cervicis muscle. Intramuscular EMG recordings were obtained unilaterally at levels C2 and C5 in 15 healthy volunteers (8 men, 7 women) who performed isometric neck extensions at 5%, 10%, and 20% of the maximal force [maximum voluntary contraction (MVC)] for 2 min each and linearly increasing force contractions from 0 to 30% MVC over 3 s. Individual motor unit action potentials were identified. The discharge rate and interspike interval variability of the motor units in the two locations did not differ. However, the recruitment threshold of motor units detected at C2 (n = 16, mean ± SD: 10.3 ± 6.0% MVC) was greater than that of motor units detected at C5 (n = 92, 6.9 ± 4.3% MVC) (P < 0.01). A significant level of short-term synchronization was identified in 246 of 307 motor unit pairs when computed within one spinal level but only in 28 of 110 pairs of motor units between the two levels. The common input strength, which quantifies motor unit synchronization, was greater for pairs within one level (0.47 ± 0.32) compared with pairs between levels (0.09 ± 0.07) (P < 0.05). In a second experiment on eight healthy subjects, interference EMG was recorded from the same locations during a linearly increasing force contraction from 0 to 40% MVC and showed significantly greater EMG amplitude at C5 than at C2. In conclusion, synaptic input is distributed partly independently and nonuniformly to different fascicles of the semispinalis cervicis muscle.

Influence of proprioceptive feedback on the firing rate and recruitment of motoneurons

We investigated the relationships of the firing rate and maximal recruitment threshold of motoneurons recorded during isometric contraction with the number of spindles in individual muscles. At force levels above 10% of maximal voluntary contraction, the firing rate was inversely related to the number of spindles in a muscle, with the slope of the relationship increasing with force. The maximal recruitment threshold of motor units increased linearly with the number of spindles in the muscle. Thus, muscles with a greater number of spindles had lower firing rates and a greater maximal recruitment threshold. These findings may be explained by a mechanical interaction between muscle fibres and adjacent spindles. During low-level (0% to 10%) voluntary contractions, muscle fibres of recruited motor units produce force twitches that activate nearby spindles to respond with an immediate excitatory feedback that reaches maximal level. As the force increases further, the twitches overlap and tend towards tetanization, the muscle fibres shorten, the spindles slacken, their excitatory firings decrease, and the net excitation to the homonymous motoneurons decreases. Motoneurons of muscles with greater number of spindles receive a greater decrease in excitation which reduces their firing rates, increases their maximal recruitment threshold, and changes the motoneuron recruitment distribution.

Discharge behaviors of trapezius motor units during exposure to low and high levels of acute psychosocial stress

This study investigated the effects of acute psychosocial stress on trapezius single motor unit discharge behaviors. Twenty-one healthy women performed feedback-controlled isometric contractions under conditions of low and high psychosocial stress in the same experimental session. Psychosocial stress was manipulated using a verbal math task combined with social evaluative threat that significantly increased perceived anxiety, heart rate, and blood pressure (P < 0.001). Motor unit discharge behaviors including the threshold and discharge rate at recruitment [7.7% (5.7%) maximal voluntary isometric contraction and 7.3 pulses per second (pps) (6.8 pps), P > 0.121, N = 103] and derecruitment [6.0% (4.4%) maximal voluntary isometric contraction and 6.5 pps (4.1 pps), P > 0.223, N = 99], the mean [11.3 pps (2.3 pps), P = 0.309, N = 106] and variability [2.5 pps (0.91 pps), P = 0.958, N = 106] of discharge rate, and the proportion of motor units exhibiting double discharges (21%, P = 0.446) did not change across stress conditions. Discharge rate modulation with changes in contraction intensity was highly variable and similar across stress conditions (P > 0.308, N = 89). Rate-rate modulation of concurrently active motor units was also highly variable (r = -0.84 to 1.00, N = 75). Estimates of DeltaF for motor unit pairs with rate-rate modulation >or=0.7 were positive and similar across stress conditions [4.7 pps (2.0 pps), P = 0.405, N = 16]. The results indicate that acute psychosocial stress does not alter trapezius motor unit discharge behaviors during a precisely controlled motor task in healthy women.

Methodological aspects of SEMG recordings for force estimation--a tutorial and review

Insight into the magnitude of muscle forces is important in biomechanics research, for example because muscle forces are the main determinants of joint loading. Unfortunately muscle forces cannot be calculated directly and can only be measured using invasive procedures. Therefore, estimates of muscle force based on surface EMG measurements are frequently used. This review discusses the problems associated with surface EMG in muscle force estimation and the solutions that novel methodological developments provide to this problem. First, some basic aspects of muscle activity and EMG are reviewed and related to EMG amplitude estimation. The main methodological issues in EMG amplitude estimation are precision and representativeness. Lack of precision arises directly from the stochastic nature of the EMG signal as the summation of a series of randomly occurring polyphasic motor unit potentials and the resulting random constructive and destructive (phase cancellation) superimpositions. Representativeness is an issue due the structural and functional heterogeneity of muscles. Novel methods, i.e. multi-channel monopolar EMG and high-pass filtering or whitening of conventional bipolar EMG allow substantially less variable estimates of the EMG amplitude and yield better estimates of muscle force by (1) reducing effects of phase cancellation, and (2) adequate representation of the heterogeneous activity of motor units within a muscle. With such methods, highly accurate predictions of force, even of the minute force fluctuations that occur during an isometric and isotonic contraction have been achieved. For dynamic contractions, EMG-based force estimates are confounded by the effects of muscle length and contraction velocity on force producing capacity. These contractions require EMG amplitude estimates to be combined with modeling of muscle contraction dynamics to achieve valid force predictions.