Gluteus medius: An intramuscular EMG investigation of anterior,middle and posterior segments during gait

Previous electromyographic (EMG) studies of gluteus medius (GMed) have not accurately quantified the function of the three proposed structurally and functionally unique segments (anterior, middle and posterior). Therefore this study used anatomically verified locations for intramuscular electrode recordings in three segments of GMed to determine whether the segments are functionally independent. Bipolar fine wire electrodes were inserted into each segment of GMed in 15 healthy individuals. Participants completed a series of four walking trials, followed by maximum voluntary isometric contractions (MVICs) in five different positions. Temporal and amplitude variables for each segment were compared across the gait cycle using ANOVA. The relative contributions of each segment to the MVIC trials were compared with non-parametric tests. All segments showed a biphasic response during the stance phase of gait. There were no differences in amplitude variables (% MVIC) between segments, but the anterior segment had a later peak during both the first and second bursts. For the MVIC trials, there were significant differences in amplitude between segments in four of the five test positions. These data indicate that GMed is composed of three functionally independent segments. This study contributes to the theoretical understanding of the role of GMed.     

Reliability and interpretation of single leg stance and maximum voluntary isometric contraction methods of electromyography normalization

Normalization of electromyographic (EMG) amplitudes is necessary in the study of human motion. However, there is a lack of agreement on the most reliable and appropriate normalization method. This study evaluated the reliability of single leg stance (SLS) and maximal voluntary isometric contraction (MVIC) normalization methods and the relationship between these measures for the gluteus maximus (GMax), gluteus medius (GMed), rectus femoris (RF), vastus lateralis (VL), hip adductor group (ADD), and biceps femoris (BF). Surface EMG was recorded in 20 subjects during three 5 s trials of SLS and MVIC. SLS and MVIC methods both demonstrated good-to-excellent reliability in all muscles (ICCs > 0.80). Intrasubject coefficients of variation were lower for the MVIC method (9–36%) than for the SLS method (20–59%). EMG amplitudes during MVIC and SLS were significantly correlated for all muscles (Pearson r’s = 0.604–0.905, p < 0.005) except GMax (r = 0.250, p = 0.288). Use of SLS normalization for the RF, VL, and BF is not recommended due to a lack of measurement precision. However, this method is justified in the GMax, GMed, and ADD and may provide a better representation of coordinated muscle function during a functional task.     

On the suitability of using surface electrode placements to estimate muscle activity of the rotator cuff as recorded by intramuscular electrodes

BackgroundElectromyography (EMG) is commonly used to assess muscle activity. Although previous studies have had moderate success in predicting individual intramuscular muscle activity from surface electrodes, extensive data does not exist for the rotator cuff. This study aimed to determine how reliably surface electrodes represent rotator cuff activity during 20 maximal exertions.MethodsFive channels of EMG were recorded on the following rotator cuff muscles: supraspinatus and infraspinatus intramuscular and surface recordings, and teres minor intramuscular recordings. An additional 3 surface electrodes were placed over the upper and middle trapezius and posterior deltoid. Subjects performed ramped maximal voluntary contractions (MVCs) for each muscle, followed by 20 isometric maximal exertions. Linear least squares best fit regressions (unconstrained and constrained with zero-intercept) were used to compare: intramuscular and surface supraspinatus and infraspinatus signals, respectively, and intramuscular teres minor and surface infraspinatus signals.FindingsRelationships existed between wire and surface electrode measurements for all rotator cuff muscles: supraspinatus (r² = 0.73); teres minor (r² = 0.61); infraspinatus (r² = 0.40), however prediction equations indicated large overestimations and offsets.InterpretationWhen appropriate multiplicative coefficients are considered, surface supraspinatus and infraspinatus electrodes may be used to estimate intramuscular supraspinatus and teres minor activations, respectively, in maximal exertions similar to those tested. However, until these relationships are better defined in other postures, intensities and exertion types, the use of surface electrodes to estimate indwelling rotator cuff activity is cautioned against.     

Comparing surface and indwelling electromyographic signals of the supraspinatus and infraspinatus muscles during submaximal axial humeral rotation

This study quantified the relationship between EMG signals recorded by surface and indwelling electrodes for the infraspinatus and supraspinatus during submaximal axial humeral rotation. Muscular activity was measured on 20 participants during 82 submaximal isometric internal or external axial humeral rotations in a range of postures and intensities. Equations to predict indwelling magnitudes from surface data were generated and the effects of humeral angle and intensity on this relationship were also evaluated.Supraspinatus surface data explained 72–76% of the variance in the indwelling data. Surface data overestimated indwelling data by up to 30% of maximal voluntary contraction (MVC). Infraspinatus surface data explained 62–64% of the variance in the indwelling data, but overestimated by 72% and 400% MVC in external and internal axial humeral rotation trials, respectively. Humeral abduction angle and exertion intensity both altered the relationship between electrode types modestly (p < 0.01) for most muscles and exertions. Better variance explanation was achieved for these submaximal exertions than previously reported values for maximal exertions.These results help inform electrode type selection for the recording of supraspinatus and infraspinatus EMG. Caution is recommended when interpreting surface recordings as indicators of indwelling recordings for exertions where the muscle studied is not a primary mover.     

A comparison of gluteus medius,gluteus minimus and tensor facia latae muscle activation during gait in post-menopausal women with and without greater trochanteric pain syndrome

The effect of greater trochanteric pain syndrome (GTPS) on gluteus medius (GMed) and minimus (GMin) activation in post-menopausal women is unknown. The aim of this study was to compare segmental muscle activation and variability of the GMed, GMin and tensor fascia latae (TFL) during gait in post-menopausal women with and without GTPS.Intramuscular electrodes were inserted into segments of GMin (x2) and GMed (x3) and a surface electrode placed on TFL. Ten control participants and 8 with GTPS completed six walking trials. Peak amplitude, average amplitude and time to peak from each phase of the gait cycle (0–30%, 30%- toe off (TO), total stance and swing) were compared between groups using independent t-tests and effect-size (ES) calculations. Variability of muscle activation was calculated using the mean coefficient of variation (CV). Reversal of anterior GMin electromyographic burst pattern and greater average muscle activity was found in the GTPS group compared to controls: 0-TO for anterior GMin (p < 0.05), anterior and middle GMed (p < 0.01); 0–30% for posterior GMin (p < 0.01) and GMed (p < 0.05). No significant differences were identified in TFL. Overall, this study found increased segmental gluteal muscle activation, decreased hip abduction strength, and reduced variability in muscle activation in post-menopausal women with GTPS, compared with controls.     

Electromyographic assessment of isometric and dynamic activation characteristics of the latissimus dorsi muscle

The aim of the current study was to analyze the activation characteristics and potential compartmentalization of the latissimus dorsi (LD) muscle during common maximal voluntary isometric contractions (MVICs) and functional dynamic tasks. Surface electromyography (sEMG) was used to measure activation magnitudes from four electrode sites (referenced to the T10, T12, L1 & L4 LD vertebral origins) across the fanning muscle belly of the LD. In addition, EMG waveforms were cross-correlated to study temporal activation timing between electrode sites (T10-T12, T12-L1, L1-L4 & T10-L4). The MVICs that were tested included a humeral adduction, humeral adduction with internal rotation, a chest-supported row and a humeral extension. Dynamic movements included sagittal lift/lowers from the floor to knee, knee to hip and hip to shoulder. No magnitude-based (p = 0.6116) or temporal-based differences were observed between electrode sites during the MVIC trials. During dynamic movements no temporal-based, but some magnitude-based differences between electrode sites were observed to be present; these differences were small in magnitude and were observed for both the maximum (p = 0.0002) and mean (p = 0.0002) EMG magnitudes. No clear pattern of compartmentalization was uncovered in the contractions studied here. In addition to these findings, it was determined that the most effective MVIC technique for LD EMG normalization purposes was a chest-supported row MVIC, paired with a T12 electrode site.     

The upper and lower segments of subscapularis muscle have different roles in glenohumeral joint functioning

Subscapularis muscle is divided into two independent segments, upper and lower (USUB and LSUB), but the role of each segment in glenohumeral functioning is unclear. We compared the electromyographic (EMG) activity of USUB and LSUB during a variety of shoulder movements, with and without an external translation force. Intramuscular electrodes were inserted in USUB and LSUB segments of 20 adults without pathology and EMG activity was measured in stabilization trials (with and without an anterior or posterior directed force at the humerus and isometric rotations) and two shoulder positions (shoulder neutral, abduction). Maximal voluntary isometric contraction (MVIC) trials were performed in abduction, internal and external rotation of the shoulder. In MVIC trials, USUB showed higher activity during internal rotation (p = 0.03), whereas LSUB showed higher activity during external rotation (p < 0.01). In stabilization trials, the interaction effects were significant for muscle segment × condition (p < 0.01), and approached significance for muscle segment × position (p = 0.06). In the neutral position, the pattern of activity for LSUB was similar to USUB. In the abducted position the LSUB, unlike USUB, was more active during external rotation (p = 0.06) and also showed increased activity in response to the posterior directed force at the humerus (p = 0.04). Our results suggest that USUB primarily acts as an agonist for internal rotation. In contrast LSUB was particularly active in external rotation in the abducted position and demonstrated increased EMG activity in response to the posteriorly directed force at the humerus in that position, suggesting more of a role in glenohumeral stabilization.     

Intramuscular fine-wire electromyography during cycling: Repeatability,normalisation and a comparison to surface electromyography

This study investigated (a) the feasibility and repeatability of intramuscular fine-wire electromyographic (fEMG) recordings from leg muscles during the repetitive, high-velocity cycling movement, (b) the influence of amplitude normalization technique on repeatability and statistical sensitivity, (c) the influence of test-retest interval duration on repeatability, and (d) differences between fEMG and surface EMG (sEMG) recordings of cycling. EMG activity of leg muscles was recorded using surface and fine-wire electrodes during one (n = 12, to investigate statistical sensitivity and compare sEMG and fEMG) or two sessions (T1 and T2, 5–20 days apart, n = 10, to investigate repeatability). fEMG recordings were feasible and there was high repeatability of fEMG recordings normalised to maximum measured EMG amplitude (MAX); mean coefficients of multiple correlation (CMC) ranged from .83 ± .13 to .88 ± .07. Data normalised to maximal (MVC) or submaximal contractions (sMVC) were less repeatable (p < .01). Statistical sensitivity was also greatest for data normalised to MAX (p < .01). Repeatability of fEMG increased with greater test-retest intervals (p < .01). The global pattern of muscle recruitment was consistent between sEMG and fEMG but sEMG recordings were characterized by additional myoelectric content. These findings support and guide the use of fEMG techniques to investigate leg muscle recruitment during cycling.     

Reliability and minimal detectable change in scapulothoracic neuromuscular activity

Alterations in scapular muscle activity, including excess activation of the upper trapezius (UT) and onset latencies of the lower trapezius (LT) and serratus anterior (SA) muscles, are associated with abnormal scapular motion and shoulder impingement. Limited information exists on the reliability of neuromuscular activity to demonstrate the efficacy of interventions. The purpose of this study was to characterize the reproducibility of scapular muscle activity (mean activity, relative onset timing) over time and establish the minimal detectable change (MDC). Surface electromyography (sEMG) of the UT, LT, SA and anterior deltoid (AD) muscles in 16 adults were captured during an overhead lifting task in two sessions, one-week apart. sEMG data were also normalized to maximum isometric contraction and the relative onset and mean muscle activity during concentric and eccentric phases of the scapular muscles were calculated. Additionally, reliability of the absolute sEMG data during the lifting task and MVIC was evaluated. Both intrasession and intersession reliability of normalized and absolute mean scapular muscle activity, assessed with intraclass correlation coefficients (ICC), ranged from 0.62 to 0.99; MDC values were between 1.3% and 11.7% MVIC and 24 to 135 mV absolute sEMG. Reliability of sEMG during MVIC was ICC = 0.82–0.99, with the exception of intersession upper trapezius reliability (ICC = 0.36). Within session reliability of muscle onset times was ICC = 0.88–0.97, but between session reliability was lower with ICC = 0.43–0.73; MDC were between 39 and 237 ms. Small changes in scapular neuromuscular mean activity (>11.7% MVIC) can be interpreted as meaningful change, while change in muscle onset timing in light of specific processing parameters used in this study is more variable.     

Biceps brachii myoelectric and oxygenation changes during static and sinusoidal isometric exercises

Surface myoelectric signal changes occurring during sustained isometric contractions have been extensively studied with quantitative surface electromyography (sEMG) and are described by means of some sEMG global variables in time and frequency domain (such as the median power spectral frequency). Recently, the possibility of studying local muscle O₂ saturation during exercise using non-invasive methods has been enhanced thanks to the use of near-infrared spectroscopy (NIRS). The purpose of this work was to combine NIRS and sEMG techniques to analyze the relationship between modifications of sEMG parameters and the underlying metabolic status of the exercising biceps brachii muscle. This relationship was tested under different isometric contraction modalities, namely static (ST) at 20, 40, 60 and 80%MVC and sinusoidal (SIN) at 40 ± 20 and 60 ± 20%MVC. Results clearly indicated the presence of an initial fast phase of muscle O₂ desaturation followed by a slow phase, regardless of the contraction modality. Moreover, the initial rate of muscle O₂ desaturation was related to the level of force output (R = 0.92), but it was independent on the contraction modality (p < 0.05). Similarly, changes in sEMG parameters were related to force level (Conduction Velocity-CV vs. Force: R = 0.87; sEMG Median Frequency-MDF vs. Force: R = 0.86). The high correlation found between CV-MDF and Tissue Oxygenation Index (TOI) slope (R = 0.73 and 0.72, respectively) suggests a strong relationship between NIRS and sEMG data. This study indicates that muscle O₂ demand during isometric contractions from low to high force levels is influenced by the type of active motor units and not from the type of isometric exercise modality.     

Effects of the pelvic compression belt on gluteus medius,quadratus lumborum,and lumbar multifidus activities during side-lying hip abduction

The aims of this study were to assess the effect of the pelvic compression belt on the electromyographic (EMG) activities of gluteus medius (GM), quadratus lumborum (QL), and lumbar multifidus (LM) during side-lying hip abduction. Thirty-one volunteers (15 men and 16 women) with no history of pathology volunteered for this study. Subjects were instructed to perform hip abduction in side-lying position with and without applying the pelvic compression belt. The pelvic compression belt was adjusted just below the anterior superior iliac spines with the stabilizing pressure using elastic compression bands. Surface EMG data were collected from the GM, QL, and LM of the dominant limb. Significantly decreased EMG activity in the QL (without the pelvic compression belt, 60.19 ± 23.66% maximal voluntary isometric contraction [MVIC]; with the pelvic compression belt, 51.44 ± 23.00% MVIC) and significantly increased EMG activity in the GM (without the pelvic compression belt, 26.71 ± 12.88% MVIC; with the pelvic compression belt, 35.02 ± 18.28% MVIC) and in the LM (without the pelvic compression belt, 30.28 ± 14.60% MVIC; with the pelvic compression belt, 37.47 ± 18.94% MVIC) were found when the pelvic compression belt was applied (p < 0.05). However, there were no significant differences of the EMG activity between male and female subjects. The findings suggest that the pelvic compression belt may be helpful to prevent unwanted substitution movement during side-lying hip abduction, through increasing the GM and LM and decreasing the QL.     

Is myoelectric activity distributed equally within the rectus femoris muscle during loaded,squat exercises?

Recent evidence suggests different regions of the rectus femoris (RF) muscle respond differently to squat exercises. Such differential adaptation may result from neural inputs distributed locally within RF, as previously reported for isometric contractions, walking and in response to fatigue. Here we therefore investigate whether myoelectric activity distributes evenly within RF during squat. Surface electromyograms (EMGs) were sampled proximally and distally from RF with arrays of electrodes, while thirteen healthy volunteers performed 10 consecutive squats with 20% and 40% of their body weight. The root mean square (RMS) value, computed separately for thirds of the concentric and eccentric phases, was considered to assess the proximo-distal changes in EMG amplitude during squat. The channels with variations in EMG amplitude during squat associated with shifts in the muscle innervation zone were excluded from analysis. No significant differences were observed between RF regions when considering squat phases and knee joint angles individually (P > 0.16) while a significant interaction between phase and knee joint angle with detection site was observed (P < 0.005). For the two loads considered, proximal RMS values were greater during the eccentric phase and for the more flexed knee joint position (P < 0.001). Our results suggest inferences on the degree of RF activation during squat must be made cautiously from surface EMGs. Of more practical relevance, there may be a potential for the differential adaption of RF proximal and distal regions to squat exercises.     

Innervation zone shift at different levels of isometric contraction in the biceps brachii muscle

Experiments were carried out to examine whether innervation zone (IZ) location remains stable at different levels of isometric contraction in the biceps brachii muscle (BB), and to determine how the proximity of the IZ affects common surface electromyography (sEMG) parameters. Twelve subjects performed maximal (MVC) and submaximal voluntary isometric contractions at 10%, 20%, 30%, 40%, 50% and 75% of MVC. sEMG signals were recorded with a 13 rows × 5 columns grid of electrodes from the short head of BB. The IZ shifted in the proximal direction by up to 2.4 cm, depending upon the subject and electrode column. The mean shift of all the columns was 0.6 ± 0.4 cm (10% vs. 100% MVC, P < 0.001). This shift biased the average values of mean frequency (+21.8 ± 9.9 Hz, P < 0.001), root mean square (?0.16 ± 0.15 mV, P < 0.05) and conduction velocity (?1.15 ± 0.93 m/s, P < 0.01) in the channels immediately proximal to the IZ. The shift in IZ could be explained by shortening of the muscle fibers, and thus lengthening of the (distal) tendon due to increasing force. These results underline the importance of individual investigation of IZ locations before the placement of sEMG electrodes, even in isometric contractions.     

EMG in rotation–flexion of the torso

The objective of this study was to determine the magnitude and phasic relationship of the torso muscles in rotation–flexion of varying degree of asymmetries of the trunk. Nineteen normal young subjects (7 males and 12 females) were stabilized on a posture stabilizing platform and instructed to assume a flexed and right rotated posture. A combination 20°, 40° and 60° of rotation and 20°, 40° and 60° of flexion resulted in nine postures. These postures were assumed in a random order. The subjects were asked to exert their maximal voluntary isometric contraction (MVC) in the plane of rotation of the posture assumed for a period of 5 s. The surface EMG from the external and internal obliques, rectus abdominis, latissimus dorsi and erector spinae at the 10th thoracic and 3rd lumbar vertebral levels was recorded. The abdominal muscles had the least response at 40° of flexion, the dorsal muscles had the highest magnitude.With increasing right rotation, the left external oblique continued to decrease its activity. The ANOVA revealed that rotation and muscles had a significant main effect on normalized peak EMG (p < 0.02) in both genders. There was a significant interaction between rotation and flexion in both genders (p < 0.02) and rotation and muscle in females. The erector spinae activity was highest at 40° flexion, due to greater mechanical disadvantage and having not reached the state of flexion–relaxation. The abdominal muscle activity declined with increasing asymmetry, due to the decreasing initial muscle length. The EMG activity was significantly affected by rotation than flexion (p < 0.02).     

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13 × 5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25 mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers’ direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16–32 mm and 16–24 mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32 mm, whereas that of monopolar MUPs ranged between 72 and 96 mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.     

Do surface electromyograms provide physiological estimates of conduction velocity from the medial gastrocnemius muscle?

Muscle fiber conduction velocity (CV) is commonly estimated from surface electromyograms (EMGs) collected with electrodes parallel to muscle fibers. If electrodes and muscle fibers are not located in parallel planes, CV estimates are biased towards values far over the physiological range. In virtue of their pinnate architecture, the fibers of muscles such as the gastrocnemius are hardly aligned in planes parallel to surface electrodes. Therefore, in this study we investigate whether physiological CV estimates can be obtained from the gastrocnemius muscle. Specifically, with a large grid of 16 × 8 electrodes we map CV estimates over the whole gastrocnemius muscle while eleven subjects exerted isometric plantar flexions at three different force levels. CV was estimated for couples of single differential EMGs and estimate locations (i.e., channels) were classified as physiological and non-physiological, depending on whether CV estimates were within the physiological range (3–6 ms^?1) or not. Physiological CV values could be estimated from a markedly small muscle region for eight participants; channels providing physiological CV estimates corresponded to about 5% of the total number of channels. As expected, physiological and non-physiological channels were clustered in distinct regions. CV estimates within the physiological range were obtained for the most distal gastrocnemius portion (ANOVA, P < 0.001), where occurrences of propagating potentials were often verified through visual analysis. For the first time, this study shows that CV might be reliably assessed from surface EMGs collected from the most distal gastrocnemius region.     

Bilateral impairments of shoulder abduction in chronic hemiparesis: Electromyographic patterns and isokinetic muscle performance

ObjectiveTo analyze electromyographic (EMG) patterns and isokinetic muscle performance of shoulder abduction movement in individuals who sustained a cerebrovascular accident (CVA).DesignTwenty-two individuals who sustained a CVA and 22 healthy subjects volunteered for EMG activity and isokinetic shoulder abduction assessments. EMG onset time, root mean square (RMS) for upper trapezius and deltoid muscles, as well as the isokinetic variables of peak torque, total work, average power and acceleration time were compared between limbs and groups.ResultsThe paretic side showed a different onset activation pattern in shoulder abduction, along with a lower RMS for both muscles (21.8 ± 13.4% of the maximal voluntary isometric contraction (MVIC) for the deltoid and 25.9 ± 15.3% MVIC for the upper trapezius, about 50% lower than the control group). The non-paretic side showed a delay in both muscles activation and a lower RMS for the deltoid (32.2 ± 13.7% MVIC, about 25% lower than the control group). Both sides of the group of individuals who sustained a CVA presented a significantly lower isokinetic performance compared to the control group (paretic side ～60% lower; non-paretic side ～35% lower).ConclusionsShoulder abduction muscle performance is impaired in both paretic and non-paretic limbs of individuals who sustained a CVA.     

Measuring voluntary quadriceps activation: Effect of visual feedback and stimulus delivery

Introduction: Quadriceps voluntary activation, assessed via the superimposed burst technique, has been extensively studied in a variety of populations as a measure of quadriceps function. However, a variety of stimulus delivery techniques have been employed, which may influence the level of voluntary activation as calculated via the central activation ratio (CAR). The purpose was to determine the effect of visual feedback, stimulus delivery, and perceived discomfort on maximal voluntary isometric contraction (MVIC) peak torque and the CAR. Methods: Quadriceps CAR was assessed in 14 individuals on two days using three stimulus delivery methods; (1) manual without visual feedback, (2) manual with visual feedback, and (3) automated with visual feedback. Results: MVIC peak torque and the CAR were not different between the automated with visual feedback (MVIC = 3.25, SE = 0.14 N m/kg; CAR = 88.63, SE = 1.75%) and manual with visual feedback (MVIC = 3.26, SE = 0.13 N m/kg, P = 0.859; CAR = 89.06, SE = 1.70%, P = 0.39) stimulus delivery methods. MVIC (2.99, SE = 0.12 N m/kg) and CAR (85.32, SE = 2.10%) were significantly lower using manual without visual feedback compared to manual with visual feedback and automated with visual feedback (CAR P < 0.001; MVIC P < 0.001). Perceived discomfort was lower in the second session (P < 0.05). Conclusion: Utilizing visual feedback ensures participant MVIC, and may provide a more accurate assessment of quadriceps voluntary activation.     

Comparison of humeral rotation co-activation of breast cancer population and healthy shoulders

Upper limb morbidities are common amongst the breast cancer population (BCP) and have a direct impact on independence. Comparing muscle co-activation strategies between BCP and healthy populations may assist in identifying muscle dysfunction and promote clinical interpretation of dysfunction, which could direct preventative and therapeutic interventions. The purposes of this study were to define humeral rotation muscle co-activation of a BCP and to compare it with a previously defined co-activation relationship of a healthy population. Fifty BCP survivors performed 18 isometric internal and external rotation exertions at various postures and intensities. Surface and intramuscular electrodes recorded shoulder muscle activity. BCP co-activation was predicted at r² = 0.77 during both exertion types. Humeral abduction angle and task intensity were important factors in the prediction of co-activation in both populations. Comparisons made between populations identified differing muscle strategies used by BCP to maintain postural control. Compared to healthy co-activation, the BCP demonstrated greater activation of internal (IR) and external rotator (ER) type muscles during their respective rotation type. The BCP demonstrated increased (⩾8.7%) activation of pectoralis major. This study has provided insight into how BCP muscles compensate during dysfunction. Continued advancement of this knowledge can provide more understanding of dysfunction, promote generation of evidence-based therapies, and can be useful in biomechanical modeling.     

Effect of pain location on spatial reorganisation of muscle activity

IntroductionWe aimed to determine whether the changes in muscle activity (in terms of both gross electromyography (EMG) and motor unit (MU) discharge characteristics) observed during pain are spatially organized with respect to pain location within a muscle which is the main contributor of the task.MethodsSurface and fine-wire EMG was recorded during matched low-force isometric plantarflexion from soleus (from four quadrants with fine-wire EMG and from the medial/lateral sides with surface EMG), both gastrocnemii heads, peroneus longus, and tibialis anterior. Four conditions were tested: two control conditions that each preceded contractions with pain induced in either the lateral (Pain_L) or medial (Pain_M) side of soleus.ResultsNeither the presence (p = 0.28) nor location (p = 0.19) of pain significantly altered gross muscle activity of any location (lateral/medial side of soleus, gastrocnemii, peroneus longus and tibialis anterior). Group data from 196 MUs show redistribution of MU activity throughout the four quadrants of soleus, irrespective of pain location. The significant decrease of MU discharge rate during pain (p < 0.0001; Pain_L: 7.3 ± 0.9–6.9 ± 1.1 Hz, Pain_M: 7.0 ± 1.1 to 6.6 ± 1.1 Hz) was similar for all quadrants of the soleus (p = 0.43), regardless of the pain location (p = 0.98). There was large inter-participant variation in respect to the characteristics of the altered MU discharge with pain.ConclusionResults from both surface and fine-wire EMG recordings do not support the hypothesis that muscle activity is reorganized in a simple systematic manner with respect to pain location.