Suprascapular nerve block results in a compensatory increase in deltoid muscle activity

A balance exists between the deltoid and rotator cuff contribution to arm elevation. Both cadaver and computer models have predicted an increase in deltoid muscle force with dysfunction of the rotator cuff. The goal of the present study was to verify this phenomenon in vivo by examining the effects of paralysis of the supraspinatus and infraspinatus muscles with a suprascapular nerve block on the electrical activity of seven shoulder muscles. Electromyographic data were collected before and after the administration of the block. The block resulted in a significant increase in muscle activity for all heads of the deltoid, with a higher percentage increase noted at lower elevation angles. Although the deltoid activity was reduced as the subjects recovered from the block, even low levels of cuff dysfunction were found to result in increased deltoid activity. These results suggest that even small disruptions in the normal function of some rotator cuff muscles (e.g., due to fatigue or impingement syndrome), may result in an increase in deltoid activity. It is possible that such compensation may result in higher superior loads at the glenohumeral joint, possibly increasing the risk of tendon damage.     

Reduction in subluxation and improved muscle function of the hemiplegic shoulder joint after therapeutic electrical stimulation.

Seventeen hemiplegic patients with chronic shoulder subluxation secondary to a cerebrovascular accident (CVA) were divided into three groups, two of which were subjected to 6 weeks of therapeutic electrical stimulation (TES) for 15 minutes twice a day, in order to assess the effectiveness of the treatment in reducing subluxation, and in improving shoulder abduction function. The third group was used as a control (C group). After 6 weeks of electrical stimulation of the supraspinatus (S group) and deltoid (D group), a significant (p<0.05) reduction in subluxation was observed in both groups when compared to the C group. The maximal force of shoulder abduction showed a tendency to increase in the S group (p<0.10). A significant increase in maximal force was also observed in the D group. In most of the TES-treated muscles, the interference pattern of EMG at maximum voluntary contraction increased. The amplitude of the EMG activity of the stimulated muscle also increased. Thus, we concluded that electrical stimulation therapy of the supraspinatus and the deltoid muscle is an effective treatment modality for shoulder subluxation and shoulder abduction function in hemiplegic patients.     

Static optimization underestimates antagonist muscle activity at the glenohumeral joint: A musculoskeletal modeling study

Static optimization is commonly employed in musculoskeletal modeling to estimate muscle and joint loading; however, the ability of this approach to predict antagonist muscle activity at the shoulder is poorly understood. Antagonist muscles, which contribute negatively to a net joint moment, are known to be important for maintaining glenohumeral joint stability. This study aimed to compare muscle and joint force predictions from a subject-specific neuromusculoskeletal model of the shoulder driven entirely by measured muscle electromyography (EMG) data with those from a musculoskeletal model employing static optimization. Four healthy adults performed six sub-maximal upper-limb contractions including shoulder abduction, adduction, flexion, extension, internal rotation and external rotation. EMG data were simultaneously measured from 16 shoulder muscles using surface and intramuscular electrodes, and joint motion evaluated using video motion analysis. Muscle and joint forces were calculated using both a calibrated EMG-driven neuromusculoskeletal modeling framework, and musculoskeletal model simulations that employed static optimization. The EMG-driven model predicted antagonistic muscle function for pectoralis major, latissimus dorsi and teres major during abduction and flexion; supraspinatus during adduction; middle deltoid during extension; and subscapularis, pectoralis major and latissimus dorsi during external rotation. In contrast, static optimization neural solutions showed little or no recruitment of these muscles, and preferentially activated agonistic prime movers with large moment arms. As a consequence, glenohumeral joint force calculations varied substantially between models. The findings suggest that static optimization may under-estimate the activity of muscle antagonists, and therefore, their contribution to glenohumeral joint stability.     

Muscle activation in the contralateral passive shoulder during isometric shoulder abduction in patients with unilateral shoulder pain.

Studies have shown an increased muscle activation at the opposite passive side during unilateral contractions. The purpose of the present study was to examine the influence of pain on muscle activation in the passive shoulder during unilateral shoulder abduction. Ten patients with unilateral rotator tendinosis of the shoulder and nine healthy controls performed unilateral maximal voluntary contractions (MVC) and sustained submaximal contractions with and without subacromial injections of local anaesthetics of the afflicted shoulder. Muscle activation was recorded by electromyography (EMG) from the trapezius, deltoid, infraspinatus and supraspinatus muscles in both shoulders. During MVCs, the EMG amplitude from muscles of the passive afflicted side was not different in patients and controls, and was not influenced by pain alterations. In contrast, the EMG amplitude from the muscles of the passive unafflicted side was lower in the patients and increased after pain reduction. During the sustained submaximal contraction the EMG amplitude increased gradually in the passive shoulder to 15-30% of the EMG amplitude observed during MVC. This response was not influenced by differences in pain. We conclude that muscle activation of the passive shoulder was closely related to the activation of the contracting muscles and thus related to central motor drive, and not directly influenced by changes in pain.     

EMG and force production of some human shoulder muscles during isometric abduction

Surface EMG was recorded in four subjects on three different occasions from the three parts of the deltoid, the clavicular part of the pectoralis major and from the infraspinatus muscles at different angles of abduction, in the frontal and scapular plane. The integrated EMG was related to the maximum values found for each muscle or muscle part during test contractions (%EMG). Linear relations can be seen for abduction angle vs %EMG. During abduction in the scapular plane the middle and posterior parts of the deltoid muscle showed significantly less activity than in the frontal plane. A simple two dimensional model to calculate the deltoid force out of total external moment at the shoulder is presented. For the middle part of the deltoid an EMG-force relation is presented. The maximal deltoid forces found during test contractions are compared with the absolute muscle force. Also, the length-force relation for the middle part of the deltoid muscle is given between 30° and 90° of abduction.     

Functional subdivision of the upper trapezius muscle during maximal isometric contractions

The electromyographic (EMG) activity pattern across the upper trapezius of 22 healthy subjects was investigated during maximal isometric contractions. Eight bipolar surface electrodes with 10 mm distance between adjacent electrode pairs were placed on a line from the clavicle to the scapula. At the region near the clavicle the highest EMG amplitudes were recorded during 90 ° arm abduction. At the more posterior parts the highest amplitudes were found both during arm abduction and shoulder elevation. A double differential recording technique which reduced the EMG cross-talk contribution supported the finding that the upper trapezius was differently activated when the arm posture was changed. The normalized EMG amplitude-force relationship during the shoulder elevation showed a curvilinear relationship on the anterior part of the upper trapezius with a slower increase in EMG amplitude than force at low force. The slope of the curve, at low force, increased gradually in the posterior direction on the upper trapezius. The EMG activity patterns across the upper trapezius indicate a flexibility in motor activation which maybe reflects a functional optimization of the contractions performed by this muscle.     

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.     

Force depression following muscle shortening in sub-maximal voluntary contractions of human adductor pollicis

Mechanical properties of skeletal muscles are often studied for controlled, electrically induced, maximal, or supra-maximal contractions. However, many mechanical properties, such as the force-length relationship and force enhancement following active muscle stretching, are quite different for maximal and sub-maximal, or electrically induced and voluntary contractions. Force depression, the loss of force observed following active muscle shortening, has been observed and is well documented for electrically induced and maximal voluntary contractions. Since sub-maximal voluntary contractions are arguably the most important for everyday movement analysis and for biomechanical models of skeletal muscle function, it is important to study force depression properties under these conditions. Therefore, the purpose of this study was to examine force depression following sub-maximal, voluntary contractions. Sets of isometric reference and isometric-shortening-isometric test contractions at 30% of maximal voluntary effort were performed with the adductor pollicis muscle. All reference and test contractions were executed by controlling force or activation using a feedback system. Test contractions included adductor pollicis shortening over 10 degrees, 20 degrees, and 30 degrees of thumb adduction. Force depression was assessed by comparing the steady-state isometric forces (activation control) or average electromyograms (EMGs) (force control) following active muscle shortening with those obtained in the corresponding isometric reference contractions. Force was decreased by 20% and average EMG was increased by 18% in the shortening test contractions compared to the isometric reference contractions. Furthermore, force depression was increased with increasing shortening amplitudes, and the relative magnitudes of force depression were similar to those found in electrically stimulated and maximal contractions. We conclude from these results that force depression occurs in sub-maximal voluntary contractions, and that force depression may play a role in the mechanics of everyday movements, and therefore may have to be considered in biomechanical models of human movement.     

Interfering effects of multitasking on muscle activity in the upper extremity.

Multitasking, where workers are required to perform multiple physical tasks with various levels of cognitive load is common in today's workplace. Simultaneous physical and mental demands are thought to cause task interference and likely increase muscle activity. To test the interfering effects of multitasking, 16 healthy participants performed hand and shoulder exertions with combinations of four grip conditions (no grip, 30% grip with low precision, 30% grip with high precision, and maximal grip) and three shoulder conditions at 90 degrees abduction (maintaining posture, 40% force-controlled moment, 40% posture-controlled moment), with and without the Stroop test while surface EMG was recorded from eight upper extremity muscles. Both 40% MVC shoulder moments increased extrinsic forearm muscle activity by 2-4% MVE (p<0.01). Grip exertion at 30% MVC reduced anterior and middle deltoid activity by 2% MVE (p<0.01). Exerting a constant force against the transducer (force-controlled) required 3-4% MVE greater middle and posterior deltoid activity (p<0.001) compared to supporting an equivalent inertial load at the same shoulder angle (posture-controlled). Performing the mental task (Stroop test) concurrently with either 40% MVC shoulder moments significantly increased trapezius activity by nearly 2% MVE (p<0.05). Interestingly, the Stroop test also reduced all deltoid activity by 1% MVE (p<0.05). The addition of both the Stroop test and force-control shoulder exertion independently reduced maximal grip force by 7% and 10% MVC, respectively. These results suggest that more complex workplace tasks may act to increase muscle load or interfere with task performance. These small but significant findings may play a role in the development of long-term musculoskeletal disorders in the workplace.     

Effects of different movement directions on electromyography recorded from the shoulder muscles while passing the target positions

PurposeWe compared electromyography (EMG) recorded from the shoulder joint muscles in the same position for different movement directions.MethodsFifteen healthy subjects participated. They performed shoulder elevation from 0° to 120°, shoulder depression from 120° to 0°, shoulder horizontal adduction from ?15° to 105°, and shoulder horizontal abduction from 105° to ?15°. The target positions were 90° shoulder elevation in the 0°, 30°, 60°, and 90° planes (0°, 30°, 60°, and 90° positions). EMG signals were recorded from the supraspinatus (SSP) muscle by fine-wire electrodes. EMG signals from the infraspinatus (ISP), anterior deltoid, middle deltoid, and posterior deltoid muscles were recorded using active surface electrodes.ResultsDuring elevation and horizontal abduction, the SSP showed significantly higher activity than that shown during depression and during horizontal adduction in the 0°, 30°, and 60° positions. During elevation, the ISP showed significantly higher activity than during depression and during horizontal adduction in the 90° position. During horizontal abduction, the ISP showed significantly higher activity than during depression in the 90° position.ConclusionsWhen the movement tasks were performed in different movement directions at the same speed, each muscle showed characteristic activity.     

Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue.

Intramuscular pressure (IMP) and electromyography (EMG) mirror muscle force in the nonfatigued muscle during static contractions. The present study explores whether the constant IMP-EMG relationship with increased force may be extended to dynamic contractions and to fatigued muscle. IMP and EMG were recorded from shoulder muscles in three sessions: 1). brief static arm abductions at angles from 0 to 90 degrees, with and without 1 kg in the hands; 2). dynamic arm abductions at angular velocities from 9 to 90 degrees /s, with and without 1 kg in the hands; and 3). prolonged static arm abduction at 30 degrees for 30 min followed by recovery. IMP and EMG increased in parallel with increasing shoulder torque during brief static tasks. During dynamic contractions, peak IMP and EMG increased to values higher than those during static contractions, and EMG, but not IMP, increased significantly with speed of abduction. In the nonfatigued supraspinatus muscle, a linear relationship was found between IMP and EMG; in contrast, during fatigue and recovery, significant timewise changes of the IMP-to-EMG ratio occurred. The results indicate that IMP should be included along with EMG when mechanical load sharing between muscles is evaluated during dynamic and fatiguing contractions.     

Obtaining maximum muscle excitation for normalizing shoulder electromyography in dynamic contractions

Muscle specific maximal voluntary isometric contractions (MVIC) are commonly used to elicit reference amplitudes to normalize electromyographic signals (EMG). It has been questioned whether this is appropriate for normalizing EMG from dynamic contractions. This study compares EMG amplitude when shoulder muscle activity from dynamic contractions is normalized to isometric and isokinetic maximal excitation as well as a hybrid approach currently used in our laboratory. Anterior, middle and posterior deltoid, upper and lower trapezius, pectoralis major, latissimus dorsi and infraspinatus were monitored during (1) manually resisted MVICs, and (2) maximum voluntary dynamic concentric contractions (MVDC) on an isokinetic dynamometer. Dynamic contractions were performed (a) at 30°/s about the longitudinal, frontal and sagittal axes of the shoulder, and (b) during manual bi-rotation of a tilted wheel at 120°/s. EMG from the wheel task was normalized to the maximum excitation from (i) the muscle specific MVIC, (ii) from any MVIC (MVIC_ALL), (iii) for any MVDC, (iv) from any exertion (maximum experimental excitation, MEE). Mean EMG from the wheel task was up to 45% greater when normalized to muscle specific isometric contractions (method i) than when normalized to MEE (method iv). Seventy-five percent of MEE’s occurred during MVDCs. This study presents an 20 useful and effective process for obtaining the greatest excitation from the shoulder muscles when normalizing dynamic efforts.     

Comparison of an EMG-based and a stress-based method to predict shoulder muscle forces

The estimation of muscle forces in musculoskeletal shoulder models is still controversial. Two different methods are widely used to solve the indeterminacy of the system: electromyography (EMG)-based methods and stress-based methods. The goal of this work was to evaluate the influence of these two methods on the prediction of muscle forces, glenohumeral load and joint stability after total shoulder arthroplasty. An EMG-based and a stress-based method were implemented into the same musculoskeletal shoulder model. The model replicated the glenohumeral joint after total shoulder arthroplasty. It contained the scapula, the humerus, the joint prosthesis, the rotator cuff muscles supraspinatus, subscapularis and infraspinatus and the middle, anterior and posterior deltoid muscles. A movement of abduction was simulated in the plane of the scapula. The EMG-based method replicated muscular activity of experimentally measured EMG. The stress-based method minimised a cost function based on muscle stresses. We compared muscle forces, joint reaction force, articular contact pressure and translation of the humeral head. The stress-based method predicted a lower force of the rotator cuff muscles. This was partly counter-balanced by a higher force of the middle part of the deltoid muscle. As a consequence, the stress-based method predicted a lower joint load (16% reduced) and a higher superior–inferior translation of the humeral head (increased by 1.2 mm). The EMG-based method has the advantage of replicating the observed cocontraction of stabilising muscles of the rotator cuff. This method is, however, limited to available EMG measurements. The stress-based method has thus an advantage of flexibility, but may overestimate glenohumeral subluxation.     

Activation-induced force enhancement in human adductor pollicis

It has been known for a long time that the steady-state isometric force after muscle stretch is bigger than the corresponding force obtained in a purely isometric contraction for electrically stimulated and maximal voluntary contractions (MVC). Recent studies using sub-maximal voluntary contractions showed that force enhancement only occurred in a sub-group of subjects suggesting that force enhancement for sub-maximal voluntary contractions has properties different from those of electrically-induced and maximal voluntary contractions. Specifically, force enhancement for sub-maximal voluntary contractions may contain an activation-dependent component that is independent of muscle stretching. To address this hypothesis, we tested for force enhancement using (i) sub-maximal electrically-induced contractions and stretch and (ii) using various activation levels preceding an isometric reference contraction at 30% of MVC (no stretch). All tests were performed on human adductor pollicis muscles. Force enhancement following stretching was found for all subjects (n = 10) and all activation levels (10%, 30%, and 60% of MVC) for electrically-induced contractions. In contrast, force enhancement at 30% of MVC, preceded by 6 s of 10%, 60%, and 100% of MVC was only found in a sub-set of the subjects and only for the 60% and 100% conditions. This result suggests that there is an activation-dependent force enhancement for some subjects for sub-maximal voluntary contractions. This activation-dependent force enhancement was always smaller than the stretch-induced force enhancement obtained at the corresponding activation levels. Active muscle stretching increased the force enhancement in all subjects, independent whether they showed activation dependence or not. It appears that post-activation potentiation, and the associated phosphorylation of the myosin light chains, might account for the stretch-independent force enhancement observed here.     

In vivo pediatric shoulder muscle volumes and their relationship to 3D strength

In the pediatric shoulder, injury and pathology can disrupt the muscle force balance, resulting in severe functional losses. As little data exists pertaining to in vivo pediatric shoulder muscle function, musculoskeletal data are crucially needed to advance the treatment of pediatric shoulder pathology/injury. Therefore, the purpose of this study was to develop a pediatric database of in vivo volumes for the major shoulder muscles and correlate these volumes with maximum isometric flexion/extension, internal/external rotation, and abduction/adduction joint moments. A methodology was developed to derive 3D shoulder muscle volumes and to divide the deltoid into sub-units with unique torque producing capabilities, based on segmentation of three-dimensional magnetic resonance images. Eleven typically developing children/adolescents (4F/7M, 12.0±3.2 years, 150.8±16.7 cm, 49.2±16.4 kg) participated. Correlation and regression analyses were used to evaluate the relationship between volume and maximum, voluntary, isometric joint torques. The deltoid demonstrated the largest (30.4±1.2%) and the supraspinatus the smallest (4.8±0.5%) percent of the total summed volume of all six muscles evaluated. The anterior and posterior deltoid sections were 43.4±3.9% and 56.6±3.9% of the total deltoid volume. The percent volumes were highly consistent across subjects. Individual muscle volumes demonstrated moderate-high correlations with torque values (0.70–0.94, p<0.001). This study presents a comprehensive database documenting normative pediatric shoulder muscle volume. Using these data a clear relationship between shoulder volume and the torques they produce was established in all three rotational degrees-of-freedom. This study furthers the understanding of shoulder muscle function and serves as a foundation for evaluating shoulder injury/pathology in the pediatric/adolescent population.     

Electromyography of pongid shoulder muscles III. Quadrupedal positional behavior

Electromyographic (EMG) recordings were taken from 14 shoulder muscles (or major parts of them) in a gorilla, a chimpanzee and an orangutan as they stood quadrupedally and tripedally, descended from elevated substrates, crutch-walked, and progressed quadrupedally on inclined and level substrates. In the African apes, low potentials commonly (but not always) occurred in the sternocostal pectoralis major, anterior deltoid, supraspinatus and subscapularis muscles during quadrupedal stance. The quadrupedal orangutan always exhibited low potentials in the pectoralis major muscle and EMG activity commonly occurred in her supraspinatus and subscapularis muscles. Quiescent tripedal stances were not accompanied by striking changes in EMG patterns from those which characterized quadrupedal stances. Per contra, eccentric loadings of the forelimb during descents from elevated substrates generally recruited notable EMG activity in the deltoid, supraspinatus and, to a lesser extent, infraspinatus muscles of the three pongid apes. The pectoralis major and caudal serratus anterior muscles were much more active in Pongo and Pan during these descents. Supportive segments of quadrupedal locomotive cycles were generally accompanied by EMG activity in the pectoralis major, intermediate and posterior deltoid and supraspinatus muscles. The intermediate and posterior deltoid muscles were characteristically active during pre-release of the hand and early swing phase. The cranial trapezius and supraspinatus muscles also may act during early swing phase. We conclude that the pectoralis major and perhaps the supraspinatus and subscapularis might serve regularly as postural muscles during static terrestrial quadrupedalism in pongid apes. The lack of dramatic differences between the EMG patterns exhibited during fist-walking versus knuckle-walking indicates that an evolutionary transformation from a shoulder complex like that of Pongo to ones like Pan or vice versa need not entail major changes in myological features.     

Muscles within muscles: the neuromotor control of intra-muscular segments

The aim of this investigation was to anatomically identify, and then determine the function of, individual segments within the human deltoid muscle. The anatomical structure of the deltoid was determined through dissection and/or observation of the shoulder girdles of 11 male cadavers (aged 65–84 years). These results indicate that the deltoid consists of seven anatomical segments (D1–D7) based upon the distinctive arrangement of each segment's origin and insertion. Radiographic analysis of a cadaveric shoulder joint suggested that only the postero-medial segment D7 has a line of action directed below the shoulder joint's axis of rotation. The functional role of each individual segment was then determined utilising an electromyographic (EMG) technique. Seven miniature (1 mm active plate; 7 mm interelectrode distance) bipolar surface electrodes were positioned over the proximal portion of each segment's muscle belly in 18 male and female subjects (18–30 years). EMG waveforms were then recorded during the production of rapid isometric shoulder abduction and adduction force impulses with the shoulder joint in 40 degrees of abduction in the plane of the scapula. Each subject randomly performed 15 abduction and 15 adduction isometric force impulses following a short familiarisation period. All subjects received visual feed back on the duration and amplitude of each isometric force impulse produced via a visual force-time display which compared subject performance to a criterion force-time curve. Movement time was 400 ms (time-to-peak isometric force) at an intensity level of 50% maximal voluntary contraction. Temporal and intensity analyses of the EMG waveforms, as well as temporal analysis of the isometric force impulses, revealed the neuromotor control strategies utilised by the CNS to control the activity of each muscle segment. The results showed that segmental neuromotor control strategies differ across the breadth of the muscle and that individual segments of the deltoid can be identified as having either “prime mover”, “synergist”, “stabiliser” or “antagonist” functions; functional classifications normally associated with whole muscle function. Therefore, it was concluded that the CNS can “fine tune” the activity of at least six discrete segments within the human deltoid muscle to efficiently meet the demands of the imposed motor task. Accepted: 15 December 1997     

Normalization of electromyogram in the neck-shoulder region

Linear and curvilinear electromyogram (EMG) normalization methods were compared among ten healthy men during a simulated work cycle demanding attention and static holding of the arm (Solitaire test). Maximal voluntary contractions (MVC) and gradually increasing contractions up to 70% of MVC were used for normalization in different arm postures. The test contractions studied included inward and outward rotations, abduction, shoulder elevation, and flexion in different arm positions. The shoulder load moment was calculated for the flexion tests using a simple two-dimensional model. The effect of arm posture on the EMG versus shoulder load moment relationship was studied on the following muscles: supraspinatus, infraspinatus, trapezius (three parts), deltoid (two parts) and pectoralis major. All muscles participated in the MVC tests performed, and its was not possible to suggest a single recommended test for each muscle. Differences in normalized EMG median values ranging up to 30% of MVC were found between linear and curvilinear normalization methods. Short-term repeatability of normalization based on a contraction with gradually increasing force was good. Arm posture affected the relationships between shoulder load moment and EMG activity of all muscles studied. Arm posture did not, however, have a significant effect on the estimated amplitude probability distribution functions during the simulated work task. Therefore, at least for the tasks studied, the principle of normalizing in the middle position of the range of movement was deemed acceptable.     

Torque-EMG-velocity relationship in female workers with chronic neck muscle pain

The present study investigated the effect of chronic neck muscle pain (defined as trapezius myalgia) on neck/shoulder muscle function during concentric, eccentric and static contraction. Forty-two female office workers with trapezius myalgia (MYA) and 20 healthy matched controls (CON) participated. Isokinetic (-60, 60 and 180 degrees s(-1)) and static maximal voluntary shoulder abductions were performed in a Biodex dynamometer, and electromyography (EMG) obtained in the trapezius and deltoideus muscles. Muscle thickness in the trapezius was measured with ultrasound. Pain and perceived exertion were registered before and after the dynamometer test. The main findings were that shoulder abduction torque (at -60 and 60 degrees s(-1)) and trapezius EMG amplitude (at -60, 0 and 60 degrees s(-1)) were significantly lower in MYA compared with CON (p<0.001-0.05). Deltoideus EMG and trapezius muscle thickness were not significantly different between the groups. While perceived exertion increased in both groups in response to the test (p<0.0001), pain increased in MYA only (p<0.0001). In conclusion, having trapezius myalgia was associated with decreased strength capacity and lowered activity of the painful trapezius muscle. The most consistent differences-in terms of both torque and EMG-were found during slow concentric and eccentric contractions. Activity of the synergistic pain free deltoideus muscle was not significantly lower, indicating specific inhibitory feedback of the painful trapezius muscle only. Parallel increase in pain and perceived exertion among MYA were observed in response to the maximal contractions, emphasizing that heavy physical exertion provokes pain increase only in conditions of myalgia.     

Direction-specific recruitment of rotator cuff muscles during bench press and row

Recent studies indicate that rotator cuff (RC) muscles are recruited in a reciprocal, direction-specific pattern during shoulder flexion and extension exercises. The main purpose of this study was to determine if similar reciprocal RC recruitment occurs during bench press (flexion-like) and row (extension-like) exercises. In addition, shoulder muscle activity was comprehensively compared between bench press and flexion; row and extension; and bench press and row exercises. Electromyographic (EMG) activity was recorded from 9 shoulder muscles sites in 15 normal volunteers. All exercises were performed at 20, 50 and 70% of subjects’ maximal load. EMG data were normalized to standard maximal voluntary contractions. Infraspinatus activity was significantly higher than subscapularis during bench press, with the converse pattern during the row exercise. Significant differences in activity levels were found in pectoralis major, deltoid and trapezius between the bench press and flexion exercises and in lower trapezius between the row and extension exercises. During bench press and row exercises, the recruitment pattern in each active muscle did not vary with load. During bench press and row exercises, RC muscles contract in a reciprocal direction-specific manner in their role as shoulder joint dynamic stabilizers to counterbalance antero-posterior translation forces.