Influences of handgrip on shoulder muscle activity

Four shoulder muscles (the supraspinatus, the infraspinatus, the middle portion of the deltoid and the descending part of the trapezius muscle) were examined using electromyography (EMG) in abducted and flexed arm positions, in nine subjects who had no history of illness from arm or shoulder. The subjects were asked to supply an intermittent isometric handgrip force of 30% and 50% of maximal voluntary contraction, in a total of eight different arm positions. The EMG activity with and without hand activity was compared in all positions. There was a statistically significant increase in the EMG activity in the supraspinatus muscle in humeral flexion from and above 60°. In the infraspinatus muscle the changes were less; a significant decrease was however noticed in abduction. In the deltoid muscle there was a significant decrease with hand activity in flexion from and above 90°. There was no statistically significant alteration regarding the EMG activity of the trapezius muscle. The result of this study implies that high demands on handgrip force, particularly while using hand tools in elevated arm positions, adds further to the already high load on some shoulder muscles. This factor should be considered in the design of manual work and in the places of work.     

The moment arms of the muscles spanning the glenohumeral joint: a systematic review

The moment arm of a muscle represents its leverage or torque‐producing capacity, and is indicative of the role of the muscle in joint actuation. The objective of this study was to undertake a systematic review of the moment arms of the major muscles spanning the glenohumeral joint during abduction, flexion and axial rotation. Moment arm data for the deltoid, pectoralis major, latissimus dorsi, teres major, supraspinatus, infraspinatus, subscapularis and teres minor were reported when measured using the geometric and tendon excursion methods. The anterior and middle sub‐regions of the deltoid had the largest humeral elevator moment arm values of all muscles during coronal‐ and scapular‐plane abduction, as well as during flexion. The pectoralis major, latissimus dorsi and teres major had the largest depressor moment arms, with each of these muscles exhibiting prominent leverage in shoulder adduction, and the latissimus dorsi and teres major also in extension. The rotator cuff muscles had the largest axial rotation moment arms regardless of the axial position of the humerus. The supraspinatus had the most prominent elevator moment arms during early abduction in both the coronal and scapular planes as well as in flexion. This systematic review shows that the rotator cuff muscles function as humeral rotators and weak humeral depressors or elevators, while the three sub‐regions of the deltoid behave as substantial humeral elevators throughout the range of humeral motion. The pectoralis major, latissimus dorsi and teres major are significant shoulder depressors, particularly during abduction. This study provides muscle moment arm data on functionally relevant shoulder movements that are involved in tasks of daily living, including lifting and pushing. The results may be useful in quantifying shoulder muscle function during specific planes of movement, in designing and validating computational models of the shoulder, and in planning surgical procedures such as tendon transfer surgery.     

Rotator cuff muscles perform different functional roles during shoulder external rotation exercises

The aim of this study was to compare activity in shoulder muscles during an external rotation task under conditions of increasing arm support to investigate whether changing support requirements would influence muscle recruitment levels, particularly in the rotator cuff (RC) muscles. Electromyographic recordings were collected from seven shoulder muscles using surface and indwelling electrodes. The dominant shoulder of 14 healthy participants were examined during dynamic shoulder external rotation performed at 90° abduction with the arm fully supported, partially supported, and unsupported. Linear regressions between arm support load and the averaged muscle activity across participants for each muscle showed infraspinatus predominantly contributing to rotating the shoulder whilst supraspinatus, deltoid, upper trapezius, and serratus anterior were predominantly functioning in support/stabilization roles. During dynamic shoulder external rotation in mid‐range abduction, the RC muscles perform different functional roles. Infraspinatus is responsible for producing external rotation torque, supraspinatus is playing a larger joint stabilizer role, and subscapularis is contributing minimally to joint stability. The results also indicate that increasing support load requirements during an external rotation task may be a functionally specific way to retrain the stabilization function of axioscapular muscles. Manipulating joint stabilization requirements while maintaining constant rotational load is a novel method of investigating the differential contribution of muscles to joint movement and stabilization during a given task. Clin. Anat. 2013. © 2012 Wiley Periodicals, Inc.     

Towards the development of a novel experimental shoulder simulator with rotating scapula and individually controlled muscle forces simulating the rotator cuff

A preclinical analysis of novel implants used in shoulder surgery requires biomechanical testing conditions close to physiology. Existing shoulder experiments may only partially apply multiple cycles to simulate postoperative, repetitive loading tasks. The aim of the present study was therefore the development of an experimental shoulder simulator with rotating scapula able to perform multiple humeral movement cycles by simulating individual muscles attached to the rotator cuff. A free-hanging, metallic humerus pivoted in a polyethylene glenoid is activated by tension forces of linear electroactuators to simulate muscles of the deltoideus (DELT), supraspinatus (SSP), infraspinatus/teres minor and subscapularis. The abductors DELT and SSP apply forces with a ratio of 3:1 up to an abduction angle of 85°. The rotating scapular part driven by a rotative electro actuator provides one-third to the overall arm abduction. Resulting joint forces and moments are measured by a 6-axis load cell. A linear increase in the DELT and SSP motors is shown up to a maximum of 150 and 50 N for the DELT and SSP, respectively. The force vector in the glenoid resulted in 253 N at the maximum abduction. The present investigation shows the contribution of individual muscle forces attached to the moving humerus to perform active abduction in order to reproducibly test shoulder implants.     

A comprehensive analysis of muscle recruitment patterns during shoulder flexion: an electromyographic study

Although flexion is a common component of the routine clinical assessment of the shoulder the muscle recruitment patterns during this movement are not clearly understood making valid interpretation of potential muscle dysfunction problematic. The purpose of this study was to comprehensively examine shoulder muscle activity during flexion in order to compare the activity levels and recruitment patterns of shoulder flexor, scapular lateral rotator and rotator cuff muscles. Electromyographic (EMG) data were recorded from 12 shoulder muscles sites in 15 volunteers. Flexion was performed in standing in the sagittal plane at no load, 20%, and 60% of each subject's maximum load. EMG data were normalized to maximum values obtained during maximum voluntary contractions. Results indicated that anterior deltoid, pectoralis major, supraspinatus, infraspinatus, serratus anterior, upper, and lower trapezius were activated at similar moderate levels. However, subscapularis was activated at low levels and significantly lower than supraspinatus and infraspinatus. Similar activity patterns across time were demonstrated in the muscles that produce flexion torque, laterally rotate the scapula, as well as supraspinatus and infraspinatus, and did not change as flexion load increased. The onset of activity in supraspinatus and anterior deltoid occurred at the same time and prior to movement of the limb at all loads with infraspinatus activity also occurring prior to movement onset at the medium and high load conditions only. Posterior rotator cuff muscles appear to be counterbalancing anterior translational forces produced during flexion and it would appear that supraspinatus is one of the muscles that consistently "initiates" flexion.     

Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non-human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm-raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long-standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.     

The influence of experimentally induced pain on shoulder muscle activity

Muscle function is altered in painful shoulder conditions. However, the influence of shoulder pain on muscle coordination of the shoulder has not been fully clarified. The aim of the present study was to examine the effect of experimentally induced shoulder pain on shoulder muscle function. Eleven healthy men (range 22–27 years), with no history of shoulder or cervical problems, were included in the study. Pain was induced by 5% hypertonic saline injections into the supraspinatus muscle or subacromially. Seated in a shoulder machine, subjects performed standardized concentric abduction (0°–105°) at a speed of approximately 120°/s, controlled by a metronome. During abduction, electromyographic (EMG) activity was recorded by intramuscular wire electrodes inserted in two deeply located shoulder muscles and by surface-electrodes over six superficially located shoulder muscles. EMG was recorded before pain, during pain and after pain had subsided and pain intensity was continuously scored on a visual analog scale (VAS). During abduction, experimentally induced pain in the supraspinatus muscle caused a significant decrease in activity of the anterior deltoid, upper trapezius and the infraspinatus and an increase in activity of lower trapezius and latissimus dorsi muscles. Following subacromial injection a significantly increased muscle activity was seen in the lower trapezius, the serratus anterior and the latissimus dorsi muscles. In conclusion, this study shows that acute pain both subacromially and in the supraspinatus muscle modulates coordination of the shoulder muscles during voluntary movements. During painful conditions, an increased activity was detected in the antagonist (latissimus), which support the idea that localized pain affects muscle activation in a way that protects the painful structure. Further, the changes in muscle activity following subacromial pain induction tend to expand the subacromial space and thereby decrease the load on the painful structures.     

Human involuntary postural aftercontractions are strongly modulated by limb position

Involuntary muscle activations called aftercontractions occur in skeletal muscles following sustained voluntary contractions. They are strongest following high-force voluntary contractions in proximal muscles. Their mechanism is unknown. Some authors have hypothesised that they are dependent on proprioceptive feedback; others believe that they are independent of such influences. These experiments tested this hypothesis by examining the effect of shoulder joint excursion magnitude and direction on aftercontraction amplitude. A 1-min maximal isometric voluntary abduction of the shoulder joint was used to evoke a postural involuntary aftercontraction in the deltoid muscle. During the 20-s aftercontraction which followed the arm was allowed to abduct in the coronal plane and active muscle shortening took place. The maximum amplitude of EMG activity during the aftercontraction of the deltoid muscle was equal to 20–50% of the EMG amplitude of the maximal voluntary contraction. The aftercontraction EMG amplitude grew as the angle of shoulder joint abduction increased. This growth ceased and the activity levelled off if arm movement was blocked. The results showed that the final EMG amplitude reached depended linearly on the final shoulder angle allowed—it did not grow purely as a function of time. Forcible adduction of the arm by the experimenter and consequent lengthening of the muscle caused the EMG of the aftercontraction to fall with decreasing shoulder joint angle. It is concluded that the neural centres controlling the involuntary aftercontraction are strongly modulated by proprioceptive feedback. Results are given as mean (SD) unless otherwise stated.     

Electromyographic analysis of the supraspinatus and deltoid muscles during 3 common rehabilitation exercises

CONTEXT: Investigators have observed electromyographic (EMG) activity of the supraspinatus muscle and reported conflicting results. OBJECTIVE: To quantify EMG activity of the supraspinatus, middle deltoid, and posterior deltoid muscles during exercises commonly used in rehabilitation. DESIGN: One-factor, repeated-measures design. SETTING: Controlled laboratory. PATIENTS OR OTHER PARTICIPANTS: Twenty-two asymptomatic subjects (15 men, 7 women) with no history of shoulder injury participated. MAIN OUTCOMES MEASURE(S): The dominant shoulder was tested. Fine-wire EMG electrodes were inserted into the supraspinatus, middle deltoid, and posterior deltoid muscles. The EMG data were collected at 960 Hz for analysis during maximal voluntary isometric contraction (MVIC) and 5 repetitions of 3 exercises: standing elevation in the scapular plane ("full can"), standing elevation in the scapular plane with glenohumeral internal rotation ("empty can"), and prone horizontal abduction at 100 degrees with glenohumeral external rotation ("prone full can"). We calculated 1-way repeated-measures analysis of variance (P < .05) and post hoc 2-tailed, paired t tests to detect significant differences in muscle activity among exercises. RESULTS: No statistical difference existed among the exercises for the supraspinatus. The middle deltoid showed significantly greater activity during the empty-can exercise (77 +/- 44% MVIC) and prone full-can exercise (63 +/- 31% MVIC) than during the full-can exercise (52 +/- 27% MVIC) (P = .001 and .017, respectively). The posterior deltoid showed significantly greater activity during the prone full-can exercise (87 +/- 53% MVIC) than during the full-can (P = .001) and the empty-can (P = .005) exercises and significantly greater activity during the empty-can exercise (54 +/- 24% MVIC) than during the full-can exercise (38 +/- 32% MVIC) (P = .012). CONCLUSIONS: While all 3 exercises produced similar amounts of supraspinatus activity, the full-can exercise produced significantly less activity of the deltoid muscles and may be the optimal position to recruit the supraspinatus muscle for rehabilitation and testing. The empty-can exercise may be a good exercise to recruit the middle deltoid muscle, and the prone full-can exercise may be a good exercise to recruit the posterior deltoid muscle.     

Moment arms of the muscles crossing the anatomical shoulder

The objective of the present study was to determine the instantaneous moment arms of 18 major muscle sub-regions crossing the glenohumeral joint during coronal-plane abduction and sagittal-plane flexion. Muscle moment-arm data for sub-regions of the shoulder musculature during humeral elevation are currently not available. The tendon-excursion method was used to measure instantaneous muscle moment arms in eight entire upper-extremity cadaver specimens. Significant differences in moment arms were reported across sub-regions of the deltoid, pectoralis major, latissimus dorsi, subscapularis, infraspinatus and supraspinatus (P < 0.01). The most effective abductors were the middle and anterior deltoid, whereas the most effective adductors were the teres major, middle and inferior latissimus dorsi (lumbar vertebrae and iliac crest fibers, respectively), and middle and inferior pectoralis major (sternal and lower-costal fibers, respectively). In flexion, the superior pectoralis major (clavicular fibers), anterior and posterior supraspinatus, and anterior deltoid were the most effective flexors, whereas the teres major and posterior deltoid had the largest extensor moment arms. Division of multi-pennate shoulder muscles of broad origins into sub-regions highlighted distinct functional differences across those sub-regions. Most significantly, we found that the superior sub-region of the pectoralis major had the capacity to exert substantial torque in flexion, whereas the middle and inferior sub-regions tended to behave as a stabilizer and extensor, respectively. Knowledge of moment arm differences between muscle sub-regions may assist in identifying the functional effects of muscle sub-region tears, assist surgeons in planning tendon reconstructive surgery, and aid in the development and validation of biomechanical computer models used in implant design.     

Cortico-motoneuronal output to intrinsic hand muscles is differentially influenced by static changes in shoulder positions

We investigated whether shoulder position influenced the recruitment properties of the abductor digiti minimi muscle (ADM) and first dorsal interosseous muscle (FDI). ADM and FDI motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were obtained in seven healthy volunteers at two different static positions of the shoulder joint (30° adduction vs 30° abduction) while the arm was passively supported at shoulder level (90° in the horizontal plane) and the elbow joint was fixed at 90°. ADM and FDI voluntary activity was also examined during (1) externally paced finger abductions at 2 Hz in the two different shoulder positions (EMG_ADM and EMG_FDI was back-averaged time-locked to the end of finger abduction) and (2) maximal voluntary abduction of the little finger and the index finger. Maximal EMG power and force were analysed in the two shoulder positions. H-reflexes from ADM and FDI were also obtained in two subjects. The ADM stimulus-response curve to TMS showed that the slope and plateau level were significantly reduced with the shoulder at 30° abduction. In contrast, the FDI stimulus-response curve to TMS was not influenced by shoulder position. The back-averaged EMG_ADM showed a significant decrease in peak amplitude and area with the shoulder at 30° abduction, while no change in EMG_FDI was observed under the same condition. Similarly, maximal EMG_ADM and force exertion by the little finger were significantly reduced with the shoulder at 30° abduction, while no such effect was observed for FDI. ADM H-reflex, but not FDI, was also decreased with shoulder abduction. These results indicate that the corticospinal pathway to ADM is less accessible to TMS and to voluntary command when the shoulder is placed at 30° abduction. In contrast, activation of FDI, whether by TMS or by volition, is not influenced by shoulder position. This finding suggests that there are differences in the corticospinal innervation to ADM and FDI, possibly due to the different role of these muscles in hand function.     

Electromyographic analysis of deltoid muscle fatigue during abduction on scapular and frontal planes

The aim of the study was to compare the fatigue rates of the deltoid muscle portions in the scapular and frontal planes. Ten healthy men without shoulder muscle impairment took part in the study. They performed isometric arm abduction for 30 seconds against a resistance of load cell while the electromyographic data were collected. The electromyographic data were transformed by the Fast Fourier Transform, to obtain the median power frequency (MDPF). The changes in MDPF of the three deltoid portions in the scapular and frontal planes were compared. The acromialis and spinalis portions fatigued during the exercises. The clavicularis portion presented no fatigue. A statistically significant difference occurred between the clavicularis and the other two portions (P < 0.05). No differences were found when the planes were compared. It represents to practice no preferential order during rehabilitation. Moreover, the acromialis and spinalis portions fatigue, although the clavicularis portion did not fatigue. The actions of other muscles of the shoulder girdle can explain this fact. Moreover, these two portions need more attention to avoid fatigue during exercises. In general, therapeutic strategies for injured patients should not only be directed towards increased force, but also towards fatigue control during shoulder exercises.     

Digital fluoroscopic assessment of the scapulohumeral rhythm

Previous studies have shown that abnormal rotation of the scapula is associated with shoulder pathology. Among the methods which have been proposed, planar x-ray measurements are probably the only methods, which enable clinicians to assess accurately and objectively the scapulohumeral function in vivo. The aim of this study was to develop a method for the assessment of scapulohumeral kinematics using digital fluoroscopy. Anteroposterior images of the right glenohumeral joint were taken, in thirty-four healthy males, with the arm at rest, 30°, 60°, 90°, 120°, 150° and maximum abduction, in the scapular plane. High inter- and intra-examiner reliability was observed regarding the arm and scapular angle measurements (ICC = 0.92–0.99). The positioning of the arm at the proposed angles was also highly accurate (< 2.3° misplacement) and reproducible (CV% < 5.3%). The mean radiation dose was 0.075 mSv (± 0.027 mSv). At the resting position the scapula was in slight downward rotation (−2.4° ± 4.3°) and the arm in slight abduction (1.5° ± 6.6°). The mean maximum scapular rotation and the mean maximum arm abduction was 61.4° (± 5.2°) and 162.4°(± 6.6°) respectively. A curvilinear relationship was found between the arm angle (AA) and the scapular angle (SA) (p < 0.0001). The AASA ratio for the entire range of abduction was 2.51. The greatest contribution of the scapula (1.71) achieved at 30°-60° of arm. The high accuracy and reliability of our method and the low radiation recordings suggests that digital fluoroscopy may be considered for further investigation of the scapulohumeral kinematics in both healthy and pathological shoulders.     

Anatomy of Shoulder Girdle Muscle Modifications and Walking Adaptation in the Scaly Chinese Pangolin (Manis Pentadactyla Pentadactyla: Pholidota) Compared with the Partially Osteoderm‐Clad Armadillos (Dasypodidae)

Because pangolins are unique mammals with a body and limbs almost entirely sheathed in hard keratinous overlapping scales and with digging and climbing abilities, the shoulder girdle muscles may differ significantly from those of other mammals including the partially osteoderm‐clad armadillos. Therefore, we conducted a functional anatomical study of the shoulder girdle muscles in Chinese pangolins (Manis pentadactyla pentadactyla, Pholidota) and some armadillo species (Dasypodidae). Our CT scans revealed that the pangolin's overlapping scales are hard structures completely encasing the limbs. The armadillo's limbs, however, are covered with small relatively soft non‐overlapping scales embedded in the skin, and articulate completely free of the hard osteodermal carapace. The attachments of some shoulder girdle muscles in the pangolin have moved from the surrounding edges of the scapula to the spine, and they, therefore, fully cover the scapula. In addition, some pangolin shoulder girdle muscles cross the shoulder joint to insert on the distal humerus, but this does not occur in armadillos. We cannot rule out the possibility that these muscle modifications represent adaptations for digging and/or climbing in pangolins. Our results and previous literature do not establish specific links between them and locomotive modes. However, we propose that the Chinese pangolin may use its derived muscular features when walking to move its armor‐restricted forelimbs more effectively by swinging its head from side to side. Anat Rec, 298:1217–1236, 2015. © 2015 Wiley Periodicals, Inc.     

Saturated muscle activation contributes to compensatory reaching strategies after stroke

The control and execution of movement could potentially be altered by the presence of stroke-induced weakness if muscles are incapable of generating sufficient power. The purpose of this study was to identify compensatory strategies during a forward (sagittal) reaching task for 20 persons with chronic stroke and 10 healthy age-matched controls. We hypothesized that the paretic anterior deltoid would be maximally activated (i.e., saturated) during a reaching task and that task completion would require activation of additional muscles, resulting in compensatory movements out of the sagittal plane. For reaching movements by control subjects, joint motion remained largely in the sagittal plane and hand trajectories were smooth and direct. Movement characteristics of the nonparetic arm of stroke subjects were similar to control subjects except for small increases in the abduction angle and the percentage that anterior deltoid was activated. In contrast, reaching movements of the paretic arm of stroke subjects were characterized by increased activation of all muscles, especially the lateral deltoid, in addition to the anterior deltoid, with resulting shoulder abduction power and segmented and indirect hand motion. For the paretic arm of stroke subjects, muscle and kinetic compensations increased with impairment severity and weaker muscles were used at a higher percentage of their available muscle activity. These results suggest that the inability to generate sufficient force with the typical agonists involved during a forward reaching task may necessitate compensatory muscle recruitment strategies to complete the task.     

Deltoid muscle volume estimated from ultrasonography: in vitro validation and correlation with isokinetic abduction strength of the shoulder

Accurate prediction of muscle parameters can be used for customization of biomechanical models. A method is presented to estimate the volume of triangular-shaped muscles from ultrasonographic measurements. The deltoid muscle was used to validate the technique in a cadaver study of 12 shoulders. The relationship between isokinetic abduction strength and estimated deltoid muscle volume of the dominant shoulder of 30 healthy male subjects was analysed. Isokinetic joint moments were quantified using the Biodex System 3. A significant relationship (R = 0.87; p < 0.001) between the calculated deltoid volume and the isokinetic abduction peak torque was established.     

Function of the shoulder muscles during arm elevation: an assessment using positron emission tomography

Although 2‐deoxy‐2‐[¹⁸F]fluoro‐D‐glucose (FDG) positron emission tomography (PET) has been used for the assessment of skeletal muscle activities, its application to the shoulder muscles is only sparse. The purpose of this study was to investigate the activities of the shoulder muscles during arm elevation using PET. Six healthy volunteers performed an arm elevation exercise before and after FDG injection. The exercise consisted of 200 repetitions of arm elevation in the scapular plane with a 0.25‐kg weight fixed to the wrist on both arms. PET examination was performed 50 min after FDG injection. For control data, PET scan was repeated for each subject on a separate day without any exercise. The volume of interest was established for each shoulder muscle. The subscapularis was divided into three portions (superior, middle, and inferior). The standardized uptake value (SUV) was calculated in each muscle to quantify its activity. The SUVs increased significantly after exercise in the deltoid, supraspinatus, and the superior portion of subscapularis. Among three divided portions of the subscapularis, the SUV of the superior one‐third was significantly greater than the rest of the muscle after exercise. Our current study clearly indicated that there were two functionally different portions in the subscapularis muscle and the superior one‐third played an important role during arm elevation in the scapular plane.     

Effects of experimental muscle pain on shoulder-abduction force steadiness and muscle activity in healthy subjects

We previously demonstrated that the steadiness of shoulder abduction is reduced in patients with subacromial impingement syndrome (SIS), which might be related to shoulder pain associated with the SIS. The aim of the present study was to examine the acute effects of experimental shoulder muscle pain on shoulder motor function in healthy subjects. The fluctuations in exerted force (force steadiness) and electromyographic (EMG) activity from eight shoulder muscles were determined during sub-maximal isometric and dynamic contractions with the shoulder abductors in nine healthy subjects (27.7 ± 4.2 years, mean ± 1 SD) before, during and after experimental pain induction. Experimental pain was induced by bolus injections of 6% hypertonic saline into the supraspinatus muscle. Experimental muscle pain reduced shoulder-abduction force steadiness on average by 21% during isometric contractions (P = 0.012) and tended to do so during concentric contractions (P = 0.083). Middle deltoid, and infraspinatus and lower trapezius muscle activity increased (3–5% EMG_max) during isometric and concentric contractions, respectively (P < 0.05). Thus, experimental shoulder muscle pain reduced the steadiness of isometric shoulder abduction and caused small changes in the abduction activation strategy. The observed effects of experimental pain on shoulder motor function differed from that observed previously in patients with SIS and chronic pain during the same types of contractions. A possible explanation may be that, even though the adopted experimental pain-paradigm may reflect the SIS in terms of the painful structures, it might not reflect the adaptations in the central nervous system seen with chronic pain.     

On the development of the shoulder girdle in Crocidura russula (Soricidae) and other placental mammals: evolutionary and functional aspects

The development of the shoulder girdle was studied in embryonic stages and a neonate of Crocidura russula using histological sections and 3-D reconstructions. Neonatal stages of Suncus etruscus and Mesocricetus auratus, both altricial placentals, were also studied. The earliest stage of C russula, in which the scapula is still partially blastematous, has already a supraspinous fossa. The dorsal portion of the scapular spine does not develop from the anterior margin of the scapula. Its mode of development varies among the placentals studied to date. In some it is completely appositional bone, in others it consists of bone formed mostly by endochondral ossification of a dorsal cartilaginous process stemming from the acromium. During development the supraspinatus muscle increases in size in proportion to the infraspinatus muscle and the humeral head increases in size in relation to the glenoid fossa. Placentals have secondary cartilage in the sternal and acromial ends of the clavicle, a derived feature absent in Marsupialia. Even the most altricial placentals have a more developed shoulder girdle at birth than any newborn marsupial studied to date.     

Arm EMG during abduction and adduction: hysteresis cycle

The electromyographic study of the muscles involved in the complex movements of the shoulder, is usually one way to quantifying the static and dynamic joint's behavior. In particular, the deltoid medium EMG produced a phenomenon similar to a hysteresis cycle when its amplitude was plotted as a function of the lateral angular position during a static, step by step, sequential abduction-adduction of the arm. Such a cycle was consistently repeated in 16 subjects (12 males and 4 females). The paired Student t-test, after comparing the mean EMG values of the rectified wave for the same arm opening angle between abduction and adduction, produced a highly significant difference (alpha<0.001) in all subjects. In all likelihood, it manifests the participation of muscles other than the deltoid medium in the overall movement (as for example, the anterior and posterior deltoids), that is, they are collaborating muscles that are different in the opening or lifting of the arm from those involved in its closure or lowering. Thus, it is concluded that a quantifiable and significant deltoid medium EMG difference has been demonstrated when the muscle is either ready to abduction or ready to adduction. The effect is fully reproducible between 0 and 90 degrees of an arm in static position within the scapular plane.