Secondary motions of the shoulder during arm elevation in patients with shoulder tightness

An analysis of secondary shoulder motions (humeral rotation, humeral head anterior/posterior translation, scapular tipping, and scapular upward/downward rotation) in subjects with anterior/posterior shoulder tightness provides the opportunity to examine the role of tightness as a means of affecting shoulder motions. Subjects with shoulder tightness (anterior, n = 12; posterior, n = 12) elevated their arms in the scapular plane. Three replicated movements were performed to the maximum motions. Kinematics data were collected by FASTRAK 3D electromagnetic system. To determine if a significant difference of the secondary motions existed between anterior/posterior shoulder tightness, two-factor mixed ANOVA models with the repeated factor of elevation angle (five elevation angles) and the independent factor of group were calculated. The relationships between the self-reported functional scores (Flexilevel Scale of Shoulder Function, FLEX-SF) and abnormal shoulder kinematics were assessed. For humeral head anterior/posterior translation, the subjects with posterior tightness demonstrated anterior humeral head translation (10 mm, p = 0.019) compared to subjects with anterior tightness. The subjects with anterior tightness demonstrated less posterior tipping (2.2°, p = 0.045) compared to subjects with posterior tightness. The humeral anterior translation had moderate relationships with FLEX-SF scores (r = ?0.535) in subjects with posterior tightness. The scapular tipping had moderate relationships with FLEX-SF scores (r = 0.432) in subjects with anterior tightness. In conclusion, the secondary motions were different between subjects with anterior and posterior shoulder tightness. During arm elevation, less scapular posterior tipping and less posterior humeral head translation in subjects with anterior and posterior shoulder tightness, respectively, are significantly related to self-reported functional disability in these subjects.     

Scapular taping alters kinematics in asymptomatic subjects

BackgroundScapular taping is frequently used in the management of shoulder pain and as a part of injury prevention strategies in sports. It is believed to alter scapular kinematics and restore normal motion. However, there is little evidence to support its use. The aim of the study was to investigate the effect of shoulder taping on the scapular kinematics of asymptomatic subjects.MethodThirteen asymptomatic subjects performed elevations in the sagittal and scapular planes with no tape and after the application of tape. A motion tracking system and a scapula locator method were used to measure the shoulder movement. Co-ordinate frames were defined for the thorax, humerus and scapula and Euler angles were used to calculate joints rotations.ResultsScapular taping increased the scapular external and upward rotations and posterior tilt in elevations in the sagittal plane (p < 0.001). In the scapular plane, taping increased scapular external rotation (p < 0.05).ConclusionsTaping affects scapulothoracic kinematics in asymptomatic subjects. The effect may be different for different planes of movement. The findings have implications on the use of taping as a preventive measure in high-risk groups. Further work is needed to assess the effect of taping on symptomatic populations.     

Differences in scapular kinematics and scapulohumeral rhythm during elevation and lowering of the arm between typical children and healthy adults

Scapular kinematics in healthy adults is well described in the literature but little is known on typical children. This study aimed to compare the three-dimensional (3-D) scapular kinematics and scapulohumeral rhythm during the elevation and lowering of the arm in the scapular plane in typical children and healthy adults. Twenty-six healthy adults (35.34 ± 11.65 years, 1.70 ± 0.10 m, 70.00 ± 12.30 kg) and 33 typical children (9.12 ± 1.51 years, 1.40 ± 0.10 m, 35.40 ± 10.45 kg) participated in this study. 3-D scapular kinematics were obtained using an electromagnetic tracking device. The subjects were asked to elevate and lower their arm in the scapular plane. Children showed less scapular protraction compared to adults at 120° during arm elevation, more anterior tilt than adults in the elevation and also at 60°, 90° and 120° during lowering of the arm. Children also showed higher scapulohumeral rhythm during lowering of the arm compared to adults from 90° to 60°. It was also found a low to little correlation between scapular position and age. The study showed small but significant differences in scapular kinematics and scapulohumeral rhythm between children and adults. These results can help clinicians to improve diagnosis and treatment protocols directed to children with dysfunction, as reference values on scapular kinematics in healthy children are also provided in this study.     

Associations between cervical and scapular posture and the spatial distribution of trapezius muscle activity

The first aim of this investigation was to quantify the distribution of trapezius muscle activity with different scapular postures while seated. The second aim of this investigation was to examine the association between changes in cervical and scapular posture when attempting to recruit different subdivisions of the trapezius muscle. Cervical posture, scapular posture, and trapezius muscle activity were recorded from 20 healthy participants during three directed shoulder postures. Planar angles formed by reflective markers placed on the acromion process, C7, and tragus were used to quantify cervical and scapular posture. Distribution of trapezius muscle activity was recorded using two high-density surface electromyography (HDsEMG) electrodes positioned over the upper, middle, and lower trapezius. Results validated the assumption that directed scapular postures preferentially activate different subdivisions of the trapezius muscle. In particular, scapular depression was associated with a more inferior location of trapezius muscle activity (r = 0.53). Scapular elevation was coupled with scapular abduction (r = 0.52). Scapular adduction was coupled with cervical extension (r = 0.35); all other changes in cervical posture were independent of changes in scapular posture. This investigation provides empirical support for reductions in static loading of the upper trapezius and improvements in neck posture through verbal cueing of scapular posture.     

Scapular muscle activity in a variety of plyometric exercises

Background: Plyometric shoulder exercises are commonly used to progress from slow analytical strength training to more demanding high speed power training in the return to play phase after shoulder injury. The aim of this study was first, to investigate scapular muscle activity in plyometric exercises to support exercise selection in practice and second, to enhance understanding of how scapular muscles are recruited during the back and forth movement phase of these exercises. Methods: Thirty-two healthy subjects performed 10 plyometric exercises while surface EMG-activity of the scapular muscles (upper (UT), middle (MT) and lower trapezius (LT) and serratus anterior (SA)) was registered. A high speed camera tracked start and end of the back and forth movement. Results: Mean scapular EMG activity during the 10 exercises ranged from 14.50% to 76.26%MVC for UT, from 15.19% to 96.55%MVC for MT, from 13.18% to 94.35%MVC for LT and from 13.50% to 98.50%MVC for SA. Anova for repeated measures showed significant differences in scapular muscle activity between exercises (p < 0.001) and between the back and forth movement (p < 0.001) within exercises. Conclusion: Plyometric shoulder exercises require moderate (31–60%MVC) to high (>60%MVC) scapular muscle activity. Highest MT/LT activity was present in prone plyometric external rotation and flexion. Highest SA activity was found in plyometric external rotation and flexion with Xco and plyometric push up on Bosu. Specific exercises can be selected that recruit minimal levels of UT activity (<15%): side lying plyometric external rotation and horizontal abduction or plyometric push up on the Bosu. The results of this study support exercise selection for clinical practice.     

The effects of taping on scapular kinematics and muscle performance in baseball players with shoulder impingement syndrome

PurposeThis study aimed to investigate the effect of elastic taping on kinematics, muscle activity and strength of the scapular region in baseball players with shoulder impingement.ScopeSeventeen baseball players with shoulder impingement were recruited from three amateur baseball teams. All subjects received both the elastic taping (Kinesio Tex^TM) and the placebo taping (3 M Micropore tape) over the lower trapezius muscle. We measured the 3-dimensional scapular motion, electromyographic (EMG) activities of the upper and lower trapezius, and the serratus anterior muscles during arm elevation. Strength of the lower trapezius was tested prior to and after each taping application. The results of the analyses of variance (ANOVA) with repeated measures showed that the elastic taping significantly increased the scapular posterior tilt at 30° and 60° during arm raising and increased the lower trapezius muscle activity in the 60–30° arm lowering phase (p < 0.05) in comparison to the placebo taping.ConclusionsThe elastic taping resulted in positive changes in scapular motion and muscle performance. The results supported its use as a treatment aid in managing shoulder impingement problems.     

Rotator cuff tendinopathy alters the muscle activity onset and kinematics of scapula

Athletes with rotator cuff (RC) tendinopathy demonstrate an aberrant pattern of scapular motion which might relate to deficits in the scapular muscles. This study aimed to determine whether alteration in scapular kinematics is associated with deficits in the activity onset of scapular muscles. Forty-three male volleyball players (17 asymptomatic and 26 with RC tendinopathy) joined the study. Three-dimensional scapular kinematics was quantified using an acromial marker cluster method. The activity onset of the upper (UT), middle (MT), and lower trapezius (LT), and serratus anterior (SA) during arm abduction was assessed with electromyography. Athletes with RC tendinopathy demonstrated less scapular upward rotation (6.6 ± 2.3 vs. 8.2 ± 1.1°, p = 0.021) in the early phase of shoulder abduction from 0° to 30° when compared to asymptomatic athletes. The tendinopathy group had delayed activity onset of LT (14.1 ± 31.4 ms vs. 74.4 ± 45.1 ms, p < 0.001) and SA (−44.9 ± 26.0 ms vs. 23.0 ± 25.2 ms, p < 0.001) relative to UT when compared to the asymptomatic group. In asymptomatic athletes, earlier activity onset of MT and LT relative to UT was associated with more scapular upward rotation during 0–30° of abduction (r = 0.665, p = 0.021) and 30–60° of abduction (r = 0.680, p = 0.015), respectively. Our findings showed the control of the scapular upward rotation is related to the activity onset of the scapular muscles in athletes.     

Effects of scapular upward rotation exercises on alignment of scapula and clavicle and strength of scapular upward rotators in subjects with scapular downward rotation syndrome

The purpose of this study was to investigate the effects of a 6-week scapular upward rotation exercise (SURE) on scapular and clavicular alignment and scapular upward rotators strength in subjects with scapular downward rotation syndrome (SDRS). Seventeen volunteer subjects with SDRS were recruited from university populations. The alignment of the scapula and clavicle was measured using radiographic analysis and compared in subjects before and after a 6-week self-SURE program. A hand-held dynamometer was used to measure the strength of the scapular upward rotators. The subjects were instructed how to perform the self-SURE program at home. The 6-week self-SURE program was divided into two sections (the first section with non-resistive SURE during weeks 1–3, and the second section with resistive SURE using thera-band during weeks 4–6). The significance of the difference between pre- and post-program was assessed using a paired t-test, with the level of statistical significance set at p < 0.05. Significant differences between pre- and post-program were found for scapular and clavicular alignment (p < 0.05). Additionally, the comparison between pre- and post-program measurements of the strength of the scapular upward rotators showed significant differences (p < 0.05). The results of this study showed that a 6-week self-SURE program is effective for improving scapular and clavicular alignment and increasing the strength of scapular upward rotator muscles in subjects with SDRS.     

Digital fluoroscopic video assessment of glenohumeral migration: Static vs. Dynamic conditions

The purpose was to compare glenohumeral (GH) migration, during dynamic shoulder elevation and statically held positions using digital fluoroscopic videos (DFV). Thirty male volunteers (25±4 years) without right shoulder pathology were analyzed using DFV (30 Hz) during arm elevation in the scapular plane. DFV were obtained at the arm at side position, 45°, 90°, and 135° for static and dynamic conditions. GH migration was measured as the distance from the center of the humeral head migrated superiorly or inferiorly relative to the center of the glenoid fossa. Inter-rater reliability was considered good; ICC (2,3) ranged from 0.83 to 0.92. A main effect was revealed for contraction type (p=0.031), in which post-hoc t-tests revealed that humeral head was significantly more superior on the glenoid fossa during dynamic contraction. A main effect was also revealed for arm angle (p<0.001), in which post-hoc t-tests revealed significantly more superior humeral head positioning at 45°, 90°, and 135° when compared to arm at side (p<0.001), as well as at 90° compared to 45° (p=0.024). There was no interaction effect between angle and contraction type (p=0.400). Research utilizing static imaging may underestimate the amount of superior GH migration that occurs dynamically.     

Fatigue-induced glenohumeral and scapulothoracic kinematic variability: Implications for subacromial space reduction

Superior humeral head translation and scapula reorientation can reduce the subacromial space. While these kinematic abnormalities exist in injured populations, the effect of muscle fatigue is unclear. Additionally, these mechanisms were typically studied independently, thereby neglecting potential covariance. This research evaluated the influence of upper extremity muscle fatigue on glenohumeral and scapulothoracic kinematics and defined their relationship. Radiography and motion tracking systems captured these kinematic relationships, during scapula plane elevation, both before and after fatigue. Fatigue-induced changes in humeral head position, scapular orientation and the minimum subacromial space width were measured. High inter-subject variability existed for each measure which precluded identification of mean differences at the population level. However, significant scapular upward rotation occurred following fatigue (p = 0.0002). Despite similar population mean results, between 39% and 57% of participants exhibited fatigue-related changes in disadvantageous orientations. Additionally, correlations between measures were generally fair (0.21–0.40) and highly dependent on elevation, likely attributed to the variable fatigue responses. Overall, the data confirms that fatigue-induced changes in kinematics poses highly variable risk of subacromial impingement syndrome across individuals. Thus, solely considering the “average” or mean population response likely underestimates potentially injurious fatigue consequences.     

Measuring humeral head translation using fluoroscopy: A validation study

Numerous techniques have been employed to monitor humeral head translation due to its involvement with several shoulder pathologies. However, most of the techniques were not validated. The objective of this study is to compare the accuracy of manual digitization and contour registration in measuring superior translation of the humeral head. Eight pairs of cadaver scapulae and humerii bones were harvested for this study. Each scapula and humerus was secured in a customized jig that allowed for control of humeral head translations and a vise that permitted rotations of the scapula about three axes. Fluoroscopy was used to take images of the shoulder bones. Scapular orientation was manipulated in different positions while the humerus was at 90° of humeral elevation in the scapular plane. Humeral head translation was measured using the two methods and was compared to the known translation. Additionally, accuracy of the contour registration method to measure 2-D scapular rotations was assessed. The range for the root mean square (RMS) error for manual digitization method was 0.27 mm - 0.43 mm and the contour registration method had a RMS error ranging from 0.18 mm - 0.40 mm. In addition, the RMS error for the scapular angle rotation using the contour registration method was 2.4°. Both methods showed acceptable errors. However, on average, the contour registration method showed lesser measurement error compared to the manual digitization method. In addition, the contour registration method was able to show good accuracy in measuring rotation that is useful in 2-D image analysis.     

Selective activation of the latissimus dorsi and the inferior fibers of trapezius at various shoulder angles during isometric pull-down exertion

The aim of this study was to determine the effect of isometric pull down exercise on muscle activity with shoulder elevation angles of 60°, 90°, and 120° and sagittal, scapular, and frontal movement planes, by electromyography (EMG) of the latissimus dorsi, inferior fibers of trapezius, and latissimus dorsi/inferior fibers of trapezius activity ratio. Fourteen men performed nine conditions of isometric pull down exercise (three conditions of shoulder elevation × three conditions of movement planes). Surface EMG was used to collect data from the latissimus dorsi and inferior fibers of trapezius during exercise. Two-way repeated analysis of variance with two within-subject factors (shoulder elevation angles and planes of movement) was used to determine the significance of the latissimus dorsi and inferior fibers of trapezius activity and latissimus dorsi/inferior fibers of trapezius activity ratio. The latissimus dorsi activity and ratio between the latissimus dorsi and the inferior fibers of trapezius were significantly decreased as shoulder elevation angle increased from 60° to 120°. The inferior fibers of trapezius activity was significantly increased with shoulder elevation angle. The EMG activity and the ratios were not affected by changes in movement planes. This study suggests that selective activation of the latissimus dorsi is accomplished with a low shoulder elevation angle, while the inferior fibers of the trapezius are activated with high shoulder elevation angles.     

Can self-report instruments of shoulder function capture functional differences in older adults with and without a rotator cuff tear?

Rotator cuff tears (RCT) are prevalent in older individuals and may compound age-associated functional declines. Our purpose was to determine whether self-report measures of perceived functional ability are valid for older patients with RCT. Twenty five subjects participated (12M/13F; age = 63.9 ± 3.0 years); 13 with RCT and 12 controls (CON). Participants completed self-report measures of shoulder function (SST, ASES, WORC) and health-related quality of life (SF-36). Isometric joint moment and range of motion (ROM) were measured at the shoulder. Relationships among functional self-reports, and between these measures and joint moment and ROM were assessed; group differences for total and subcategory scores were evaluated. There were significant correlations among self-reports (r_s = 0.62–0.71, p ⩽ 0.02). For RCT subjects, ASES was associated with all joint moments except adduction (p ⩽ 0.02); SST, ASES, and WORC were associated with abduction and external rotation ROM (p ⩽ 0.04). For RCT subjects, SST and WORC were associated with SF-36 physical function subcategory scores (p ⩽ 0.05). The RCT group scored worse than CON on all functional self-reports (p < 0.01) and WORC and ASES subcategories (p < 0.01). In conclusion, SST, ASES, and WORC demonstrate utility and discriminant validity for older individuals by distinguishing those with RCT, but this work suggests prioritizing ASES given its stronger association with functional group strength.     

Motion of the shoulder complex in individuals with isolated acromioclavicular osteoarthritis and associated with rotator cuff dysfunction: Part 1 – Three-dimensional shoulder kinematics

This study described the three-dimensional shoulder motion during the arm elevation in individuals with isolated acromioclavicular osteoarthritis (ACO) and ACO associated with rotator cuff disease (RCD), as compared to controls. Seventy-four participants (ACO = 23, ACO + RCD = 25, Controls = 26) took part of this study. Disability was assessed with the DASH, three-dimensional kinematics were collected during arm elevation in the sagittal and scapular planes, and pain was assessed with the 11-point numeric pain rating scale. For each kinematic variable and demographic variables, separate linear mixed-model 2-way ANOVAs were performed to compare groups. Both ACO groups had higher DASH and pain scores. At the scapulothoracic joint, the isolated ACO group had greater internal rotation than control, and the ACO + RCD group had greater upward rotation than both other groups. At the sternoclavicular joint, both groups with ACO had less retraction, and the isolated ACO group had less elevation and posterior rotation. At the acromioclavicular joint, the isolated ACO group had greater upward rotation, and both ACO groups had greater posterior tilting. Patients with ACO had altered shoulder kinematics, which may represent compensatory responses to reduce pain and facilitate arm motion during arm elevation and lowering.     

Are chronic neck pain,scapular dyskinesis and altered scapulothoracic muscle activity interrelated?: A case-control study with surface and fine-wire EMG

ObjectivesThe function of the scapula is important in normal neck function and might be disturbed in patients with neck pain. The surrounding muscular system is important for the function of the scapula. To date, it is not clear if patients with idiopathic neck pain show altered activity of these scapulothoracic muscles. Therefore, the objective of this study was to investigate differences in deeper and superficial lying scapulothoracic muscle activity between patients with idiopathic neck pain and healthy controls during arm elevation, and to identify the influence of scapular dyskinesis on muscle activity.MethodsScapular dyskinesis was rated with the yes/no method. The deeper lying (Levator Scapulae, Pectoralis Minor (Pm) and Rhomboid major) and superficial lying (Trapezius and Serratus Anterior) scapulothoracic muscles’ activity was investigated with fine-wire and surface EMG, respectively, in 19 female subjects with idiopathic neck pain (age 28.3 ± 10.1 years, average duration of neck pain 45.6 ± 36.3 months) and 19 female healthy control subjects (age 29.3 ± 11.7 years) while performing scaption and towel wall slide. Possible interactions or differences between subject groups, scapular dyskinesis groups or phases of the task were studied with a linear mixed model.ResultsHigher Pm activity during the towel wallslide (p = 0.024, mean difference 8.8 ± 3.3% MVIC) was shown in patients with idiopathic neck pain in comparison with healthy controls. For the MT, a significant group 1 dyskinesis interaction effect was found during scaption which revealed that patients with neck pain and scapular dyskinesis showed lower Middle Trapezius (MT) activity in comparison with healthy controls with scapular dyskinesis (p = 0.029, mean difference 5.1 ± 2.2% MVIC).ConclusionsIn the presence of idiopathic neck pain, higher Pm activity during the towel wallslide was found. Patients with neck pain and scapular dyskinesis showed lower MT activity in comparison with healthy controls with scapular dyskinesis during scaption. Scapular dyskinesis did not have a significant influence on scapulothoracic muscle activity.     

Electromyographic analysis during pull,forward punch,elevation and overhead throw after conservative treatment or capsular shift at patient with multidirectional shoulder joint instability

PurposeThe aim of this study was to compare the muscle activity of patients with multidirectional instability treated in a conservative or complex manner (capsular shift with postoperative rehabilitation) and the muscle activity of stable shoulder joints before and after treatment during pull, push, and elevation of upper extremities and during overhead throw.ScopeThe study was carried out on 34 patients with multidirectional shoulder instability treated non-operatively, on 31 patients with multidirectional shoulder instability treated operatively, and on 50 healthy subjects. Signals were recorded by surface EMG from eight different muscles. The mean and standard deviation of the maximum amplitude of normalized voluntary electrical activity for the different movement types and time broadness values during overhead throw were determined for each muscle in all groups and compared with each other.ConclusionThe centralization of the glenohumeral joint and the reduction of instability is attempted to be ensured by the organism through increasing the role of rotator cuff muscles (p = 0.009) and decreasing the role of the deltoid, biceps brachii, and pectoralis maior muscles (p = 0.007). At patients after short-term and long-term conservative treatment, the maximum amplitude of normalized voluntary electrical activity of stabilizer muscles is significantly higher (p = 0.006), and that of accelerator muscles is significantly lower (p = 0.005) and the time broadness is significantly longer (p = 0.01) than that of the control group. At patients after complex treatment (open capsular shift with postoperative conservative rehabilitation) the characteristic of the muscle pattern is similar (p = 0.19) to the control group.The complex treatment resolves the labral ligamentous abnormalities by operative treatment and restores the impaired muscular control by postoperative rehabilitation, whereas the conservative treatment restores only the muscular control.     

Neuromuscular control of scapula muscles during a voluntary task in subjects with Subacromial Impingement Syndrome. A case-control study

Imbalance of neuromuscular activity in the scapula stabilizers in subjects with Subacromial Impingement Syndrome (SIS) is described in restricted tasks and specific populations. Our aim was to compare the scapular muscle activity during a voluntary movement task in a general population with and without SIS (n = 16, No-SIS = 15).Surface electromyography was measured from Serratus anterior (SA) and Trapezius during bilateral arm elevation (no-load, 1 kg, 3 kg). Mean relative muscle activity was calculated for SA and the upper (UT) and lower part of trapezius (LWT), in addition to activation ratio and time to activity onset. In spite of a tendency to higher activity among SIS 0.10–0.30 between-group differences were not significant neither in ratio of muscle activation 0.80–0.98 nor time to activity onset 0.53–0.98.The hypothesized between-group differences in neuromuscular activity of Trapezius and Serratus was not confirmed. The tendency to a higher relative muscle activity in SIS could be due to a pain-related increase in co-activation or a decrease in maximal activation. The negative findings may display the variation in the specific muscle activation patterns depending on the criteria used to define the population of impingement patients, as well as the methodological procedure being used, and the shoulder movement investigated.     

Interlimb symmetry of dynamic knee joint stiffness and co-contraction is maintained in early stage knee osteoarthritis

Individuals with knee OA often exhibit greater co-contraction of antagonistic muscle groups surrounding the affected joint which may lead to increases in dynamic joint stiffness. These detrimental changes in the symptomatic limb may also exist in the contralateral limb, thus contributing to its risk of developing knee osteoarthritis. The purpose of this study is to investigate the interlimb symmetry of dynamic knee joint stiffness and muscular co-contraction in knee osteoarthritis.Muscular co-contraction and dynamic knee joint stiffness were assessed in 17 subjects with mild to moderate unilateral medial compartment knee osteoarthritis and 17 healthy control subjects while walking at a controlled speed (1.0 m/s). Paired and independent t-tests determined whether significant differences exist between groups (p < 0.05).There were no significant differences in dynamic joint stiffness or co-contraction between the OA symptomatic and OA contralateral group (p = 0.247, p = 0.874, respectively) or between the OA contralateral and healthy group (p = 0.635, p = 0.078, respectively). There was no significant difference in stiffness between the OA symptomatic and healthy group (p = 0.600); however, there was a slight trend toward enhanced co-contraction in the symptomatic knees compared to the healthy group (p = 0.051).Subjects with mild to moderate knee osteoarthritis maintain symmetric control strategies during gait.     

Spontaneous whole body rotations and classical dance expertise: How shoulder–hip coordination influences supporting leg displacements

The link between supporting leg stability and individual trunk strategies used during spontaneous whole-body rotations was studied according to visual and kinesthetic imagery styles for classical dancers and untrained female participants. Shoulders–hip angles in the horizontal plane and supporting leg (SL) displacements were analyzed with three-dimensional kinematic at the beginning and end of the four turns, identified according to their SL (left vs. right) and turn direction (clockwise, CW vs. counterclockwise, CCW). To begin a turn in CCW on left SL, all the participants turned shoulders before hips (?25° angle), p < 0.01. Untrained participants yielded the reverse (+30° angle) in CW – their non-preferred turn - whereas dancers maintained their trunk en bloc. In the turn slow down, in their preferred direction all the participants adopted en bloc behavior to avoid imbalance. Dancers kept this pattern in CCW but untrained participants separated shoulders and hips, p < 0.01; on left SL (+20° angle) hips finished before shoulders and on right SL (?25° angle), shoulders finished before hips. Both mental imagery styles and spatial context link reduction of shoulder–hip angle and stability of SL. Daily expertise, not only dance training, facilitates the en bloc shoulder–hip coordination to maintain equilibrium.     

Multi-channel electromyography during maximal isometric and dynamic contractions

Motor unit behavior differs between contraction types at submaximal contraction levels, however is challenging to study during maximal voluntary contractions (MVCs). With multi-channel surface electromyography (sEMG), mean physiological characteristics of the active motor units can be extracted. Two 8-electrode sEMG arrays were attached on biceps brachii muscle (one on each head) to examine behavior of sEMG variables during isometric, eccentric and concentric MVCs of elbow flexors in 36 volunteers.On average, isometric (364 ± 88 N) and eccentric (353 ± 74 N) MVCs were higher than concentric (290 ± 73 N) MVC (p < 0.001). Mean muscle fiber conduction velocity (CV) was highest during eccentric MVC (4.42 ± 0.49 m/s) than concentric (4.25 ± 0.49 m/s, p < 0.01) and isometric (4.14 ± 0.45 m/s, p < 0.001) MVCs. Furthermore, eccentric MVC showed lower sEMG amplitude at the largest elbow joint angles (120–170°) and higher CV at the smallest (70–150°) elbow joint angles (p < 0.05–0.001) than concentric MVC.The differences in CV and sEMG amplitude between the MVCs suggest that the control strategy of motor units differs between the contraction types during MVCs, and is dependent on the muscle length between the dynamic MVCs.