Scapular muscle activation and co-activation following a fatigue task

Favorable Clinical Outcomes After Humeral Head Depressor Muscle Coactivation Training With EMG for Patients With Arthroscopic Rotator Cuff Repair: A Randomized Controlled Trial

BACKGROUND

The coactivation (Co-A) of shoulder muscles that contribute to humeral head depression can lead to mechanical unloading of the subacromial structures during abduction and thus can be beneficial for patients with arthroscopic rotator cuff repair (ARCR). The present study aims to examine the effectiveness of humeral head depressor muscle Co-A (DM-Co-A) training on clinical outcomes in a sample of patients with ARCR.

HYPOTHESIS

We hypothesized that DM-Co-A training with medioinferior vector during glenohumeral exercises can improve clinical results in the rehabilitation of ARCR.

STUDY DESIGN

Randomized controlled single-blind study.

LEVEL OF EVIDENCE

Level 1B.

METHODS

A total of 27 patients who underwent ARCR after a medium-sized rotator cuff tear and completed their Phase 1 training with ≥80% compliance were included. Together with 14 weeks of conservative treatment (6 weeks of Phase 2 training and 8 weeks of Phase 3 training), synchronized "DM-Co-A Training" was applied to the treatment group with an electromyography (EMG) biofeedback (EMG-BF) device. Patients in the treatment group were asked to voluntarily activate the humeral head depressor muscles guided by visual and auditory feedback of the EMG-BF device during the Phase 2 and Phase 3 conservative treatment exercises performed by the control group. Demographic characteristics of the participants were recorded. Visual analog scale and universal goniometer were used to assess pre- and posttreatment pain severity and joint range of motion, respectively. The Disabilities of Arm, Shoulder and Hand Questionnaire, Revised Oxford Shoulder Score, Modified Constant-Murley Shoulder Score, and the Western-Ontario Rotator Cuff Index were used to assess functionality.

RESULTS

There was a clinically meaningful improvement in pain severity, active ROM excluding internal rotation, and functionality in the treatment group compared with the control group (P < 0.05).

CONCLUSION

A 14-week duration DM-Co-A with EMG may be beneficial in the postoperative rehabilitation of patients after ARCR.

Scapular muscle endurance, shoulder pain, and functionality in patients with rotator-cuff-related shoulder pain: a matched, case-control study

BACKGROUND

Deficiency in scapular muscle endurance (SME) is a risk factor for rotator-cuff-related shoulder pain (RCRSP). However, the exact relationship among SME, pain, and functionality remains unclear. This study aims to compare SME, pain, and functionality in RCRSP patients to those in age-sex-matched healthy controls.

METHODS

Twenty-three patients with RCRSP and 23 age-sex matched healthy controls were included in the study. SME was measured using a 1-kg dynamometer. Self-reported pain level was assessed using a visual analog scale. The Functional Impairment Test-Hand, Neck, Shoulder, and Arm (FIT-HaNSA) was also used to assess functional impairment.

RESULTS

The control group had higher SME and total FIT-HaNSA scores than the patient group (P<0.05). There was a statistically significant and positive correlation between SME and FIT-HaNSA scores in both groups (P<0.05).

CONCLUSIONS

SME was affected by RCRSP. Pain and functional impairment were correlated with low SME. Level of evidence: IV.

Variability of Time- and Frequency-Domain Surface Electromyographic Measures in Non-Fatigued Shoulder Muscles

OCCUPATIONAL APPLICATIONSLocalized Muscle Fatigue (LMF) can be monitored or predicted based on the relative change in the values of surface electromyography (sEMG) measures with respect to the "fresh" or no-fatigue condition. Quantification of LMF based on relative change, though, relies on the assumption that the sEMG measures recorded in a no-fatigue condition can serve as an appropriate reference. Results of this study indicate that sEMG measures in a no-fatigue condition are affected by various work-related factors and provide further guidance on the variability of commonly used time- and frequency-domain sEMG measures to assist the ergonomist in improving the accuracy of LMF assessment.

A physiological and biomechanical investigation of three passive upper-extremity exoskeletons during simulated overhead work

The objective of this study was to evaluate three passive upper-extremity exoskeletons relative to a control condition. Twelve subjects performed an hour-long, simulated occupational task in a laboratory setting. Independent measures of exoskeleton, exertion height (overhead, head height), time, and their interactions were assessed. Dependent measures included changes in tissue oxygenation (ΔTSI) in the anterior deltoid and middle trapezius, peak resultant lumbar spine loading, and subjective discomfort in various body regions. A statistically significant reduction in ΔTSI between exoskeleton and control was only observed in one instance. Additionally, neither increases in spinal loading nor increases in subjective discomfort ratings were observed for any of the exoskeletons. Ultimately, the exoskeletons offered little to no physiological benefit for the conditions tested. However, the experimental task was not highly fatiguing to the subjects, denoted by low ΔTSI values across conditions. Results may vary for tasks requiring constant arm elevation or higher force demands. Practitioner summary This study quantified the benefits of upper-extremity exoskeletons using NIRS, complementary to prior studies using EMG. The exoskeletons offered little to no physiological benefit for the conditions tested. However, the experimental task was not highly fatiguing, and results may vary for an experimental task with greater demand on the shoulders. Abbreviations: WMSD: work-related musculoskeletal disorder; EMG: electromyography; NIRS: near-infrared spectroscopy; NIR: near-infrared; Hb: haemoglobin; Mb: myoglobin; TSI: tissue saturation index; ATT: adipose tissue thickness.

The effects of muscle fatigue on scapulothoracic joint position sense and neuromuscular performance

BACKGROUND

Fatigue of the scapular musculature might affect the sensorimotor system and neuromuscular control and therefore impair the dynamic alignment of the scapula. The purpose of this study was to investigate the effects of scapular muscle fatigue on joint position sense and the neuromuscular performance of the scapulothoracic joint.

METHODS

Thirty healthy subjects were recruited. The joint position sense (measured as reposition errors during the scapular elevation and protraction task) of the scapulothoracic joint and scapular kinematics and muscle activation (% of maximum voluntary isometric contraction) during scaption, shoulder elevation in the scapular plane, before and after the fatigue task (modified push-up) were measured. The repeated measures analysis of variance (ANOVA) was used to assess the effects of fatigue on scapular reposition error and neuromuscular control.

RESULTS

Although joint reposition sense did not change after fatigue, muscle activation of the serratus anterior increased significantly (18.5%-26.3%, p = 0.006, elevation task; 15.2%-27.4%, p = 0.037, protraction task). At 120° scaption, the scapula showed a decreased posterior tilt (13.8°-10.3°, p < 0.001), increased protraction (10.2°-13.3°, p = 0.004) and lateral rotation (52.8°-54.4°, p = 0.005). Muscle activation of the serratus anterior increased (65.9%-84.6%, p = 0.002) during 90°-120°scaption.

CONCLUSION

Scapular muscle fatigue did not alter the scapulothoracic joint position sense, but increased serratus anterior activation and resulted in a more anteriorly tilted, protracted, and laterally rotated scapula during scaption. Whether or not these fatigue-related changes are linked to the development of shoulder problems needs to be investigated further.

Analysis of Muscle Load-Sharing in Patients With Lateral Epicondylitis During Endurance Isokinetic Contractions Using Non-linear Prediction

The aim of this paper is to analyze muscle load-sharing in patients with Lateral Epicondylitis during dynamic endurance contractions by means of non-linear prediction of surface EMG signals. The proposed non-linear cross-prediction scheme was used to predict the envelope of an EMG signal and is based on locally linear models built in a lag-embedded Euclidean space. The results were compared with a co-activation index, a common measure based on the activation of a muscle pair. Non-linear prediction revealed changes in muscle coupling, that is load-sharing, over time both in a control group and Lateral Epicondylitis (p < 0.05), even when subjects did not report pain at the end of the exercise. These changes were more pronounced in patients, especially in the first part of the exercise and up to 50% of the total endurance time (p < 0.05). By contrast, the co-activation index showed no differences between groups. Results reflect the changing nature of muscular activation strategy, presumably because of the mechanisms triggered by fatigue. Strategies differ between controls and patients, pointing to an altered coordination in Lateral Epicondylitis.

A Survey of Assistive Technologies for Assessment and Rehabilitation of Motor Impairments in Multiple Sclerosis

Multiple sclerosis (MS) is a disease that affects the central nervous system, which consists of the brain and spinal cord. Although this condition cannot be cured, proper treatment of persons with MS (PwMS) can help control and manage the relapses of several symptoms. In this survey article, we focus on the different technologies used for the assessment and rehabilitation of motor impairments for PwMS. We discuss sensor-based and robot-based solutions for monitoring, assessment and rehabilitation. Among MS symptoms, fatigue is one of the most disabling features, since PwMS may need to put significantly more intense effort toward achieving simple everyday tasks. While fatigue is a common symptom across several neurological chronic diseases, it remains poorly understood for various reasons, including subjectivity and variability among individuals. To this end, we also investigate recent methods for fatigue detection and monitoring. The result of this survey will provide both clinicians and researchers with valuable information on assessment and rehabilitation technologies for PwMS, as well as providing insights regarding fatigue and its effect on performance in daily activities for PwMS.

Muscle fatigue assessment during robot-mediated movements

BACKGROUND

Several neuromuscular disorders present muscle fatigue as a typical symptom. Therefore, a reliable method of fatigue assessment may be crucial for understanding how specific disease features evolve over time and for developing effective rehabilitation strategies. Unfortunately, despite its importance, a standardized, reliable and objective method for fatigue measurement is lacking in clinical practice and this work investigates a practical solution.

METHODS

40 healthy young adults performed a haptic reaching task, while holding a robotic manipulandum. Subjects were required to perform wrist flexion and extension movements in a resistive visco-elastic force field, as many times as possible, until the measured muscles (mainly flexor and extensor carpi radialis) exhibited signs of fatigue. In order to analyze the behavior and the characteristics of the two muscles, subjects were divided into two groups: in the first group, the resistive force was applied by the robot only during flexion movements, whereas, in the second group, the force was applied only during extension movements. Surface electromyographic signals (sEMG) of both flexor and extensor carpi radialis were acquired. A novel indicator to define the Onset of Fatigue (OF) was proposed and evaluated from the Mean Frequency of the sEMG signal. Furthermore, as measure of the subjects' effort throughout the task, the energy consumption was estimated.

RESULTS

From the beginning to the end of the task, as expected, all the subjects showed a decrement in Mean Frequency of the muscle involved in movements resisting the force. For the OF indicator, subjects were consistent in terms of timing of fatigue; moreover, extensor and flexor muscles presented similar OF times. The metabolic analysis showed a very low level of energy consumption and, from the behavioral point of view, the test was well tolerated by the subjects.

CONCLUSION

The robot-aided assessment test proposed in this study, proved to be an easy to administer, fast and reliable method for objectively measuring muscular fatigue in a healthy population. This work developed a framework for an evaluation that can be deployed in a clinical practice with patients presenting neuromuscular disorders. Considering the low metabolic demand, the requested effort would likely be well tolerated by clinical populations.

An electromyography study of muscular endurance during the posterior shoulder endurance test

The primary purpose was to determine if there is a difference between the median frequency slopes of 5 posterior shoulder muscles during the initial portion of the Posterior Shoulder Endurance Test (PSET) at the 90° and 135° shoulder abduction positions. Fifty-five healthy volunteers (31 females) participated. The median frequency of the posterior deltoid (PD), upper trapezius (UT), middle trapezius (MT), lower trapezius (LT), and infraspinatus (INF) was measured during the PSET at 90° and 135° of shoulder abduction. External torque of 13 ± 1 Nm was used for females and 21 ± 1 Nm for males. A fixed effect multi-variable regression model was used to investigate the median frequency slopes. Males and females were analyzed separately. Median frequency slopes demonstrated fatigue in all 5 of the muscles. The PD fatigued greater than the UT in males (p = 0.0215) and greater than the LT in females (p = 0.008). The time to task failure (TTF) was greater at 90° than 135° for females and males (p = 0.016; p = 0.0193) respectively. The PSET causes fatigue in all of the muscles that were tested, with the PD fatiguing at a greater rate compared to one muscle for each sex. This investigation supports using TTF as a clinical measure of shoulder girdle endurance at 90° shoulder abduction.

Relationship between scapular muscle and core endurance in healthy subjects

BACKGROUND

Scapular muscle endurance and core endurance reportedly influence shoulder injury risk. The exact relationship between scapular muscle endurance and core endurance, and how they impact one another in the healthy subjects remain unclear.

OBJECTIVE

To investigate the relationship between scapular muscle endurance and core endurance in healthy subjects.

METHODS

Fifty healthy volunteers (23 males, 27 females; mean age 20.42 ± 1.04 years) were participated in this study. Endurance of the serratus anterior and trapezius muscles was assessed using the scapular muscle endurance test. Sorensen test (endurance of trunk extensor muscles), trunk flexor endurance test, and side bridge test (endurance of lateral core muscles) were conducted to assess the core endurance. Pearson's product moment correlations examined relationships between scapular muscle endurance and each of the core endurance tests scores.

RESULTS

Scapular muscle endurance test scores showed a positive correlation with the side bridge test scores (r = 0.414; p = 0.003). No significant correlation was found between scapular muscle endurance test scores and the other core endurance tests scores (p > 0.05).

CONCLUSIONS

There appears to be a link between the scapular muscle endurance and lateral core muscles in healthy subjects; however, more research is needed to provide a definitive answer on the nature of this relationship. Further studies involving patients with shoulder pathology are warranted.

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

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

Can clinical observation differentiate individuals with and without scapular dyskinesis?

Background:

Altered scapular rotation and position have been named scapular dyskinesis. Visual dynamic assessment could be applied to classify this alteration based on the clinical observation of the winging of the inferior medial scapular border (Type I) or of the prominence of the entire medial border (Type II), or by the excessive superior translation of the scapula (Type III).

Objective:

The aim of this study was to determine if there were differences in scapular rotations (Type I and II) and position (Type III) between a group of subjects with scapular dyskinesis, diagnosed by the clinical observation of an expert physical therapist, using a group of healthy individuals (Type IV).

Method:

Twenty-six asymptomatic subjects volunteered for this study. After a fatigue protocol for the periscapular muscles, the dynamic scapular dyskinesis tests were conducted to visually classify each scapula into one of the four categories (Type IV dyskinesis-free). The kinematic variables studied were the differences between the maximum rotational dysfunctions and the minimum value that represented both normal function and a small dysfunctional movement.

Results:

Only scapular anterior tilt was significantly greater in the type I dyskinesis group (clinical observation of the posterior projection of the inferior angle of the scapula) when compared to the scapular dyskinesis-free group (p=0.037 scapular and p=0.001 sagittal plane).

Conclusions:

Clinical observation was considered appropriate only in the diagnoses of dyskinesis type I. Considering the lower prevalence and sample sizes for types II and III, further studies are necessary to validate the clinical observation as a tool to diagnose scapular dyskinesis.

Shoulder kinetics and ultrasonography changes after performing a high-intensity task in spinal cord injury subjects and healthy controls

STUDY DESIGN

This is a prospective and comparative study between two groups.

OBJECTIVES

The objective of this study was to compare the changes in shoulder joint forces and their moments, as well as any possible ultrasound changes, when subjects with spinal cord injury (SCI) and healthy controls (CG) undertake a high-intensity manual wheelchair propulsion test.

SETTING

This study was conducted in an inpatient SCI rehabilitation center.

METHODS

A group of 22 subjects with SCI at level T2 or below who use a manual wheelchair (MWU), categorized as AIS grade A or B, were compared with a CG of 12 healthy subjects. Subjects in each group performed a high-intensity wheelchair propulsion test. The variables analyzed were shoulder joint forces and the moments at the beginning and at the end of the test. Ultrasound variables before and after the propulsion test were also analyzed. Correlations were also drawn between the ultrasonography and demographic variables.

RESULTS

In both groups, peak shoulder forces and moments increased after the test in almost all directions. No differences in the ultrasound parameters were found. A greater long-axis biceps tendon thickness (LBTT) was associated with more shoulder pain according to WUSPI or VAS (r=0.428, P<0.05 and r=0.452, P<0.05, respectively).

CONCLUSIONS

Shoulder joint forces and moments increase after an intense propulsion task. In subjects with SCI, these increases center on forces with less chance of producing subacromial damage. No changes are produced in ultrasonography variables, whereas a poorer clinical and functional evaluation of the shoulder of the MWUs appears to be related to a thicker long-axis biceps tendon.

Investigation of Fatigability during Repetitive Robot-Mediated Arm Training in People with Multiple Sclerosis

BACKGROUND

People with multiple sclerosis (MS) are encouraged to engage in exercise programs but an increased experience of fatigue may impede sustained participation in training sessions. A high number of movements is, however, needed for obtaining optimal improvements after rehabilitation.

METHODS

This cross-sectional study investigated whether people with MS show abnormal fatigability during a robot-mediated upper limb movement trial. Sixteen people with MS and sixteen healthy controls performed five times three minutes of repetitive shoulder anteflexion movements. Movement performance, maximal strength, subjective upper limb fatigue and surface electromyography (median frequency and root mean square of the amplitude of the electromyography (EMG) signal of the anterior deltoid) were recorded during or in-between these exercises. After fifteen minutes of rest, one extra movement bout was performed to investigate how rest influences performance.

RESULTS

A fifteen minutes upper limb movement protocol increased the perceived upper limb fatigue and induced muscle fatigue, given a decline in maximal anteflexion strength and changes of both the amplitude and the median frequency of EMG the anterior deltoid. In contrast, performance during the 3 minutes of anteflexion movements did not decline. There was no relation between changes in subjective fatigue and the changes in the amplitude and the median frequency of the anterior deltoid muscle, however, there was a correlation between the changes in subjective fatigue and changes in strength in people with MS. People with MS with upper limb weakness report more fatigue due to the repetitive movements, than people with MS with normal upper limb strength, who are comparable to healthy controls. The weak group could, however, keep up performance during the 15 minutes of repetitive movements.

DISCUSSION AND CONCLUSION

Albeit a protocol of repetitive shoulder anteflexion movements did not elicit a performance decline, fatigue feelings clearly increased in both healthy controls and people with MS, with the largest increase in people with MS with upper limb weakness. Objective fatigability was present in both groups with a decline in the muscle strength and increase of muscle fatigue, shown by changes in the EMG parameters. However, although weak people with multiple sclerosis experienced more fatigue, the objective signs of fatigability were less obvious in weak people with MS, perhaps because this subgroup has central limiting factors, which influence performance from the start of the movements.

Effects of repetitive shoulder activity on the subacromial space in manual wheelchair users

This study investigated (1) the effect of repetitive weight-relief raises (WR) and shoulder external rotation (ER) on the acromiohumeral distance (AHD) among manual wheelchair users (MWUs) and (2) the relationship between shoulder pain, subject characteristics, and AHD changes. Twenty-three MWUs underwent ultrasound imaging of the nondominant shoulder in an unloaded baseline position and while holding a WR position before and after the WR/ER tasks. Paired t-tests and Spearman correlational analysis were used to assess differences in the AHD before and after each task and the relationships between pain, subject characteristics, and the AHD measures. A significant reduction in the subacromial space (P<0.01) occurred when subjects performed a WR position compared to baseline. Individuals with increased years of disability had greater AHD percentage narrowing after WR (P=0.008). Increased shoulder pain was associated with AHD percentage narrowing after ER (P≤0.007). The results support clinical practice guidelines that recommend MWUs limit WR to preserve shoulder function. The isolated repetitive shoulder activity did not contribute to the changes of subacromial space in MWUs. The ultrasonographic measurement of the AHD may be a target for identifying future interventions that prevent pain.

Shoulder muscle fatigue during repetitive tasks as measured by electromyography and near-infrared spectroscopy

OBJECTIVE

The objective of this study was to quantify shoulder muscle fatigue during repetitive exertions similar to motions found in automobile assembly tasks.

BACKGROUND

Shoulder musculoskeletal disorders (MSDs) are a common and costly problem in automotive manufacturing.

METHOD

Ten subjects participated in the study. There were three independent variables: shoulder angle, frequency, and force. There were two types of dependent measures: percentage change in near-infrared spectroscopy (NIRS) measures and change in electromyography (EMG) median frequency. The anterior deltoid and trapezius muscles were measured for both NIRS and EMG. Also, EMG was collected on the middle deltoid and biceps muscles.

RESULTS

The results showed that oxygenated hemoglobin decreased significantly due to the main effects (shoulder angle, frequency, and force). The percentage change in oxygenated hemoglobin had a significant interaction attributable to force and repetition for the anterior deltoid muscle, indicating that as repetition increased, the magnitude of the differences between the forces increased. The interaction of repetition and shoulder angle was also significant for the percentage change in oxygenated hemoglobin. The median frequency decreased significantly for the main effects; however, no interactions were statistically significant.

CONCLUSIONS

There was significant shoulder muscle fatigue as a function of shoulder angle, task frequency, and force level. Furthermore, percentage change in oxygenated hemoglobin had two statistically significant interactions, enhancing our understanding of these risk factors.

APPLICATION

Ergonomists should examine interactions of force and repetition as well as shoulder angle and repetition when evaluating the risk of shoulder MSDs.

Scapular orientation following repetitive prone rowing: implications for potential subacromial impingement mechanisms

While fatigue of the rotator cuff demonstrably causes superior humeral head migration and concomitant risk of impingement, the relationship between specific muscular fatigue, scapular dyskinesis and impingement risk is less clear. The purpose of this study was to examine changes in scapular orientation following a simulated prone rowing fatiguing protocol that targeted the scapula stabilizing muscles while attempting to alleviate rotator cuff muscular demands. Scapular orientation and muscle activity were collected from participants before and immediately after the fatiguing task. This task fatigued both the stabilizing (upper and middle trapezius, and latissimus dorsi) and rotator cuff (supraspinatus, and infraspinatus) muscles. The upper extremity muscle fatigue pattern caused by the protocol did not elicit any significantly changes in three-dimensional scapular position with all post-fatigue changes being ≤ 1° (p = 0.17-0.58). These results indicated that scapular reorientation is likely not the dominant mechanism of fatigue-induced subacromial impingement development. However, the substantial variability present in the kinematics prevents complete exclusion of scapular dyskinesis as a secondary causal mechanism of impingement.

Scapular-muscle performance: two training programs in adolescent swimmers

CONTEXT

Swimming requires well-balanced scapular-muscle performance. An additional strength-training program for the shoulders is pursued by swimmers, but whether these muscle-training programs need to be generic or specific for endurance or strength is unknown.

OBJECTIVE

To evaluate isokinetic scapular-muscle performance in a population of adolescent swimmers and to compare the results of training programs designed for strength or muscle endurance.

DESIGN

Controlled laboratory study.

SETTING

University human research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Eighteen adolescent swimmers.

INTERVENTION(S)

Each participant pursued a 12-week scapular-training program designed to improve either muscle strength or muscle endurance.

MAIN OUTCOME MEASURE(S)

Bilateral peak force, fatigue index, and protraction/retraction strength ratios before and after the scapular-training program.

RESULTS

Scapular protraction/retraction ratios were slightly higher than 1 (dominant side  =  1.08, nondominant side  =  1.25, P  =  .006). Side-to-side differences in retraction strength were apparent both before and after the training program (P  =  .03 and P  = .05, respectively). After the training program, maximal protraction (P < .05) and retraction (P < .01) strength improved on the nondominant side. Peak force and fatigue index were not different between the training groups. The fatigue indexes for protraction on both sides (P < .05) and retraction on the nondominant side (P  =  .009) were higher after the training program.

CONCLUSIONS

We describe the scapular-muscle characteristics of a group of adolescent swimmers. Both muscle-strength and muscle-endurance programs improved absolute muscle strength. Neither of the strength programs had a positive effect on scapular-muscle endurance. Our results may be valuable for coaches and physiotherapists when they are designing exercise programs for swimmers.

Shoulder muscle endurance: the development of a standardized and reliable protocol

Background

Shoulder muscle fatigue has been proposed as a possible link to explain the association between repetitive arm use and the development of rotator cuff disorders. To our knowledge, no standardized clinical endurance protocol has been developed to evaluate the effects of muscle fatigue on shoulder function. Such a test could improve clinical examination of individuals with shoulder disorders. Therefore, the purpose of this study was to establish a reliable protocol for objective assessment of shoulder muscle endurance.

Methods

An endurance protocol was developed on a stationary dynamometer (Biodex System 3). The endurance protocol was performed in isotonic mode with the resistance set at 50% of each subject's peak torque as measured for shoulder external (ER) and internal rotation (IR). Each subject performed 60 continuous repetitions of IR/ER rotation. The endurance protocol was performed by 36 healthy individuals on two separate occasions at least two days apart. Maximal isometric shoulder strength tests were performed before and after the fatigue protocol to evaluate the effects of the endurance protocol and its reliability. Paired t-tests were used to evaluate the reduction in shoulder strength due to the protocol, while intraclass correlation coefficients (ICC) and minimal detectable change (MDC) were used to evaluate its reliability.

Results

Maximal isometric strength was significantly decreased after the endurance protocol (P < 0.001). The total work performed during the last third of the protocol was significantly less than the first third of the protocol (P < 0.05). The test-retest reliability of the post-fatigue strength measures was excellent (ICC >0.84).

Conclusions

Changes in muscular performance observed during and after the muscular endurance protocol suggests that the protocol did result in muscular fatigue. Furthermore, this study established that the resultant effects of fatigue of the proposed isotonic protocol were reproducible over time. The protocol was performed without difficulty by all volunteers and took less than 10 minutes to perform, suggesting that it might be feasible for clinical practice. This protocol could be used to induce local muscular fatigue in order to evaluate the effects of fatigue on shoulder kinematics or to evaluate changes in shoulder muscle endurance following rehabilitation.

The specificity of fatiguing protocols affects scapular orientation: Implications for subacromial impingement

BACKGROUND

shoulder impairments are often associated with altered scapular kinematics. As muscles control scapular movement, functionally altering muscle performance through fatigue may produce scapular kinematics that mimic those of injured patients. The aim of this study was to examine if changes in scapular tilt, rotation and pro/retraction following two different upper extremity fatiguing protocols have any implications with respect to subacromial impingement.

METHODS

scapular orientation was monitored during posturally constrained static holds (at 0°, 45° and 90° of humeral elevation) before and after two fatiguing protocols, one global and one local. Both protocols are associated with producing changes in shoulder skeletal arrangement.

FINDINGS

following the global fatiguing protocol, there was significantly more scapular posterior tilt (P<0.01) and upward rotation (P<0.02), particularly at 90° humeral elevation. No changes in scapular orientation occurred following the local fatiguing protocol.

INTERPRETATION

scapular orientation changes following muscle fatigue acted to increase the subacromial space. Thus, the rotator cuff muscles, not the scapular stabilizers, have more influence on actively preventing mechanical subacromial impingement. The lack of evidence of reduction of the subacromial space thus implicates superior humeral head translation as a more likely primary mechanism of the initiation of subacromial impingement.

Superior humeral head migration occurs after a protocol designed to fatigue the rotator cuff: a radiographic analysis

HYPOTHESIS

Awkward postures and repetitive work have been suggested to lead to shoulder fatigue, which may in turn decrease the subacromial space. The aim of this study was to quantify changes in humeral head position relative to the glenoid after rotator cuff fatigue. We hypothesized that the humeral head would migrate superiorly with fatigue due to the inability of the rotator cuff muscles to balance the upward pull of the deltoid.

MATERIALS AND METHODS

Four anterior-posterior radiographs (at 0°, 45°, 90°, and 135° of elevation in the scapular plane) of the glenohumeral joint were taken before and after a fatiguing task. The fatiguing task was a simulated job task intended to exhaust the entire rotator cuff.

RESULTS

The position of the humeral head with respect to the glenoid cavity was significantly affected both by arm elevation angle and fatigue state. In the prefatigued state, increasing arm elevation angle was related to superior translation until 90°, after which the humeral head moved inferiorly to a more central position. In the postfatigued state, the inability of the rotator cuff to centralize the humeral head led to increasing translations with higher elevations.

DISCUSSION

Superior humeral head migration was associated with the fatigued state. This implies that overhead or repetitive work, or both, may accelerate the development of subacromial impingement through reduction of the subacromial space.

CONCLUSIONS

Continuous overhead work demonstrably created rotator cuff fatigue, which apparently inhibited the ability of the shoulder musculature to resist upward humeral translation.

Shoulder biomechanics: today's consensus and tomorrow's perspectives

Shoulder biomechanics is a fast growing field, which is progressively expanding its focus to include more applied research. The papers included in this Special Issue confirm this trend. After a classification of the papers as dealing with fundamental or applied research through theoretical or experimental methods, in this Editorial we tried to summarize the elements of consensus and the open issues discussed during the last International Shoulder Group meeting, held in Bologna (Italy) in 2008.