Between Two Rocks and in a Hard Place: Reflecting on the Biomechanical Basis of Shoulder Occupational Musculoskeletal Disorders.

OBJECTIVE: The aim was to review the biomechanical origins of occupational shoulder damage, while considering the complexity of shoulder mechanics and musculoskeletal consequences of diverse task demands. BACKGROUND: Accessible measures of physical exposures are the primary focus of occupational shoulder assessments and analyses. This approach has led to guidelines and intervention strategies that are often inadequate for mitigating shoulder disorders amongst the complexity of modern workplace demands. Integration of complex shoulder mechanics into occupational assessments, analyses, and interventions is critical for reducing occupational shoulder injury risk. METHOD: This narrative review describes shoulder biomechanics in the context of common injury mechanisms and consequent injuries, with a particular focus on subacromial impingement syndrome. Several modulators of shoulder injury risk are reviewed, including fatigue, overhead work, office ergonomics considerations, and pushing and pulling task configurations. RESULTS: Relationships between work requirements, muscular demands, fatigue, and biomechanical tissue loads exist. This review highlights that consideration of specific workplace factors should be integrated with our knowledge of the intricate arrangement and interpersonal variability of the shoulder complex to proactively evaluate occupational shoulder demands and exposures. CONCLUSION: A standard method for evaluating shoulder muscle exposures during workplace tasks does not exist. An integrated approach is critical for improved work design and prevention of shoulder tissue damage and accompanying disability. APPLICATION: This review is particularly relevant for researchers and practitioners, providing guidance for work design and evaluation for shoulder injury prevention by understanding the importance of the unique and complex mechanics of the shoulder.

Assessing potential trade-offs between the lower back and shoulders: influence of lift training intervention on joint demands.

Background. Many of the approaches available for modifying manual materials handling (MMH) exertion emphasize lower back protection but often do not consider how interventions affect other body regions. This study focused on the influence of lift training on resultant joint moments and muscular demand trade-offs between the lower back and shoulders during MMH tasks. Methods. Three recommended lifting techniques (straddle lift, pivot technique and tripod lift) were compared to a priori (untrained) self-selected lifting techniques. Results. Mean and cumulative resultant moments indicated that using the lifting techniques evaluated in this investigation protected the shoulders more than the lower back. Mean and peak shoulder muscle activity also decreased following training (p < 0.05). Although there were no peak and mean changes to lower back muscle activity (p>0.05), there was a significant decrease in cumulative lower back muscle activity (p < 0.05). Reported perceived exertion values decreased following training across the lifting techniques for all evaluated body regions (p < 0.05). Conclusion. Overall, the recommended MMH techniques protected both the lower back and the shoulders, and no exposure trade-offs between them were identified.

Use of a Novel EEG-Based Objective Test, the Cognalyzer®, in Quantifying the Strength and Determining the Action Time of Cannabis Psychoactive Effects and Factors that May Influence Them Within an Observational Study Framework.

INTRODUCTION: Current methods to detect recent delta-9-tetrahydrocannabinol (THC) use cannot objectively quantify its psychoactive effects (PE). The Cognalyzer®, an electroencephalography (EEG)-based method, detects and quantifies the strength of THC-induced PE on a scale from 0 to 100%. This study assesses the relationship between the magnitude of Cognalyzer® PE predictions and reported subjective drug effects for 4-h post-cannabis inhalation. METHODS: Seventy-five participants were enrolled in the study. Prior to ad libitum cannabis inhalation, an EEG recording episode was completed. Immediately after inhalation, the Drug Effects Questionnaire (DEQ) was administered and another EEG recording performed. For 25 participants, the study ended. For 50 participants, assessments were repeated at 30-min intervals for 4 h. EEG files were blinded and analyzed using two versions of the Cognalyzer® algorithm. The relationship between the Cognalyzer® PE level results and the DEQ was assessed using generalized linear models and multiple regression. RESULTS: There were significant PE increases from pre-cannabis for up to 3.5 h. Mean reports of feeling drug effects were > 0 at all post-inhalation time points (p ≤ 0.024). Furthermore, there were significant relationships between the Cognalyzer® PE and self-reported perception of drug effects (p ≤ 0.001). Subgroup analysis showed that Cognalyzer® PE levels were impacted by cannabis use history, subjective ratings of drug effects, oral fluid THC concentration and the cannabis product inhaled. CONCLUSION: The findings show that the Cognalyzer® can be used to objectively determine the strength of cannabis psychoactive effects that cannabis products create on consumers and how it changes depending on their experience with cannabis. The Cognalyzer® can be used to conduct scientific consumer research to generate trustworthy informational material about the psychoactive experience of cannabis products. For clinical research, the Cognalyzer® can be used to study the pharmacodynamics of cannabinoids or delivery systems, such as nano-emulsifications.

Level of exoskeleton support influences shoulder elevation, external rotation and forearm pronation during simulated work tasks in females.

Despite growing literature, limited research details the influence of passive upper limb exoskeletons on upper limb kinematics. Two bolting tasks and a tracing task were completed at two heights (overhead and between waist and overhead height) for four exoskeleton conditions (no exoskeleton, and 3 levels of exoskeleton assistance) by female participants. Motion capture data, ratings of perceived exertion and discomfort, and task duration were recorded. Exoskeleton condition increased minimum shoulder elevation by 35-36% (Δ10.5-10.7°) at 1.81 kg and 2.72 kg of support, mean shoulder external rotation by 316% (Δ24.6°) at 0.91 kg of support and mean forearm pronation by 30.9% (Δ14.6°) at 0.91 kg of support. Exoskeleton condition reduced ratings of perceived exertion and discomfort, but not significantly. Task duration was unaffected. Exoskeleton use at any of three different settings modestly affected some joint kinematics for the tasks examined, which may merit consideration when deciding on occupational exoskeleton implementation.

A Randomized, Triple-Blind, Placebo-Controlled, Crossover Study to Investigate the Efficacy of a Single Dose of AlphaWave® L-Theanine on Stress in a Healthy Adult Population.

INTRODUCTION: Stress is a complex life occurrence essential for survival and goal achievement but can be damaging in excess. Because of the high prevalence of stress in North America, a safe supplement that effectively reduces stress is in demand. The objective of this study was to investigate the efficacy and safety of AlphaWave® L-Theanine on whole-scalp and frontal alpha power, midline theta power, and salivary cortisol in healthy, moderately stressed adults. METHODS: This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of AlphaWave® L-Theanine (200 mg) or placebo was administered. To induce stress, a mental arithmetic test (MAT) was administered before and after the dose. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, adverse events, clinical chemistry, and hematology were assessed to evaluate efficacy and safety. RESULTS: Increases in heart rate, blood pressure, and self-reported stress and state anxiety indicated that participants experienced stress during the MAT. AlphaWave® L-Theanine led to a greater increase in frontal region and whole-scalp alpha power 3 h post-dose compared to placebo (p ≤ 0.050). Within groups, there were increases in alpha power, at 3 h with AlphaWave® L-Theanine, over the whole recording and during the eyes-open portions (p ≤ 0.048) of the alpha task. The changes in alpha wave activity are supported by greater decreases in salivary cortisol 1 h post-dose (p < 0.001) with AlphaWave® L-Theanine compared to placebo. CONCLUSION: This study was conducted during the SARS-CoV-2 pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave® L-Theanine significantly increased frontal region alpha power compared to placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response. AlphaWave® L-Theanine was found to be safe and well tolerated by participants. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT04706494.

Stress is a complex part of life that is essential for survival and achieving goals. Too much stress, however, can be damaging. There is a high prevalence of stress in North America, creating a demand for a safe and effective supplement to reduce it. This study investigated the effectiveness and safety of AlphaWave^® L-Theanine on brainwaves and salivary cortisol in healthy, moderately stressed adults facing an acute stressor. This was a randomized, triple-blind, placebo-controlled, crossover study that consisted of two study periods with a 7-day washout. A single dose of 200 mg of AlphaWave^® L-Theanine or placebo was administered before and after a mental arithmetic test to elicit acute stress. Electroencephalogram, salivary cortisol, blood pressure, heart rate, self-reported stress, and safety were assessed to evaluate efficacy and safety. This study was conducted during the coronavirus pandemic, which has had a rapid and significant effect on both physical and mental health around the world. A single dose of AlphaWave^® L-Theanine had significant positive effects on brainwaves, salivary cortisol, and self-reported state anxiety compared to the placebo in response to an acute stress challenge. These changes are indicative of relaxation in the brain and suggest a calming response in a moderately stressed but otherwise healthy population. AlphaWave^® L-Theanine was found to be safe and well tolerated by participants.

Higher body mass index and body fat percentage correlate to lower joint and functional strength in working age adults.

As the prevalence of obesity grows worldwide, it becomes an increasing concern in working populations. Ergonomists are faced with the challenge of accommodating workplace layouts to include this worker demographic. This study investigated the relationship between shoulder and low back isometric joint strengths across body mass index (BMI) groups. Additionally, relationships between body fat percentage (BF%), absolute strength, and strength normalized to body mass were examined. Ninety, healthy, working age participants performed 11 functional and isometric joint strength exertions. BMI group influenced normalized strength, as the obese 2+ (BMI >35.0) group had up to 63.1% lower joint strength than all other BMI groups (p < 0.05). Significant strong to moderate negative linear relationships existed between BF% and normalized strength for both males and females, and relationships were stronger for females. These strength deficits highlight the importance of considering body composition during ergonomics analyses and configuration of occupational tasks.

Sensitivity, Specificity and Accuracy of a Novel EEG-Based Objective Test, the Cognalyzer®, in Detecting Cannabis Psychoactive Effects.

INTRODUCTION: Current standards for identifying recent cannabis use are based on body fluid testing. The Cognalyzer® is a novel electroencephalography (EEG) measurement device and algorithm designed to objectively characterize brainwave alterations associated with cannabis. The objective of this study was to assess the accuracy, sensitivity and specificity levels of the Cognalyzer® to characterize brainwave alterations following cannabis inhalation. METHODS: Seventy-five participants, aged 19-55 years, were enrolled, and oral fluid samples were collected pre-cannabis inhalation. EEG and subjective drug effects questionnaire (DEQ) were administered pre- and post-ad libitum cannabis inhalation. Fifty participants remained in the clinic for 4 h post-inhalation. Blinded analyses of the EEG files were conducted by Zentrela Inc. using two versions (V1 and V2) of the Cognalyzer® algorithm. Pre- vs. post-inhalation comparison status was characterized by the Cognalyzer® and summarized for: sensitivity, specificity, accuracy, percent false positive, percent false negative and positive and negative predictive value. The null hypothesis was tested using McNemar's test. Cognalyzer® results pre- and post-inhalation were combined with the oral fluid tetrahydrocannabinol (THC) concentration to evaluate potential to improve current drug testing. RESULTS: The two versions of the Cognalyzer® algorithm had similar diagnostic results. Diagnostic outcomes were improved when participants with missing EEG recordings or electrode placement errors were removed. The Cognalyzer® accuracy was 85.5% and 83.9%, sensitivity was 87.1% and 88.7%, and specificity was 83.9% and 79.0% for algorithm V1 and V2, respectively. Combining Cognalyzer® results with oral fluid concentrations reduced false-positive oral fluid test results by up to 49%. CONCLUSION: The Cognalyzer® characterized brainwave alterations associated with cannabis inhalation with high levels of accuracy in a population of participants with varied cannabis inhalation histories, relative to the comparison standard of pre- vs. post-inhalation status. The Cognalyzer® allows the results to be generalized to the larger population addressing a limitation in currently accepted standards.

Erratum to "Changing the tone of clinical study design in the cannabis industry".

[This corrects the article on p. 4 in vol. 11, PMID: 32104589.].

Sub-regional activation of supraspinatus and infraspinatus muscles during activities of daily living is task dependent.

The supraspinatus and infraspinatus muscles each have multiple sub-regions that may activate differentially in activities of daily living. Awareness of these differential demands critically informs rehabilitation of rotator cuff muscle following injury, particularly if centered on recovering and strengthening the rotator cuff to perform daily tasks. This study quantified muscle activation of supraspinatus and infraspinatus sub-regions during the performance of six activities of daily living. Twenty-three participants (mean: 22.6 ± 2.6 years) completed the following tasks: opening a jar, reaching at shoulder height, overhead reaching, pouring water from a pitcher, eating with a spoon, and combing hair. Indwelling electromyography was collected from the anterior and posterior supraspinatus and superior, middle, and inferior infraspinatus. Tasks requiring high arm elevations (e.g. reaching at shoulder and overhead height) activated anterior supraspinatus between 21 and 28% MVC. The posterior supraspinatus consistently activated between 10 and 30% MVC across all tasks. All sub-regions of infraspinatus activated highly (between 18 and 25% MVC) in tasks requiring high arm elevations in flexion. These findings may be leveraged to define effective measures to increase rotator cuff function in daily tasks.

Joint moment trade-offs across the upper extremity and trunk during repetitive work.

Individuals can coordinate small kinematic changes at several degrees of freedom simultaneously in the presence of fatigue, leaving it unclear how overall biomechanical demands at each joint are altered. The purpose of this study was to evaluate trade-offs in joint moments between the trunk, shoulder, and elbow during repetitive upper extremity work. Participants performed four simulated workplace tasks cyclically until meeting fatigue termination criteria. Emergent fatigue-induced adaptations to repetitive work resulted in task-dependent trade-offs in joint moments. In general, reduced shoulder moments were compensated for by increased elbow and trunk joint moment contributions. Although mean joint moment changes were modest (range: 1-3 Nm) across participants, a wide distribution of responses was observed, with standard deviations exceeding 10 Nm. Re-distributing biomechanical demands across joints may alleviate constant tissue loads and facilitate continued task performance with fatigue but may be at the expense of increasing demands at adjacent joints.

Changing the Tone of Clinical Study Design in the Cannabis Industry.

Cannabis (also known as marijuana) is the most frequently used psychoactive substance globally. Cannabis exerts therapeutic functions for many indications and has vast potential as a health and wellness product. Advances in our understanding of the composition and pharmacological properties of cannabis have revealed interactions between cannabis, an individuals' circadian rhythms and and their endocannabinoid signaling. Exogenously administered cannabinoids can bidirectionally entrain central and peripheral clocks that comprise circadian rhythms, and malfunctions in the endocannabinoid system are reported to impact neurological processes. Therefore, it is necessary to account for the circadian rhythm when designing clinical trials examining the pharmacological properties of cannabis-based products for health and wellness to limit its potential confounding impact on results. Consideration of the entrainment capabilities of the endocannabinoid system is warranted when designing clinical trials.

Postural and muscular adaptations to repetitive simulated work.

Complex repetitive tasks are common in the workplace and have been associated with upper extremity disorders. The purpose of this study was to examine the progressive effects of highly repetitive work on joint kinematics and muscle activity of the trunk and upper extremity. Fifteen healthy men performed 60 one-minute cycles of 4 simulated automotive-related tasks. Electromyography of eight muscles and kinematics of the trunk and right upper extremity were collected. Data were analysed at 12-min intervals and divided into a complete work cycle. The time to complete the work cycle decreased by 6.3 s over the trials. Peak shoulder flexion decreased and peak elbow flexion increased during the work cycle. Muscle activity magnitude and variability was influenced by time during the repetitive tasks. This study found adaptations to highly repetitive but light work in only 1 h; redistributing muscle demands within the shoulder over time may reduce muscle fatigue development. Practitioner Summary: While the work was not strenuous, we were able to demonstrate muscular and postural adaptations in a single hour of simulated work. By evaluating both the whole work cycle and the sub-tasks, we aim to develop new methods for evaluating the risk of complex tasks in prolonged repetitive work.

Using EMG Amplitude and Frequency to Calculate a Multimuscle Fatigue Score and Evaluate Global Shoulder Fatigue.

OBJECTIVE: The authors developed a function to quantify fatigue in multiple shoulder muscles by generating a single score using relative changes in EMG amplitude and frequency over time. BACKGROUND: Evaluating both frequency and amplitude components of the electromyographic signal provides a more complete evaluation of muscle fatigue than either variable alone; however, little effort has been made to combine time and frequency domains for the evaluation of myoelectric fatigue. METHOD: Surface EMG was measured from 14 shoulder muscles while participants performed simulated, repetitive work tasks until exhaustion. Each 60-s work cycle consisted of four tasks (dynamic push, dynamic pull, static drill, static force target matching task) scaled to participants' anthropometrics and strength. The function was generated to calculate a multimuscle fatigue score (MMFS) based on changes in EMG frequency, amplitude, and the number of muscles showing signs of myoelectric fatigue (increase in EMG amplitude; decrease in EMG frequency). RESULTS: The function was evaluated through changes in MMFS over time: first (31.8 ± 14.6), middle (47.6 ± 25.3), last (58.6 ± 35.5) reference exertions ( p < .05). The evaluation of the relationships between MMFS and changes in strength ( r = -0.510) and MMFS and perceived fatigue (RPF) ( r = 0.298) showed significant relationships over time ( p < .05). MMFS scores increased over time ( p < .05) with significant relationships between MMFS and strength changes and RPF ( p < .05). CONCLUSION AND APPLICATION: The MMFS allows for comparisons between workplace tasks, which can aid in workplace design to mitigate the development of fatigue.

Muscular and kinematic adaptations to fatiguing repetitive upper extremity work.

Repetitive work is common in the modern workplace and the effects are often studied using fatigue protocols; however, there is evidence that the manner in which fatigue develops impacts the kinematic and muscular response to reduced physical capacity. The purpose of this study was to simultaneously evaluate muscular and kinematic adaptations during fatiguing, repetitive work until exhaustion. We measured muscle activity in 13 muscles in the shoulder and trunk, and captured full body kinematics while participants completed simulated, repetitive work tasks. Every 12 min, reference data were collected to quantify fatigue. This sequence continued until they reached the termination criteria. Participants displayed significant signs of muscle fatigue, loss of strength and increased perceived fatigue (p < 0.05). Analysis revealed a significant effect of time on posture and muscle activity that was both task and time dependent, and variable both between and within individuals. Participants were able to compensate for reduced physical capacity and maintain task performance through coordinated compensation strategies.

Upper body kinematic and muscular variability in response to targeted rotator cuff fatigue.

The rotator cuff muscles are prominent stabilizers of the shoulder and are vulnerable to muscle fatigue. Rotator cuff fatigue may result in subacromial impingement (SAI) through the superior translation of the humeral head. Scapular changes have been reported inconsistently, but may prevent SAI. The purpose of this study was to quantify changes in scapular kinematics, as well as muscle activity during glenohumeral motions following targeted rotator cuff fatigue. Ten healthy men completed four planar glenohumeral motions (cross-flexion, frontal, scapular, and sagittal plane elevation) prior to and immediately following a rotator cuff fatigue protocol on two separate days. Scapular kinematics and muscle activity of thirteen muscles were recorded. Scapular protraction decreased significantly with fatigue during scapular plane elevation (p < 0.001; ηp2 = 0.74). Although not significant, large effect sizes were found with decreased scapular protraction during elevation in the frontal (p = 0.012; ηp2 = 0.52) and sagittal planes (p = 0.007; ηp2 = 0.58), as well as decreased scapular medial rotation during cross-flexion (p = 0.008; ηp2 = 0.56). Between-subject variability (standard deviations from 2.3° to 14.5°) and within-subject day-to-day differences (upwards of 10° deviation in the opposite direction) were high among all kinematic changes following fatigue. Considerable day-to-day differences in scapular stabilizer muscle activity in response to fatigue were present. Due to the degrees of freedom at the upper extremity, individuals can employ a variety of compensatory strategies to fatigue. The variable compensatory strategies across the scapular stabilizers resulted in individual-specific scapular kinematic changes that could act as either impingement-sparing or impingement-promoting. The high variance in day-to-day differences within-subjects indicates that kinematic and muscular responses to fatigue may be adaptive within individuals over time.

Submaximal normalizing methods to evaluate load sharing changes in the shoulder during repetitive work.

The EMG-force relationship changes with muscle fatigue, making interpretation of load sharing between muscles challenging. The purpose of this investigation was to evaluate the efficacy of normalizing EMG to repeated, static, submaximal exertions to mitigate fatigue artifacts in EMG amplitude (increased amplitude without muscle force change). Participants completed repetitive work tasks, in 60-second cycles, until exhaustion. Surface EMG was recorded from 11 shoulder muscles (anterior, middle and posterior deltoids, infraspinatus, upper, middle and lower trapezius, latissimus dorsi, serratus anterior, sternal and clavicular heads of pectoralis major). Every 12 min, participants completed 4 submaximal reference exertions. Reference exertion EMG data were used in 6 normalizing methods including 1 standard (normalized to initial reference exertion) and 5 novel methods: (i) Fatigue Only, (ii) Linear, (iii) Cubic, (iv) Points Forward, and (v) Points Forward/Backward. Data normalized with each novel method were compared to the Standard Method using mixed effects modelling. Significant differences depended on the muscle and the number of time points included (p < .05). The cubic model correlated better to the actual data points than linear predicted values. The novel cubic normalizing method created muscle activity ratios that appear to mitigate the fatigue effects and better reflect muscular loads during fatiguing work.

Dynamic and static shoulder strength relationship and predictive model.

Static strength is typically used to standardize occupational tasks in an effort to limit over-exertion injuries; however, workplace tasks are commonly dynamic in nature. The purpose of this investigation was to assess factors influencing isokinetic shoulder strength and to develop predictive equations for isokinetic shoulder flexion and extension strength using isometric strength. Fifteen women performed a set of concentric isokinetic and isometric shoulder flexion and extension maximal exertions across a series of movement planes, angular velocities, and grip types. Data were used to generate two stepwise multiple regression models for predicting isokinetic shoulder flexion and exertion strength across the various exertion parameters. The final regression models explained a high degree of variance in predicting isokinetic shoulder flexion (R2 = 0.59) and extension (R2 = 0.67) with a subset of four and five inputs, respectively. The predictive equations can help establish acceptable force limits for workplace tasks requiring dynamic actions using more easily attainable static forces.

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.

Adaptations to isolated shoulder fatigue during simulated repetitive work. Part II: Recovery.

The shoulder allows kinematic and muscular changes to facilitate continued task performance during prolonged repetitive work. The purpose of this work was to examine changes during simulated repetitive work in response to a fatigue protocol. Participants performed 20 one-minute work cycles comprised of 4 shoulder centric tasks, a fatigue protocol, followed by 60 additional cycles. The fatigue protocol targeted the anterior deltoid and cycled between static and dynamic actions. EMG was collected from 14 upper extremity and back muscles and three-dimensional motion was captured during each work cycle. Participants completed post-fatigue work despite EMG manifestations of muscle fatigue, reduced flexion strength (by 28%), and increased perceived exertion (∼3 times). Throughout the post-fatigue work cycles, participants maintained performance via kinematic and muscular adaptations, such as reduced glenohumeral flexion and scapular rotation which were task specific and varied throughout the hour of simulated work. By the end of 60 post-fatigue work cycles, signs of fatigue persisted in the anterior deltoid and developed in the middle deltoid, yet perceived exertion and strength returned to pre-fatigue levels. Recovery from fatigue elicits changes in muscle activity and movement patterns that may not be perceived by the worker which has important implications for injury risk.

Adaptations to isolated shoulder fatigue during simulated repetitive work. Part I: Fatigue.

Upper extremity muscle fatigue is challenging to identify during industrial tasks and places changing demands on the shoulder complex that are not fully understood. The purpose of this investigation was to examine adaptation strategies in response to isolated anterior deltoid muscle fatigue while performing simulated repetitive work. Participants completed two blocks of simulated repetitive work separated by an anterior deltoid fatigue protocol; the first block had 20 work cycles and the post-fatigue block had 60 cycles. Each work cycle was 60s in duration and included 4 tasks: handle pull, cap rotation, drill press and handle push. Surface EMG of 14 muscles and upper body kinematics were recorded. Immediately following fatigue, glenohumeral flexion strength was reduced, rating of perceived exertion scores increased and signs of muscle fatigue (increased EMG amplitude, decreased EMG frequency) were present in anterior and posterior deltoids, latissimus dorsi and serratus anterior. Along with other kinematic and muscle activity changes, scapular reorientation occurred in all of the simulated tasks and generally served to increase the width of the subacromial space. These findings suggest that immediately following fatigue people adapt by repositioning joints to maintain task performance and may also prioritize maintaining subacromial space width.