Publications by authors named "Clark R Dickerson"

98 Publications

Wrapping technique and wrapping height interact to modify physical exposures during manual pallet wrapping.

Appl Ergon 2021 May 15;93:103378. Epub 2021 Feb 15.

Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. Electronic address:

Occupational tasks often involve musculoskeletal demands that contribute to injury risk. In pallet wrapping tasks, 36% of workplace claims involve over exertion and repetitive exposures (Workplace Safety and Prevention Services, 2012). Tools that modify how the wrap is handled by workers have been introduced to help mitigate over exertion and extreme postures wherever possible. A novel device has been introduced that places the required tension on the roll to mitigate these factors. However, the effect of reducing the necessary tension on the roll during pallet wrapping in limiting muscular demand and extreme postures is unknown. Fourteen healthy university aged participants completed 12 wrapping trials on a simulated pallet (2 repetitions of 2 wrapping techniques (device, hand wrapping) at 3 wrapping heights (low, medium, high)). Surface electromyography (sEMG) was measured on 6 shoulder and 2 low back muscles; anterior and middle deltoids, biceps brachii, infraspinatus, supraspinatus, upper trapezius, and erector spinae (T8 & L3). Kinematic data were collected for the torso and upper extremity and global to torso, and torso to upper arm angles were computed. Repeated measures ANOVAs were performed for the following experimental factors: 1) technique used (device or hand wrapping) and 2) the wrapping height (low, medium, high) for each muscle (8), angle (5), rating of perceived discomfort (1) and rating of perceived exertion (1). Pallet wrapping without a device required greater trunk flexion at lower heights and thoracohumeral elevation at higher wrapping heights as compared to using the device. Muscular activation increased when using the device, specifically at the higher and lower heights. Posture and muscular demands during pallet wrapping tasks are sensitive to both wrapping technique and wrapping height. Ergonomics interventions such as this device may mitigate postural risks associated with manual material handling tasks.
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http://dx.doi.org/10.1016/j.apergo.2021.103378DOI Listing
May 2021

Upper extremity and trunk body segment parameters are affected by BMI and sex.

J Biomech 2021 Mar 9;117:110230. Epub 2021 Jan 9.

Department of Kinesiology, University of Waterloo, Waterloo, Canada. Electronic address:

Although body mass index (BMI) relates to body segment parameters (BSPs), unknowns persist over whether: 1) BSPs relate to BMI group classifications, 2) sex influences BMI/BSP relationships, and 3) simple anthropometric measures sufficiently predict BSPs. Dual energy X-ray absorptiometry (DXA) scans and anthropometric measures were obtained from 76 participants (33M, 43F) of varying body composition. Trunk, neck and head (TNH), arm, forearm and hand masses were obtained from DXA scans and center of mass locations (COM) estimated from geometric models. Groups with larger BMIs had lesser hand and forearm mass (%total body mass; p<0.001) and greater TNH mass (p=0.014). Males had greater hand, forearm, and arm masses (p<0.05). TNH COM was inferior in groups with larger BMI (p<0.01) and in males (p=0.006). In females, arm COM was distal in Obese II/III versus Normal (p=0.024). Two sets of linear models were created to predict BSPs; a simplified set with only BMI, height, weight, sex and mass potential predictors and a complex set with additional anthropometric measures. Complex arm and TNH mass models (arm R=0.43, TNH R=0.61) explained more variance than simplified models (arm R=0.1, TNH R=0.33). Complex hand mass, forearm mass and TNH COM models had smaller R increases versus simplified models (hand=0.05, forearm=0.06, TNH=0.08). Explained variance in forearm COM (R=0.2) and arm COM (R=0.27) complex models was low, suggesting a constant may provide reasonable estimates. Certain BSPs can be estimated using simplified measures, whereas prediction of other BSPs markedly improves if additional anthropometric measures are included.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110230DOI Listing
March 2021

The rate of tendon failure in a collagen fibre recruitment-based model.

J Mech Behav Biomed Mater 2021 03 24;115:104273. Epub 2020 Dec 24.

University of Waterloo, Department of Kinesiology, Canada. Electronic address:

Accurate characterization of the mechanical response of collagenous tissues is critical for investigations into mechanisms of soft tissue injury. These tissues are inherently viscoelastic, exhibiting strain-rate dependent stiffnesses, creep, and stress-relaxation. The strain-rate features of the failure portion of the stress-strain curve are less well developed. Collagen-distribution based models are improving and capable of reproducing the non-linear aspects of the elastic response of soft tissues, but still require parameterization of failure regions. Therefore, the purpose of this investigation, was to determine whether the parameters characterizing the rate of damage accumulation in a collagen-distribution model are proportional to strain rate. Fifty rat tail tendons were subjected to one of five different strain rates (0.01, 0.05, 0.1, 0.15, 0.20 s) until failure in an uni-axial strain test. To test the hypothesis that the parameters associated with damage rate are proportional to strain rate, a collagen distribution model was employed with the parameters describing the rate of fibre damage being obtained by least-squares and regressed against the strain rate. The breaking function was found to be proportional to strain rate, with a proportionality constant of 60.7 s. Properties characterizing the failure portion of the stress-strain curves for rat tail tendons are also reported. The Young's Modulus did not vary with strain rate and was found to be 103.3 ± 49.5 MPa. Similarly, failure stresses and strains did not vary across the strain rates tested, and were 15.6 ± 6.1 MPa and 32.2 ± 9.1%, respectively.
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http://dx.doi.org/10.1016/j.jmbbm.2020.104273DOI Listing
March 2021

Standard bipolar surface EMG estimations mischaracterize pectoralis major activity in commonly performed tasks.

J Electromyogr Kinesiol 2021 Feb 5;56:102509. Epub 2020 Dec 5.

Department of Kinesiology, University of Waterloo, Waterloo, Canada. Electronic address:

The pectoralis major assists in several shoulder movements, such as humeral vertical and horizontal adduction, flexion, extension, and internal rotation. Despite its involvement in numerous functional activities, its role in typical shoulder function is ambiguous. Due to this, its purpose in arm movement is largely diminished. However, mounting evidence associates pectoralis major injuries to long-term debilitating arm disability. Therefore, a more deliberate investigation of its role in typical shoulder function is paramount. The purpose of this paper is to outline the current limitations in the acquisition and characterization of pectoralis major activation using standard bipolar surface electromyography. Macroscopic level analyses are used to investigate pectoralis major activation in eight tasks at low (15-25% of maximal voluntary effort (MVE) and moderate (50% MVE) efforts in healthy males. Virtually derived bipolar EMG amplitudes are quantified for the clavicular and the upper sternocostal regions based on the common locations used to acquire EMG signals from classic EMG. HD-sEMG amplitudes from three pectoralis major regions (i.e. clavicular, upper, and lower sternocostal) were compared to virtually derived bipolar EMG amplitudes (i.e. clavicular and upper sternocostal) to determine if current EMG methods misestimate pectoralis major activity. Current findings indicate that classic EMG recordings mischaracterize pectoralis major activation in several tasks and effort levels, highlighting the importance of acquiring signals from multiple pectoralis major regions.
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http://dx.doi.org/10.1016/j.jelekin.2020.102509DOI Listing
February 2021

Examining assessment methods of scapular motion: Comparing results from planar elevations and functional task performance.

Clin Biomech (Bristol, Avon) 2020 12 24;80:105203. Epub 2020 Oct 24.

School of Rehabilitation Science, College of Medicine, University of Saskatchewan, Canada. Electronic address:

Background: Scapular kinematics of breast cancer survivors are most often evaluated during arm elevation. However, known compensations exist during functional task performance. The purpose of this study was to determine if scapular kinematics of breast cancer survivors during arm elevation are related to scapular kinematics during functional task performance.

Methods: Scapular kinematics of 25 non-cancer controls and 25 breast cancer survivors (split by presence of impingement pain) during arm elevation in 3 planes and 3 reaching and lifting functional tasks were measured. Scapular upward rotation and scapulohumeral rhythm (SHR) at 30° increments of arm elevation were calculated. Between-group differences of upward rotation during arm elevation were evaluated with one-way ANOVAs (p < 0.05). The association of upward rotation angle and SHR during arm elevation and functional tasks was tested with Pearson correlations (p < 0.05).

Findings: Scapular upward rotation was reduced for the breast cancer survivor with pain at lower levels of arm elevation in each plane by up to 7.1° (p = .014 to 0.049). This is inconsistent with functional task results, in which upward rotation decrements occurred at higher levels of arm elevation. Upward rotation angles and SHR during arm elevation had an overall weak-to-moderate relationship (r = 0.003 to 0.970, p = .001 to 0.048) to values from functional tasks. Arm elevation during sagittal plane elevation demonstrated scapular upward rotation that was most closely associated to upward rotation during functional task performance.

Interpretation: Inconsistent relationships suggests that clinical evaluations should adopt basic functional movements for scapular motion assessment to complement simple arm elevations.
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http://dx.doi.org/10.1016/j.clinbiomech.2020.105203DOI Listing
December 2020

Development of a comparative chimpanzee musculoskeletal glenohumeral model: implications for human function.

J Exp Biol 2020 11 18;223(Pt 22). Epub 2020 Nov 18.

Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada N2L 3G1.

Modern human shoulder function is affected by the evolutionary adaptations that have occurred to ensure survival and prosperity of the species. Robust examination of behavioral shoulder performance and injury risk can be holistically improved through an interdisciplinary approach that integrates anthropology and biomechanics. Coordination of these fields can allow different perspectives to contribute to a more complete interpretation of biomechanics of the modern human shoulder. The purpose of this study was to develop a novel biomechanical and comparative chimpanzee glenohumeral model, designed to parallel an existing human glenohumeral model, and compare predicted musculoskeletal outputs between the two models. The chimpanzee glenohumeral model consists of three modules - an external torque module, a musculoskeletal geometric module and an internal muscle force prediction module. Together, these modules use postural kinematics, subject-specific anthropometrics, a novel shoulder rhythm, glenohumeral stability ratios, hand forces, musculoskeletal geometry and an optimization routine to estimate joint reaction forces and moments, subacromial space dimensions, and muscle and tissue forces. Using static postural data of a horizontal bimanual suspension task, predicted muscle forces and subacromial space were compared between chimpanzees and humans. Compared with chimpanzees, the human model predicted a 2 mm narrower subacromial space, deltoid muscle forces that were often double those of chimpanzees and a strong reliance on infraspinatus and teres minor (60-100% maximal force) over other rotator cuff muscles. These results agree with previous work on inter-species differences that inform basic human rotator cuff function and pathology.
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http://dx.doi.org/10.1242/jeb.225987DOI Listing
November 2020

Design and Preliminary Implementation of an Air Microfluidics Enabled Soft Robotic Knee Brace Towards the Management of Osteoarthritis.

Annu Int Conf IEEE Eng Med Biol Soc 2020 07;2020:4502-4505

A dynamic and low-profile unloader tibiofemoral knee brace is designed and prototyped by synergizing concepts from the fields of microfluidics and soft robotics. Microfluidics provides strategies for miniaturization and multiplexing while soft robotics afford the tools to create soft fluidic actuators and allow compliant and inherently safe robotic assistance as part of clothing. The unloader knee brace provides dynamic response during the gait cycle, where a three-point leverage torque is provided only during the stance phase to contribute to joint stability when required and enhance comfort and compliance.Clinical Relevance- This novel soft robotic brace has the potential to reduce device abandonment due to aesthetics, user non-compliance and discomfort due to a constant three-point leverage torque during the gait cycle. Also, this air microfluidics enabled soft robotic knee brace could be expanded upon to improve the efficacy of braces in general and augment the effects of physical therapy, rehabilitation and treatment of musculoskeletal conditions.
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http://dx.doi.org/10.1109/EMBC44109.2020.9175677DOI Listing
July 2020

Determining best practices for manual pill crushing through evaluation of upper extremity muscle exposures.

Appl Ergon 2021 Jan 1;90:103275. Epub 2020 Oct 1.

Department of Kinesiology, University of Waterloo, Canada. Electronic address:

Nurses in long-term care homes often crush pills into a fine powder using a manual pill crushing device. This study provides novel quantitative data on muscle loading experienced during pill crushing. The influence of surface height, number of pills and device orientation were studied in twelve muscles of the upper extremity. Variations in the work surface height and number of pills crushed resulted in static shoulder and forearm muscle activations that exceeded recommended static limits. In most cases, working at approximately a 50th percentile female's hip height (87 cm) reduced the level of muscle activity, often to below the EMG-based exposure limit, compared to higher heights. A perpendicularly oriented device required substantially lower muscle activity in some shoulder muscles, with marginal differences occurring in muscles of the elbow and wrist. These data can inform practical design and work practice recommendations to reduce muscular activity while performing this important healthcare task.
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http://dx.doi.org/10.1016/j.apergo.2020.103275DOI Listing
January 2021

Estimating Muscle Forces for Breast Cancer Survivors During Functional Tasks.

J Appl Biomech 2020 Sep 22:1-8. Epub 2020 Sep 22.

University of Waterloo.

Breast cancer survivors have known scapular kinematic alterations that may be related to the development of secondary morbidities. A measure of muscle activation would help understand the mechanisms behind potential harmful kinematics. The purpose of this study was to define muscle force strategies in breast cancer survivors. Shoulder muscle forces during 6 functional tasks were predicted for 25 breast cancer survivors (divided by impingement pain) and 25 controls using a modified Shoulder Loading Analysis Module. Maximum forces for each muscle were calculated, and 1-way analysis of variance (P < .05) was used to identify group differences. The differences between maximum predicted forces and maximum electromyography were compared with repeated-measures analysis of variance (P < .05) to evaluate the success of the model predictions. Average differences between force predictions and electromyography ranged from 7.3% to 31.6% but were within the range of previously accepted differences. Impingement related pain in breast cancer survivors is associated with increased force of select shoulder muscles. Both pectoralis major heads, upper trapezius, and supraspinatus peak forces were higher in the pain group across all tasks. These force prediction differences are also associated with potentially harmful kinematic strategies, providing a direction for possible rehabilitation strategies.
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http://dx.doi.org/10.1123/jab.2020-0047DOI Listing
September 2020

The Effect of Shoulder Mobilization on Scapular and Shoulder Muscle Activity During Resisted Shoulder Abduction: A Crossover Study of Asymptomatic Individuals.

J Manipulative Physiol Ther 2020 10 26;43(8):832-844. Epub 2020 Jul 26.

Centre for Health, Activity and Rehabilitation Research, School of Physiotherapy, University of Otago, Dunedin, Otago, New Zealand. Electronic address:

Objective: The primary aim was to investigate the effect of inferior shoulder mobilization on scapular and shoulder muscle activity during resisted shoulder abduction in asymptomatic individuals.

Methods: This was a lab-based, repeated-measures, crossover, randomized controlled study. Twenty-two participants were recruited. The order of experimental conditions was randomized. Each participant performed 5 repetitions of resisted shoulder abduction before and after the control and mobilization (grade +IV inferior shoulder mobilization, 3 sets, 60 seconds) conditions. Surface electromyography recorded the muscle activity of anterior, middle, and posterior deltoid; supraspinatus; infraspinatus; upper and lower trapezius; serratus anterior; and latissimus dorsi muscles.

Results: Muscle activity levels reduced for infraspinatus (11.3% MVIC, 95% CI: 1.7-20.8), middle (22.4% MVIC, 95% CI: 15.9-28.8) and posterior deltoid (8.7 % MVIC, 95% CI: 4.6-12.9), and serratus anterior (-28.1% MVIC, 95% CI: 15.6-40.8) muscles after the mobilization condition during the eccentric phase of shoulder abduction. No carryover effects were observed, and within-session reliability was excellent (intraclass correlation coefficient scores ranging from 0.94 to 0.99).

Conclusion: Our findings suggest that inferior glenohumeral mobilization reduces activity levels of some scapular and shoulder muscles. Given the exploratory nature of our study, changes in muscle activity levels may have been found by chance. Confirmatory studies are required.
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http://dx.doi.org/10.1016/j.jmpt.2019.11.006DOI Listing
October 2020

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

J Electromyogr Kinesiol 2020 Oct 17;54:102450. Epub 2020 Jul 17.

Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. Electronic address:

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.
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http://dx.doi.org/10.1016/j.jelekin.2020.102450DOI Listing
October 2020

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

Int J Occup Saf Ergon 2020 Jun 22:1-8. Epub 2020 Jun 22.

Department of Kinesiology, University of Waterloo, Canada.

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. Three recommended lifting techniques (straddle lift, pivot technique and tripod lift) were compared to a priori (untrained) self-selected lifting techniques. 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 ( < 0.05). Although there were no peak and mean changes to lower back muscle activity (>0.05), there was a significant decrease in cumulative lower back muscle activity ( < 0.05). Reported perceived exertion values decreased following training across the lifting techniques for all evaluated body regions ( < 0.05). Overall, the recommended MMH techniques protected both the lower back and the shoulders, and no exposure trade-offs between them were identified.
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http://dx.doi.org/10.1080/10803548.2020.1733343DOI Listing
June 2020

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

Hum Factors 2020 Jan 21:18720819896191. Epub 2020 Jan 21.

7991 York University, Canada.

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.
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http://dx.doi.org/10.1177/0018720819896191DOI Listing
January 2020

The ability of surface electromyography to represent supraspinatus anterior and posterior partition activity depends on elevation angle, hand load and plane of elevation.

J Biomech 2020 01 18;99:109526. Epub 2019 Nov 18.

Department of Kinesiology, University of Waterloo, Waterloo, Canada. Electronic address:

This study examined relationships between electromyography recorded from indwelling electrodes of the anterior and posterior supraspinatus and a surface supraspinatus electrode. Twenty male and twenty female participants completed full range humeral elevations in three planes of elevation (0/40/90°) and three hand loads (unloaded/20%/40% of maximal elevation strength). EMG activation was combined with motion capture to determine activation at instantaneous activation angles, and linear regressions of anterior and posterior indwelling electrodes relative to the surface electrode determined relationships between these signals. Regressions between surface and indwelling signals were affected by plane of elevation, elevation angle, load intensity and participant sex, but no interactions existed. Surface signals underestimated activation at low elevation angles for both regions, and up to 45% in the anterior supraspinatus (p < 0.01), then overestimated activation at higher elevation angles. Surface EMG underestimated indwelling signals by up to 15% in unloaded conditions, while overestimating the posterior region by up to 17% in the 40% load condition (p < 0.01). Sex effects showed increased overestimation by surface signals in the posterior region in males by 21% (p < 0.01). Better agreement existed for the anterior region than the posterior region across postures, but postural relationships should be considered when choosing electrodes for this muscle.
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http://dx.doi.org/10.1016/j.jbiomech.2019.109526DOI Listing
January 2020

Impingement pain affects kinematics of breast cancer survivors in work-related functional tasks.

Clin Biomech (Bristol, Avon) 2019 Dec 19;70:223-230. Epub 2019 Oct 19.

School of Rehabilitation Science, College of Medicine, University of Saskatchewan, Canada. Electronic address:

Background: Breast cancer survivors may encounter upper limb morbidities post-surgery. It is currently unclear how these impairments affect arm kinematics, particularly during functional task performance. This investigation examined upper body kinematics during functional tasks for breast cancer survivors and an age-matched control group.

Methods: Fifty women (aged 35-65) participated: 25 breast cancer survivors who had undergone mastectomy and 25 age-range matched controls. Following basic clinical evaluation, including shoulder impingement tests, motion of the torso and upper limbs were tracked during six upper limb-focused functional tasks from which torso, scapular, and thoracohumeral angles were calculated. Between-group differences were evaluated with independent t-tests (p < .05). The breast cancer group was then divided based upon impingement tests and differences between the three new groups were tested with one-way ANOVAs (p < .05).

Findings: Breast cancer survivors had higher disability scores, lower range of motion, and lower performance scores. The largest kinematic differences existed between the breast cancer survivors with impingement pain and the two non-pain groups. During overhead tasks, right peak scapular upward rotation was significantly reduced (d = 0.80-1.11) in the breast cancer survivors with impingement pain. This group also demonstrated trends of decreased peak humeral abduction and internal rotation at extreme postures (d = 0.54-0.78). These alterations are consistent with kinematics considered high risk for rotator cuff injury development.

Interpretation: Impingement pain in breast cancer survivors influences functional task performance and may be more important to consider than self-reported disability when evaluating pain and potential injury development.
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http://dx.doi.org/10.1016/j.clinbiomech.2019.10.001DOI Listing
December 2019

Verification of intramuscular electromyography electrode placement for neuromuscular partitions of infraspinatus.

J Electromyogr Kinesiol 2019 Dec 9;49:102354. Epub 2019 Sep 9.

University of Toronto, Department of Surgery, Division of Anatomy, Toronto, Ontario, Canada.

The infraspinatus muscle is composed of three neuromuscular partitions: superior, middle and inferior. Although methods for fine-wire EMG electrode insertion into these partitions have been developed and used, it has yet to be verified. The purpose of this cadaveric EMG needle placement study was to assess the accuracy and reproducibility of a protocol used to target the three partitions of infraspinatus. On seven shoulder specimens, two investigators inserted needles into each superior, middle and inferior partition according to a previously developed protocol. Each was blinded to the other's insertion sites. The specimens were dissected and the location of each needle was digitized and modeled in 3D. Of the 42 needles that were inserted, 32 were placed in the targeted partition. The highest accuracy rate occurred for the middle partition (100%), followed by the inferior (71.4%) and then the superior (57.1%). When the needles were not placed in the targeted partition, they were located in the neighboring partition within infraspinatus or the teres minor muscle. The current study showed the middle partition could be targeted accurately, whereas the superior and inferior partitions were more challenging. Ultrasound guidance may be necessary to ensure accurate placement into all parts of infraspinatus.
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http://dx.doi.org/10.1016/j.jelekin.2019.102354DOI Listing
December 2019

Kinematic and EMG analysis of horizontal bimanual climbing in humans.

J Biomech 2019 Jul 22;92:11-18. Epub 2019 May 22.

University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. Electronic address:

Climbing is an increasingly popular recreational and competitive behavior, engaged in a variety of environments and styles. However, injury rates are high in climbing populations, especially in the upper extremity and shoulder. Despite likely arising from an arboreal, climbing ancestor and being closely related to primates that are highly proficient climbers, the modern human shoulder has devolved a capacity for climbing. Limited biomechanical research exists on manual climbing performance. This study assessed kinematic and muscular demands during a bimanual climbing task that mimicked previous work on climbing primates. Thirty participants were recruited - 15 experienced and 15 inexperienced climbers. Motion capture and electromyography (EMG) measured elbow, thoracohumeral and trunk angles, and activity of twelve shoulder muscles, respectively, of the right-side while participants traversed across a horizontal climbing apparatus. Statistical parametric mapping was used to detect differences between groups in kinematics and muscle activity. Experienced climbers presented different joint motions that more closely mimicked the kinematics of climbing primates, including more elbow flexion (p = 0.0045) and internal rotation (p = 0.021), and less thoracohumeral elevation (p = 0.046). Similarly, like climbing primates, experienced climbers generally activated the shoulder musculature at a lower percentage of maximum, particularly during the exchange from support to swing and swing to support phase. However, high muscle activity was recorded in all muscles in both participant groups. Climbing experience coincided with a positive training effect, but not enough to overcome the high muscular workload of bimanual climbing. Owing to the evolved primary usage of the upper extremity for low-force, below shoulder-height tasks, bimanual climbing may induce high risk of fatigue-related musculoskeletal disorders.
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http://dx.doi.org/10.1016/j.jbiomech.2019.05.023DOI Listing
July 2019

The influence of posture variation on electromyographic signals in females obtained during maximum voluntary isometric contractions: A shoulder example.

J Electromyogr Kinesiol 2019 May 31:102319. Epub 2019 May 31.

Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. Electronic address:

Surface electromyography (sEMG) is commonly used to estimate muscle demands in occupational tasks. To allow for comparisons, sEMG amplitude is normalized to muscle specific maximum voluntary contractions (MVCs) performed in a standardized set of postures. However, maximal sEMG amplitude in shoulder muscles is highly dependent on arm posture and therefore, normalizing task related muscular activity to standard MVCs may lead to misinterpretation of task specific muscular demands. Therefore, the purpose of this study was to investigate differences in commonly monitored shoulder muscles using normalized sEMG amplitude between maximal exertions at different hand locations and across force exertion directions relative to standard MVCs. sEMG was recorded from the middle deltoid, pectoralis major sternal head, infraspinatus, latissimus dorsi, and upper trapezius. Participants completed standardized muscle-specific MVCs and two maximal exertions in 5 hand locations (low left, low right, high left, high right, and central) in each of the four force directions (push, pull, up, and down). Peak sEMG was analyzed in the direction(s) that elicited the highest signal for each muscle. All muscles differed by location (p < 0.05). Latissimus dorsi had the greatest activation during pulls (32-135% MVC); upper trapezius and middle deltoid while exerting upwards (73-103% and 42-78% MVC, respectively); infraspinatus while pushing (38-79% MVC); and pectoralis major activation was the highest during downwards exertions (48-84% MVC). Normalization of location specific maximal exertions to standard muscle specific MVCs underestimated maximal activity across 90% of the tasks in all shoulder muscles tested, except for latissimus dorsi where amplitudes were overestimated in low right hand location. Normalization of location specific muscle activity to standard muscle specific MVCs often underestimates muscle activity in task performance and is cautioned against if the goal is to accurately estimate muscle demands.
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http://dx.doi.org/10.1016/j.jelekin.2019.05.020DOI Listing
May 2019

Regional activation of supraspinatus and infraspinatus sub-regions during dynamic tasks performed with free weights.

J Electromyogr Kinesiol 2019 May 11:102308. Epub 2019 May 11.

University of Saskatchewan, College of Medicine, School of Rehabilitation Science, Saskatoon, Saskatchewan, Canada. Electronic address:

Growing evidence supports the existence of distinct anatomical sub-regions within supraspinatus and infraspinatus, but only recently has attention turned to exploring their potential functional differences. Using indwelling fine-wire electromyography, muscle activity was investigated from these sub-regions in 15 participants (mean 34 yr, 170 cm, 71.9 kg) during dynamic external rotation (ER), abduction, flexion, and scaption tasks with and without free weights corresponding to 50% and 75% of the participant's five repetition maximum. Electromyography data were normalized to isometric and isokinetic maximal voluntary contractions and activation ratios for each sub-region compared. Differences in mean regional activation ratios for supraspinatus and infraspinatus varied by arm posture, but were not influenced by load. Relative activation of posterior supraspinatus was greater during an ER task performed in side lying compared to an ER task performed with 90° of humeral elevation in seated and prone postures. Relative activation of superior infraspinatus was greater during an ER task in prone and side lying postures compared to flexion and scaption. Similar results were found when comparing regional muscle activation ratios for infraspinatus between tasks regardless of normalization method employed. These findings may impact exercise selection in the non-operative management of rotator cuff tears.
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http://dx.doi.org/10.1016/j.jelekin.2019.05.009DOI Listing
May 2019

The utility of the acromion marker cluster (AMC) in a clinical population.

J Electromyogr Kinesiol 2019 Apr 15:102298. Epub 2019 Apr 15.

Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Canada.

Introduction: The acromion marker cluster (AMC) is a non-invasive scapular motion tracking method. However, it lacks testing in clinical populations, where unique challenges may present. This investigation resolved the utility of the AMC approach in a compromised clinical population.

Methods: The upper body of breast cancer survivors (BCS) and controls were tracked via motion capture and scapular landmarks palpated and recorded using a digitizer at static neutral to maximum elevation postures. The AMC tracked the scapula during dynamic maximum arm abduction. Both single (SC) and double calibration (DC) methods were applied to calculate scapular angles. The influences of calibration method, elevation, and group on mean and absolute error with two-way fixed ANOVAs with interactions (p < 0.05). Root mean square errors (RMSE) were calculated and compared.

Results: DC improved AMC estimation of palpated scapular orientation over SC, especially at higher arm elevations; RMSE averaged 11° higher for SC than DC at maximum elevation, but the methods were only 2.2° different at 90° elevation. DC of the AMC yielded mean error values of ∼5-10°. These approximate errors reported for AMC with young, lean adults.

Conclusions: The AMC with DC is a non-invasive method with acceptable error for measuring scapular motion of BCS and age-matched controls.
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http://dx.doi.org/10.1016/j.jelekin.2019.04.002DOI Listing
April 2019

Examining upper extremity muscle demand during selected push-up variants.

J Electromyogr Kinesiol 2019 Feb 28;44:165-172. Epub 2018 Dec 28.

Department of Kinesiology, University of Waterloo, Waterloo, Canada. Electronic address:

Push-ups are regularly adapted for specific muscular demands. The push-up plus (PUP) has been used to emphasize serratus anterior activation. Alterations in body posture have been suggested for targeted activation of muscles surrounding the shoulder and rotator cuff, but little data exists to inform these changes. The purpose of this research was to examine upper extremity muscle activity changes when varying aspects of the PUP. Healthy male participants (n = 20) performed PUP trials using combinations of hand contact area (knuckles/palms), humeral rotation (neutral/60° internal/60° external) and hand location (shoulder height/±30% arm length superior/inferior) at a fixed cadence. Electromyographic (EMG) activation from 14 muscles of the right upper extremity during each trial were examined, as well as ratios of activation for 7 muscle pairs. Palm contact and a superior hand location increased serratus activation, but absolute EMG changes between these effects were marginal (standard error = 3.8). Each independent variable altered mean EMG for most muscles as a main effect, with the largest changes in infraspinatus, upper and lower trapezius, and anterior deltoid. Altering the standard PUP does not seem to enhance serratus activation, but diverse responses in other muscles have training implications. These changes in muscle strategies can tailor the use of modified PUP exercises to target individual muscles or groups.
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http://dx.doi.org/10.1016/j.jelekin.2018.12.008DOI Listing
February 2019

Arm posture influences on regional supraspinatus and infraspinatus activation in isometric arm elevation efforts.

J Electromyogr Kinesiol 2019 Feb 15;44:108-116. Epub 2018 Dec 15.

University of Waterloo, Faculty of Applied Health Sciences, Department of Kinesiology, Digital Industrial Ergonomics and Shoulder Evaluation Laboratory (DIESEL), Waterloo, Ontario, Canada. Electronic address:

This study aimed to evaluate the effect of arm posture on activation of the anterior and posterior regions of supraspinatus and the superior and middle regions of infraspinatus during resisted isometric arm elevations. Thirty-one healthy participants performed 18 isometric resistance exertions against a force cube in three elevation planes (flexion, scaption, abduction) and three elevation angles (30°, 90°, 150°) in maximal and sub-maximal resistance conditions. EMG data were obtained using four pairs of fine wire electrodes. The mean activation of each region and the activation ratios were compared across postures using ANOVAs. Supraspinatus anterior was significantly more active during abduction and scaption, and in higher elevation angles, while the posterior region showed similar activation levels across postures. Infraspinatus regions were more active during flexion with more relative activation of the infraspinatus superior at 90° flexion. The results suggest that regional activation of supraspinatus and infraspinatus should be considered for assessment and rehabilitation purposes. In any clinical setting where it is important to reduce the stress on the supraspinatus anterior, isometric flexion exercises performed with arm in low elevation angles could provide the opportunity to strengthen the posterior region of supraspinatus with limited stress on the anterior region. Beside external rotation exertions, resisted flexion tests may be useful for evaluation of infraspinatus regions.
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http://dx.doi.org/10.1016/j.jelekin.2018.12.005DOI Listing
February 2019

Activation of Supraspinatus and Infraspinatus Partitions and Periscapular Musculature During Rehabilitative Elastic Resistance Exercises.

Am J Phys Med Rehabil 2019 05;98(5):407-415

From the Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada.

Objective: The purpose of this study was to quantify the activation of partitions within supraspinatus and infraspinatus and some periscapular muscles during four resistance exercises with elastic bands.

Design: Twenty-seven right-handed healthy volunteers (age, 22.5 ± 2.7 yrs) were recruited. Intramuscular electromyography from supraspinatus (anterior and posterior) and infraspinatus (superior and middle) and surface electromyography data from the upper, middle, and lower trapezius and serratus anterior were recorded during four elastic resistance exercises (Y, T, W, L). Kinematics were recorded synchronously. Electromyography values were presented as percentage of maximal voluntary isometric contraction and compared across exercises using analysis of variance. Muscle activation ratios were also calculated.

Results: The mean activations of all rotator cuff partitions were more than 40% maximal voluntary isometric contraction, except middle infraspinatus during the T exercise (29.3% maximal voluntary isometric contraction). Serratus anterior activity was significantly higher during the Y exercise (P < 0.008). Lower trapezius was activated more than 80% maximal voluntary isometric contraction in all four exercises with higher contributions compared with the upper trapezius.

Conclusions: The investigated exercises induced moderate to high activation in supraspinatus and infraspinatus partitions and very high activation in lower trapezius. YTWL exercises are appropriate for strengthening of some rotator cuff and periscapular muscles and for late stages of shoulder rehabilitation.
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http://dx.doi.org/10.1097/PHM.0000000000001116DOI Listing
May 2019

Algorithmically detectable directional changes in upper extremity motion indicate substantial myoelectric shoulder muscle fatigue during a repetitive manual task.

Ergonomics 2019 Mar 10;62(3):431-443. Epub 2019 Jan 10.

a Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Waterloo, Ontario, Canada.

Repetitive workplace tasks are associated with fatigue-induced changes to shoulder muscular strategies, potentially altering kinematics and elevating susceptibility to tissue overexposures. Accessible and reliable methods to detect shoulder muscle fatigue in the workplace are therefore valuable. Detectable changes in joint motion may provide a plausible fatigue identification method. In this investigation, the onset of the first kinematic changes, as identified by a symbolic motion representation (SMSR) algorithm, and the onset of substantial surface electromyography (sEMG) mean power frequency (MPF) fatigue were not significantly different, both occurring around 10% of task duration. This highlights the potential utility of SMSR identified directional changes in joint motion during repetitive tasks as a cue of substantial muscle fatigue, enabling ergonomics responses that can mitigate shoulder muscular fatigue accumulation and its associated deleterious physical effects. Practitioner Summary: The onset of substantial muscle fatigue during a repetitive dynamic task was assessed using kinematics and myoelectric-based techniques. Algorithmically detectable directional changes in upper extremity joint motion occurred with the onset of substantial muscle fatigue, highlighting the potential of this as a useful approach for workplace fatigue identification.
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http://dx.doi.org/10.1080/00140139.2018.1536808DOI Listing
March 2019

An evaluation of off-axis manual forces and upper extremity joint moments during unilateral pushing and pulling exertions.

Ergonomics 2019 Jan 21;62(1):52-64. Epub 2019 Jan 21.

a Department of Kinesiology, Faculty of Applied Health Sciences , University of Waterloo , Waterloo , Ontario , Canada.

This study quantified changes in off-axis manual force production and upper extremity joint moments during sub-maximal one-handed push and pull tasks. Off-axis forces in the up/down and left/right directions were quantified in the presence or absence of constraints placed upon the direction of manual force application and/or arm posture. Resultant off-axis forces of 13.1% and 9.4% were produced for pulls and pushes, respectively. Off-axis forces during pulling were oriented downwards and to the right and were associated with a decreased should flexion moment when posture was constrained. Off-axis forces in the up/down direction were minimized with increased on-axis force level. Off-axis forces during pushing tended to be oriented to the left and were associated with increased elbow flexion moment when off-axis forces were allowed. By not accounting for these off-axis forces, we may not be accurately reflecting actionable muscle- and joint-level loading characteristics derived from biomechanically-based proactive ergonomics assessment approaches. Practitioner Summary: Constrained arm postures and directions of manual force application influence the production of off-axis forces. As inaccurate estimation of true manual forces can markedly influence actionable outcomes of proactive ergonomic assessments, this study suggests that simplification of these estimates is insufficient and potentially misleading.
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http://dx.doi.org/10.1080/00140139.2018.1525501DOI Listing
January 2019

Regional activation of anterior and posterior supraspinatus differs by plane of elevation, hand load and elevation angle.

J Electromyogr Kinesiol 2018 Dec 29;43:14-20. Epub 2018 Aug 29.

Department of Kinesiology, University of Waterloo, 200 University Avenue W, Waterloo, ON N2L 3G1, Canada. Electronic address:

The supraspinatus is one of the muscles of the rotator cuff, and growing research on fibre type composition and mechanical advantages in specific postures suggest this muscle may have distinct anterior and posterior regions. Activation differences between these regions may identify important functional differences. This research quantified muscular activation of these regions throughout a range of motion with differing hand loads. Forty participants completed paced humeral elevations in 7 planes of elevation (0/15/30/40/60/75/90°) using 3 hand loads (unloaded arm/20%/40% maximal elevation strength). Indwelling electromyography collected muscle activity of the anterior and posterior supraspinatus. Hand load and elevation angle interacted to affect activity of the anterior supraspinatus in most planes of elevation - by up to 41 %MVC (p < 0.01), but in few planes for the posterior region. Plane of elevation influenced anterior and posterior region activation by up to 17 %MVC and 13 %MVC, respectively (p < 0.01). Increasing hand loads increased activation in both regions (p < 0.01), but more so for the anterior region. These differences may indicate differences in function between the two regions. The sustained activation in the smaller posterior supraspinatus may indicate this region as primarily a glenohumeral stabilizer, while the larger anterior region acts to achieve glenohumeral motion.
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http://dx.doi.org/10.1016/j.jelekin.2018.08.003DOI Listing
December 2018

Workstation configuration and container type influence upper limb posture in grocery bagging.

Appl Ergon 2018 Nov 30;73:206-213. Epub 2018 Jul 30.

Department of Health Sciences, University of Saskatchewan, Saskatoon, SK, Canada; Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. Electronic address:

Introduction: Repetitive movements and awkward postures are two persistent injury risk factors for grocery store cashiers. Due to the recent rise in popularity of environmentally-friendly grocery bagging options, current recommendations for cashiers are likely outdated. Correspondingly, the objective of this study was to examine the effects of cashier-specific work demands, workstation configuration, and container type on upper limb postures during typical job activities.

Methods: Fifteen experienced cashiers bagged groceries at varying combinations of workstation height (low, medium, high) and container type (reusable bins, reusable bags, plastic bags). Upper limb movement was quantified with motion capture and amplitude probability distribution functions of humeral elevation and humeral axial internal rotation were used to assess the static (10th percentile), median (50 percentile), and peak (90 percentile) postural demands, which were then interpreted in the context of existing postural guidelines.

Results: High workstation height and reusable bags increased right arm elevation at peak posture by 15.7° compared to the low workstation height and reusable bin combination. However, reusable bins increased internal rotation demands of the right arm by 4.3° compared to other container types. Left arm elevation and internal rotation were consistently lower than right arm angles.

Conclusion: Cashiers are encouraged to adjust the workstation to decrease the arm elevation and internal rotation required by higher workstation heights and tall containers, and to use both arms for scanning and packing, when possible, to reduce undesirable arm postures.
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http://dx.doi.org/10.1016/j.apergo.2018.07.012DOI Listing
November 2018

Fast Forward-Dynamics Tracking Simulation: Application to Upper Limb and Shoulder Modeling.

IEEE Trans Biomed Eng 2019 02 17;66(2):335-342. Epub 2018 May 17.

Objective: Musculoskeletal simulation can be used to estimate muscle forces in clinical movement studies. However, such simulations typically only target movement measurements and are not applicable to force exertion tasks which are commonly used in rehabilitation therapy. Simulations can also produce nonphysiological joint forces or be too slow for real-time clinical applications, such as rehabilitation with real-time feedback. The objective of this study is to propose and evaluate a new formulation of forward-dynamics assisted tracking simulation that incorporates measured reaction forces as targets or constraints without any additional computational cost.

Methods: We illustrate our method with idealized proof-of-concept models and evaluate it with two upper limb cases: Tracking of hand reaction forces during an isometric force-generation task and constraining glenohumeral joint reaction forces for stability during arm elevation.

Results: We show that the addition of reaction force optimization terms within our simulations generates plausible muscle force predictions for these tasks, which are strongly related to reaction forces in addition to movement. Execution times for all models tested were not different when run with or without the reaction force optimization term, ensuring that the simulations are fast enough for real-time clinical applications.

Conclusion: Our novel reaction force optimization term leads to more realistic shoulder reaction forces, without any additional computational costs.

Significance: Our formulation is not only valuable for shoulder simulations, but could be used in various clinical situations (e.g., for different joints and rehabilitation therapy tasks) where the direction and/or magnitude of reaction forces are of interest.
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http://dx.doi.org/10.1109/TBME.2018.2838020DOI Listing
February 2019

The effect of aging and contextual information on manual asymmetry in tool use.

Exp Brain Res 2018 Aug 8;236(8):2347-2362. Epub 2018 Jun 8.

Department of Kinesiology, University of Waterloo, 200 University Avenue West, Waterloo, ON, N2L 3G1, Canada.

Healthy aging affects manual asymmetries in simple motor tasks, such as unilateral reaching and aiming. The effects of aging on manual asymmetries in the performance of a complex, naturalistic task are unknown, but are relevant for investigating the praxis system. This study examined how aging influences manual asymmetry in different contexts in a tool manipulation task. Fifty healthy, right-hand-dominant young (N = 29; 21.41 ± 2.87 years), and elderly (N = 21; mean: 74.14 ± 6.64 years) participants performed a 'slicing' gesture in response to a verbal command in two contexts: with (tool) and without the tool (pantomime). For interjoint relationships between shoulder plane of elevation and elbow flexion, a HAND × AGE × CONTEXT interaction existed (F = 4.746, p = 0.035). In pantomime, interjoint control deviated more in the left (non-dominant) than the right (dominant) limb in the elderly adult group (Wilcoxon, p = 0.010). No such differences existed in the young adult group (Wilcoxon, p = 0.471). Furthermore, contextual information reduced interjoint deviation in young adults when the task was performed with the right (dominant) hand (Wilcoxon, p = 0.001) and in the elderly adults when the task was performed with the left (non-dominant) hand (Wilcoxon, p = 0.012). The presence of the tool did not reduce interjoint deviation for the right hand in the elderly group (Wilcoxon, p = 0.064) or the left hand in the young group (Wilcoxon, p = 0.044). Deviation within trials (i.e., intrasubject deviation) in elbow flexion was higher in the elderly relative to the young adult group (p = 0.003). Finally, resultant peak velocities were smaller (p = 0.002) and cycle duration longer (p < 0.0001) in the elderly adult group. This study provides novel evidence that aging affects manual asymmetries and sensorimotor control in a naturalistic task and warrants that aging research considers the context in which the task is performed.
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http://dx.doi.org/10.1007/s00221-018-5304-5DOI Listing
August 2018

Female maximal push/pull strength capabilities by humeral abduction angle in bilateral exertions.

Appl Ergon 2018 Jul 20;70:136-141. Epub 2018 Mar 20.

Department of Kinesiology, University of Waterloo, 200 University Avenue W, Waterloo, N2L 3G1, ON, Canada. Electronic address:

Pushing or pulling an object constitutes the majority of manual materials handling tasks. Anthropometric differences between workers alter pushing and pulling strategies at fixed heights, potentially modifying exposures and causing overexertion. Capability normalized to arm posture rather than work height remains unknown, and the purpose of this research was to quantify maximal pushing and pulling strength and upper extremity joint moments using fixed humeral abduction angles and a flexed arm position. Twenty university-aged females completed maximal pushes and pulls at 0°, 30°, 60°, 90°, and 120° of humeral abduction with an elbow to 90°. Abduction angle was the largest modifier of both push and pull force generating capability (p < 0.01), with increasing abduction reducing force capability by up to 30%. While push exertions on average exceeded pulls by 17 N (p < 0.01), the difference is much smaller than previous reports. Ergonomists should consider humeral angle of the worker rather than work heights, as individuals working in increased abduction decreases capability and increases potential overexertion injury risk.
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http://dx.doi.org/10.1016/j.apergo.2018.02.023DOI Listing
July 2018