Age-related changes in normal isometric shoulder strength

Older age is associated with decreased overall shoulder strength but not direction-specific differences in the three-dimensional feasible torque space

Shoulder strength is reduced in older adults but has only been assessed in planar motions that do not reflect the diverse requirements of daily tasks. We quantified the impact of age on strength spanning the three degrees of freedom relevant to shoulder function, referred to as the feasible torque space. We hypothesized that the feasible torque space would differ with age and expected this age-effect to reflect direction-specific deficits. We measured strength in 32 directions to characterize the feasible torque space of the shoulder in participants without shoulder pain or tendinous pathology (n = 39, 19-86 years). We modeled the feasible torque space for each participant as an ellipsoid, computed the ellipsoid size and direction-specific metrics (ellipsoid position, orientation, and shape), and then tested the effect of age on each metric. Age was negatively associated with ellipsoid size (a measure of overall strength magnitude; -0.0033 ± 0.0007 (Nm/kg)/year, p < 0.0001). Contrary to our expectation, the effect of age on the direction-specific metrics did not reach statistical significance. The effect of age did not differ significantly between male and female participants. Three-dimensional strength measurements allowed us to constrain the direction of participants' maximum torque production and characterize the entire feasible torque space. Our findings support a generalized shoulder strengthening program to address age-related shoulder weakness in those without pain or pathology. Clinical exam findings of imbalanced weakness may suggest underlying pathology beyond an effect of age. Longitudinal studies are needed to determine the positive or negative impact of our results.

The effects of exoskeleton use on human response to simulated overhead tasks with vibration

The use of occupational exoskeletons has grown fast in manufacturing industries in recent years. One major scenario of exoskeleton use in manufacturing is to assist overhead, power hand tool operations. This preliminary work aimed to determine the effects of arm-supporting exoskeletons on shoulder muscle activity and human-hand tool coupling in simulated overhead tasks with axially applied vibration. An electromagnetic shaker capable of producing the random vibration spectrum specified in ISO 10819 was hung overhead to deliver vibrations. Two passive, arm-supporting exoskeletons, with one (ExoVest) transferring load to both the shoulder and pelvic region while the second one (ExoStrap) transferring load primarily to the pelvic region, were used in testing. Testing was also done with the shaker placed in front of the body to better understand the posture and exoskeleton engagement effects. The results collected from 6 healthy male subjects demonstrate the dominating effects of the overhead working posture on increased shoulder muscle activities. Vibration led to higher muscle activities in both agonist and antagonist shoulder muscles to a less extent. Exoskeleton use reduced the anterior deltoid and serratus anterior activities by 27% to 43%. However, wearing the ExoStrap increased the upper trapezius activities by 23% to 38% in the overhead posture. Furthermore, an increased human-shaker handle coupling was observed in the OH posture when wearing the ExoVest, indicating a more demanding neuromuscular control.

Choosing breast-conserving therapy or mastectomy and subpectoral implant breast reconstruction: implications for pectoralis major function

BACKGROUND

An increasing number of women are choosing mastectomy and subpectoral implant (SI) breast reconstruction over breast-conserving therapy (BCT). It is unclear to what extent these procedures differ in their effect on the pectoralis major (PM). The purpose of this study was to assess the impact of choosing BCT or SI breast reconstruction on PM function.

METHODS

Ultrasound shear wave elastography images were acquired from the PM fiber regions and surface electromyography obtained activity from six shoulder muscles, while 14 BCT participants, 14 SI participants, and 14 age-matched controls remained at rest or generated submaximal shoulder torques.

RESULTS

BCT and SI participants were significantly weaker in shoulder adduction, while BCT participants were also weaker in internal and external rotation (all p ≤ 0.003). PM function was altered following either BCT or SI. In all treatment groups, the clavicular fiber region contributed primarily to flexion, and the sternocostal primarily contributed to adduction. However, healthy participants utilized the clavicular region more during adduction and the sternocostal region more during flexion when compared to BCT or SI participants (all p ≤ 0.049). The still intact clavicular region increased its contributions to flexion torques in SI participants compared to controls (p = 0.016). Finally, BCT and SI participants compensated for changes in PM function using synergistic shoulder musculature.

CONCLUSION

Both BCT and SI breast reconstruction result in significant long-term upper extremity strength deficits. Our results suggest changes to the underlying function of the PM and the adoption of unique but inadequate neuromuscular compensation strategies drive these deficits.

Sex differences in strength at the shoulder: a systematic review

Background

Understanding differential strength capability between sexes is critical in ergonomics and task design. Variations in study designs and outcome measures generates challenges in establishing workplace guidelines for strength requirements to minimize upper extremity risk for workers. The purpose of this systematic review was to collate and summarize sex differences in strength at the shoulder across movement directions and contraction types.

Methods

A total of 3,294 articles were screened from four databases (Embase, Medline, SCOPUS, and Web of Science). Eligibility criteria included observational studies, direct measurement of muscular joint, and healthy adult participants (18-65 years old). Strength outcome measures were normalized to percentages of male outputs to allow comparisons across articles.

Results

A total of 63 studies were included within the final review. Majority of articles observed increased strength in males; the gap between male-female strength was greater in flexion and internal/external rotation, with females generating ~30% of male strength; scaption strength ratios were most consistent of the movement groups, with females generating 55-62% of male strength.

Conclusion

Sex strength differences should be considered as an important factor for workplace task design as women are more at risk for occupational-related injuries than men in equivalent strength requirements. Differences in strength were not synonymous across motions; females demonstrated increased disparity relative to male strength in horizontal flexion/extension, forward flexion and internal/external rotation. Some movements had an extremely limited pool of available studies for examination which identified critical research gaps within the literature. Collating and quantifying strength differences is critical for effective workstation design with a range of users to mitigate potential overexertion risk and musculoskeletal injury.

Scalable musculoskeletal model for dynamic simulations of upper body movement

A musculoskeletal (MSK) model is a valuable tool for assessing complex biomechanical problems, estimating joint torques during motion, optimizing motion in sports, and designing exoskeletons and prostheses. This study proposes an open-source upper body MSK model that supports biomechanical analysis of human motion. The MSK model of the upper body consists of 8 body segments (torso, head, left/right upper arm, left/right forearm, and left/right hand). The model has 20 degrees of freedom (DoFs) and 40 muscle torque generators (MTGs), which are constructed using experimental data. The model is adjustable for different anthropometric measurements and subject body characteristics: sex, age, body mass, height, dominant side, and physical activity. Joint limits are modeled using experimental dynamometer data within the proposed multi-DoF MTG model. The model equations are verified by simulating the joint range of motion (ROM) and torque; all simulation results have a good agreement with previously published research.

Scalable musculoskeletal model for dynamic simulations of lower body movement

A musculoskeletal (MSK) model is an important tool for analysing human motions, calculating joint torques during movement, enhancing sports activity, and developing exoskeletons and prostheses. To enable biomechanical investigation of human motion, this work presents an open-source lower body MSK model. The MSK model of the lower body consists of 7 body segments (pelvis, left/right thigh, left/right leg, and left/right foot). The model has 20 degrees of freedom (DoFs) and 28 muscle torque generators (MTGs), which are developed from experimental data. The model can be modified for different anthropometric measurements and subject body characteristics, including sex, age, body mass, height, physical activity, and skin temperature. The model is validated by simulating the torque within the range of motion (ROM) of isolated movements; all simulation findings exhibit a good level of agreement with the literature.

The effect of handedness on upper extremity isometric strength symmetry

BACKGROUND

Understanding upper extremity strength symmetry can have important implications for ergonomics assessment and design. Few studies have examined isometric joint strength symmetry of left-handed individuals, or examined how handedness can influence strength. As such, the purpose of this study was to investigate the influence of handedness on dominant/non-dominant (D/ND) strength ratio in several moment directions about the shoulder, elbow, wrist, and hand. It was hypothesized that the isometric strength symmetry of left-handed individuals would be significantly different from that of right-handed individuals.

METHODS

The study recruited 28 participants to perform a series of maximal voluntary isometric contractions (MVCs) with both arms for various efforts about the shoulder, elbow, and wrist, as well as handgrip for a total of 68 MVCs. Strength symmetry ratios were computed, and a two-way mixed-model ANOVA evaluated the effects of handedness and MVC test on strength symmetry.

INTERPRETATION

Significant differences in D/ND ratios between right and left-handed individuals were found for 11 of the 17 explored exertion directions. Left-handed individuals tended to possess greater strength in their non-dominant limb, while right-handed individuals tended to have greater strength in their dominant limb. Left- and right-handed individuals often significantly differed in D/ND ratio, suggesting that separate normative values should be created to account for handedness when considering return-to-work or strength-based ergonomics design criterion.

Age- and sex-related differences in upper-body joint and endpoint kinematics during a drinking task in healthy adults

Background

The objective kinematic assessments of activities of daily living are desired. However, the limited knowledge regarding age- and sex-related differences prevents the adaptation of these measurements to clinical settings and in-home exercises. Therefore, this study aimed to determine the effects of age and sex on joint and endpoint kinematics during a common activity of daily living, specifically, drinking from a glass.

Methods

In total, 32 healthy adults (18 males and 14 females) aged 22-77 years performed a drinking task comprising reaching for a glass, bringing it forward and sipping, returning it, and placing the hand back to the starting position, which was recorded using a three-dimensional motion-capturing system. A two-way analysis of variance was used to statistically compare joint angles at five different time points and endpoint kinematic variables in the four drinking phases between older and younger age groups and sexes.

Results

Wrist radial deviation was greater in older adults than in younger participants at all five different time points (F = 5.16-7.34, p ≤ 0.03, η2 = 0.14-0.21). Moreover, lesser shoulder abduction and greater shoulder internal rotation and forearm pronation when moving and returning the hand to the starting position were observed in the female group than in the male group (F = 4.21-20.03, p ≤ 0.0497, η2 = 0.13-0.41). Trunk flexion was lower in the female group than in the male group at all time points (F = 4.25-7.13, p ≤ 0.0485, η2 = 0.12-0.19). Regarding endpoint kinematics, the performance time in the reaching phase was longer in older adults than in younger individuals (F = 4.96, p = 0.03, η2 = 0.14). Furthermore, a shorter time while returning the hand to the starting position was observed in the female group than in the male group (F = 9.55, p < 0.01, η2 = 0.22).

Conclusions

The joint kinematics of drinking were partially characterized by an age effect, whereas endpoint kinematics were scattered in all drinking phases. Sex-related effects in most upper-body motions and postures may cause rapid motions in females. Therefore, clinicians could use this knowledge for precise assessments and to suggest feasible in-home exercises.

Normative Values of Isometric Shoulder Strength Among Healthy Adults

Background

Normative data is useful for comparing measured values of strength with population norms and can avoid the issues associated with limb symmetry index. The available normative shoulder strength values are limited by constraints on research designs and variability in subject groups which prevents this data being successfully extrapolated to the greater population.

Purpose

The purpose of this study was to establish normative isometric strength values for various movements of the shoulder that are specific to function and rotator cuff strength. A secondary goal of this study was to analyze the effect of age, gender, weight, height, activity level and arm dominance on shoulder strength.

Design

Observational cohort study.

Methods

Subjects in four age groups (20-29, 30-39, 40-49, 50-59) were included in this study-200 males (40.0 ± 11.6 years, 179.1 ± 6.5 cm, 81 ± 13.0 kg) and 200 females (40.1 ± 11.5 years, 165.3 ± 7.4sm, 64.4 ± 11.6 kg). Bilateral isometric strength measurements were taken with a handheld dynamometer testing seven shoulder movements. Tables of normative strength data were constructed. Multivariate analyses were performed to analyze the effects of age, gender, weight, height and activity level on isometric shoulder strength.

Results

Men were stronger than women (p<0.001). Age was not associated with most strength measures with the exception of dominant arm abduction (p<0.004), non-dominant arm abduction (p<0.028) and non-dominant arm scapular plane abduction (p<0.004) which had a negative association with strength. Weight was positively associated with strength (p<0.001). Activity level was positively associated with all strength measures (p<0.05) except dominant sided abduction (p=0.056). There were no statistically significant differences between dominant and non-dominant sides.

Conclusion

This normative data may be useful to the clinician, as it permits a standard against which to compare shoulder strength for various age groups. Clinicians can have confidence that the uninvolved limb, if symptom free, can be used as an adequate benchmark for strength measures.

Levels of Evidence

Level 3©The Author(s).

Load-Induced Glenohumeral Translation After Rotator Cuff Tears: Protocol for an In Vivo Study

BACKGROUND

Rotator cuff tears are a common shoulder injury, but they sometimes remain undiagnosed, as symptoms can be limited. Altered shoulder biomechanics can lead to secondary damage and degeneration. In biomechanical analyses, the shoulder (ie, the glenohumeral joint) is normally idealized as a ball-and-socket joint, even though a translation is often observed clinically. To date, no conclusive changes in glenohumeral translation have been reported in patients with rotator cuff tears, and it is unknown how an additional handheld weight that is comparable to those used during daily activities will affect glenohumeral translations in patients with rotator cuff tears.

OBJECTIVE

This study aims to assess the load-induced glenohumeral translation (liTr) in patients with rotator cuff tears and its association with the load-induced changes in muscle activation (liMA).

METHODS

Patients and asymptomatic controls will be recruited. Participants will fill out health questionnaires and perform 30° arm abduction and adduction trials, during which they will hold different handheld weights of a maximum of 4 kg while motion capture and electromyographic data are collected. In addition, fluoroscopic images of the shoulders will be taken for the same movements. Isometric shoulder muscle strength for abduction and rotation will be assessed with a dynamometer. Finally, shoulder magnetic resonance images will be acquired to assess muscle status and injury presence. The dose-response relationship between additional weight, liTr, and liMA will be evaluated.

RESULTS

Recruitment and data collection began in May 2021, and they will last until the recruitment target is achieved. Data collection is expected to be completed by the end of 2022. As of November 2022, data processing and analysis are in progress, and the first results are expected to be submitted for publication in 2023.

CONCLUSIONS

This study will aid our understanding of biological variations in liTr, the influence of disease pathology on liTr, the potential compensation of rotator cuff tears by muscle activation and size, and the association between liTr and patient outcomes. The outcomes will be relevant for diagnosis, treatment, and rehabilitation planning in patients with rotator cuff tears.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04819724; https://clinicaltrials.gov/ct2/show/NCT04819724.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

DERR1-10.2196/43769.

Impact of Age on Shoulder Range of Motion and Strength

Background

Total shoulder arthroplasty (TSA) is a surgical technique commonly used to treat patients with arthritis and rotator cuff deficiency. Its purpose is to reduce pain and improve shoulder function, namely range of motion (ROM) and strength. While shoulder ROM and strength have been studied extensively in patients with various shoulder pathologies, there is a dearth of knowledge with regard to the asymptomatic population.

Methods

A cross-sectional study was conducted in the outpatient orthopaedic clinic following institutional review board approval. Patients 18 years of age and older with at least one asymptomatic and healthy shoulder with no prior history of shoulder surgery, injury, or pain were enrolled in the study. Demographic information, ROM, and strength measurements were collected for 256 shoulders, evenly stratified into groups by age and sex. A goniometer was used to measure forward elevation, abduction, and external rotation, and a handheld dynamometer was utilized for measuring strength. Statistical evaluation was conducted using Pearson correlations, analysis of variance, and Bonferroni and Mann–Whitney post hoc tests, with P < .01 indicating a significant difference.

Results

Abduction strength (P < .001), external rotation strength (P < .001), and internal rotation strength (P < .001) were negatively correlated with age when viewing the data as a whole and after stratification of males and females. Age and shoulder ROM, namely abduction (P < .001) and forward elevation (P < .001), were also significantly negatively correlated, although internal rotation decreased with age as well. When comparing across age groups, abduction (P = .001) and forward elevation (P = .001) were significantly higher in group 1 (18-35) when compared to group 4 (66+), but external rotation was not significantly different between these groups. External rotation (P = .001) was only significantly different between groups 2 (36-50) and 4. Variation in external rotation strength was also found. Group 4 was found to have significantly less strength than all 3 of the other groups.

Conclusion

Shoulder strength significantly decreased with age, with abduction strength and external rotation strength displaying the strongest negative correlations. Decreases in strength were most prominent in patients 66 years of age and above. Shoulder ROM was not as tightly correlated with age, although abduction, forward elevation, and internal rotation were found to generally decrease over time. Differences in external rotation were not clinically significant. These correlations provide useful controls for patients of various ages regarding their clinical outcomes when presenting with shoulder pathology. Variations in current literature allow this study to verify the impact of age on shoulder ROM and strength.

An evaluation of upper limb strength and range of motion of breast cancer survivors immediately following treatment

BACKGROUND

There is a growing number of breast cancer survivors from improved cancer treatments. However, treatments often impair upper limb function, specifically range of motion and strength, reducing quality of life and function. The primary purpose of this study was to quantify differences in strength and range of motion following treatment. The secondary purpose aimed to measure the activation of each upper limb muscle in the completion of tasks.

METHODS

29 breast cancer survivors were categorized into two groups based on time-since-treatment: 1) up to 1-year post-treatment, and 2) 1 to 2 years post-treatment. Participants completed maximal strength and range of motion tasks. During trials eight muscles were monitored bilaterally. Maximal force output was taken during strength trials, and kinematics were monitored during range of motion trials. A 2 by 2 mixed ANOVA (limb (affected, unaffected) x time-since-treatment) examined interaction and main effects of these factors on task peak force, angle and mean activation.

FINDINGS

Time-since-treatment influenced strength (flexion, extension, internal and external rotation) and range of motion (flexion, scapular abduction), wherein the group further from treatment had 11.5-15.5° less range of motion and 27.7-43.6 N less force production. A main effect of time-since-treatment influenced muscular behaviours during both tasks, where activation was higher in the group 1-2 years from treatment.

INTERPRETATION

Effects of treatment may manifest in a delayed manner whereby strength and range of motion are reduced in breast cancer survivors to a greater extent in those who are past 1 year of treatment cessation.

In Vivo Static Retraction and Dynamic Elongation of Rotator Cuff Repair Tissue After Surgical Repair: A Preliminary Analysis at 3 Months

Background:

Rotator cuff repair is a common orthopaedic procedure that provides pain relief for many patients, but unfortunately, an estimated 20% to 70% of repair procedures will fail. Previous research has shown that elongation (ie, retraction) of a repaired tendon is common even in patients with a repair construct that appears intact on magnetic resonance imaging. However, it is unknown how this repair tissue functions under dynamic conditions.

Purpose:

To quantify static retraction and maximum dynamic elongation of repair tissue after rotator cuff repair.

Study Design:

Case series; Level of evidence, 4.

Methods:

Data from 9 patients were analyzed for this study. During surgery, a 3.1-mm tantalum bead was sutured to the supraspinatus tendon, medial to the repair site. Glenohumeral kinematics were assessed at 1 week (static) and 3 months (static and during scapular-plane abduction) after surgery using a biplanar videoradiographic system. The 3-dimensional position of the bead was calculated relative to the tendon’s insertion on the humerus (ie, bead-to-insertion distance). Static retraction was calculated as the change in the bead-to-insertion distance under static conditions between 1 week and 3 months after surgery, and maximum dynamic elongation was calculated as the maximal positive change in the bead-to-insertion distance during dynamic motion relative to the start of motion. The magnitudes of static retraction and maximum dynamic elongation were assessed with 1-sample t tests.

Results:

At 3 months after surgery, static retraction occurred in all patients by a mean of 10.0 ± 9.1 mm (P = .01 compared with no elongation). During scapular-plane abduction, maximum dynamic elongation averaged 1.4 ± 1.0 mm (P < .01 compared with no elongation). Descriptively, dynamic elongation consistently took 1 of 2 forms: an initial increase in the bead-to-insertion distance (mean, 2.0 ± 0.6 mm) before decreasing until the end of motion or an immediate and substantial decrease in the bead-to-insertion distance at the onset of motion.

Conclusion:

Repair tissue elongation (static retraction and maximum dynamic elongation) appeared to be a common and significant finding at 3 months after arthroscopic rotator cuff repair. Dynamic elongation of repair tissue during scapular-plane abduction exhibited 1 of 2 distinct patterns, which may suggest different patterns of supraspinatus mechanical and neuromuscular function.

Rate of improvement in shoulder strength after anatomic and reverse total shoulder arthroplasty

Background

Methods

Results

Conclusion

Presurgical tear characteristics and estimated shear modulus as predictors of repair integrity and shoulder function one year after rotator cuff repair

Background

Rotator cuff repair provides pain relief for many patients; however, retears are relatively common and affect approximately 20%-70% of patients after repair. Although magnetic resonance imaging (MRI) offers the ability to assess tissue characteristics such as tear size, retraction, and fatty infiltration, it provides little insight into the quality of the musculotendinous tissues the surgeon will encounter during surgery. However, shear wave elastography (SWE) could provide an indirect assessment of quality (ie, stiffness) by measuring the speed of shear waves propagating through tissue. The objective of this study was to determine the extent to which estimated shear modulus predicts repair integrity and functional outcomes 1 year after rotator cuff repair.

Methods

Thirty-three individuals scheduled to undergo arthroscopic rotator cuff repair were enrolled in this study. Before surgery, shear modulus of the supraspinatus tendon and muscle was estimated using ultrasound SWE. MRIs were obtained before and 1 year after surgery to assess tear characteristics and repair integrity, respectively. Shoulder strength, range of motion, and patient-reported pain and function were assessed before and after surgery. Functional outcomes were compared between groups and across time using a two-factor mixed model analysis of variance. Stepwise regression with model comparison was used to investigate the extent to which MRI and shear modulus predicted repair integrity and function at 1 year after surgery.

Results

At 1 year after surgery, 56.5% of patients had an intact repair. No significant differences were found in any demographic variable, presurgical tear characteristic, or shear modulus between patients with an intact repair and those with a recurrent tear. Compared with presurgical measures, patients in both groups demonstrated significant improvements at 1 year after surgery in pain (P < .01), self-reported function (P < .01), range of motion (P < .01), and shoulder strength (P < .01). In addition, neither presurgical MRI variables (P > .16) nor shear modulus (P > .52) was significantly different between groups at 1 year after surgery. Finally, presurgical shear modulus generally did not improve the prediction of functional outcomes above and beyond that provided by MRI variables alone (P > .22).

Conclusion

Although SWE remains a promising modality for many clinical applications, this study found that SWE-estimated shear modulus did not predict repair integrity or functional outcomes at 1 year after surgery, nor did it add to the prediction of outcomes above and beyond that provided by traditional presurgical MRI measures of tear characteristics. Therefore, it appears that further research is needed to fully understand the clinical utility of SWE for musculoskeletal tissue and its potential use for predicting outcomes after surgical rotator cuff repair.

Factors Predicting Postoperative Range of Motion and Muscle Strength one Year after Shoulder Arthroplasty

Background

Shoulder arthroplasty improves shoulder range of motion (ROM), strength and function in patients with advanced shoulder disease. However, clinical outcomes vary and are not always predictable among patients. Pre-operative factors and patients' characteristics may influence improvement after surgery. This study examined the impact of the pre-operative objective measures range of motion (ROM) and strength, age, sex, and comorbidities on shoulder ROM, strength status and the amount of improvement one year following shoulder arthroplasty.

Methods

140 patients were assessed pre-operatively and one year after shoulder arthroplasty in this prospective cohort study. Pearson's correlations and multiple regression analyses were performed to test the impact of potential predictors on abduction, flexion, internal rotation and external rotation ROM as well as on shoulder abductors, flexors, internal rotators and external rotators strength at one year.

Results

Pre-operative ROM significantly predicted 10% - 37% of the improvement in ROM after surgery. Less pre-operative ROM was associated with a greater improvement in ROM. Less pre-operative muscle strength was associated with a greater improvement in strength after surgery. Pre-operative shoulder muscles predicted 28% - 38% of the strength status at one year, and 24% - 43% of the improvement in strength postoperatively. Older age was associated with less improvement in ROM and strength at one year. With other predictors, age explained 37% of the change in ROM and 36% of the change in strength. Male sex was associated with greater improvement in muscle strength. Sex significantly predicted 24% - 36% of the change in strength.

Conclusion

Pre-operative ROM and strength, age, and sex are significant predictors of the improvement in the shoulder ROM and strength one year after shoulder arthroplasty. The improvement in these measures is expected to decline with age and men are expected to gain more strength than women following this surgical intervention.

Sex Differences in Rotator Cuff Muscles' Response to Various Work-Related Factors

Occupational ApplicationsDespite the frequency and cost of rotator cuff injuries among male and female workers, very little is known about the strength, endurance, and perceived exertion and electromyographic response of rotator cuff muscles to different exertion levels. In this study, sex differences were studied using muscle-specific maximal and submaximal exertions of the supraspinatus, infraspinatus, and teres minor muscles. Females showed lower strength and endurance, but higher muscle activity and perceived exertion compared to males. The baseline data presented in this paper can assist ergonomic practitioners in determining the worker capacity to ensure that physically-demanding shoulder exertions can be performed without incurring injurious stress. Such data is also essential to establish population norms for the better design of workplace tasks.

Predictors of work-related musculoskeletal symptoms in shoulders among nursing assistants working in nursing homes

Nursing assistants (NAs) working in nursing homes (NHs) are at higher risk for work-related musculoskeletal symptoms (WRMSs) than their counterparts working in other health care settings. Worldwide, NAs have ranked shoulders in the top three body parts at risk of WRMSs. However, factors associated with their shoulder WRMSs are currently unknown. The aim of this study was to identify these associated risk factors among NAs working in NHs. 440 NAs from 47 nursing homes (with 60-90% response rate from each nursing home), recruited by convenience sampling, participated in this cross-sectional study in 2014-2015. A validated and reliable questionnaire was used for data collection. Information on demographic, job content questionnaire (JCQ), perceived physical exertion (PE), workstyle, ergonomic and manual handling knowledge and other work-related factors was collected using a self-administered questionnaire. 53% of the participants reported experiencing with WRMSs in their shoulders. Nine associated factors of shoulder WRMSs were identified using bivariate analysis. With the adjustment of age and gender using multivariable logistic regression, body mass index (OR = .931, 95% CI [.874-.991]), job title of health workers (OR = 2.72, 95% CI [1.18-6.25]) and workstyle-working through pain (OR = 1.06, 95% CI [1.01-1.11]) remained as predictors. Effort should be directed at integrating "workstyle intervention" into lifestyle physical activity training for NAs.

Shoulder Rotational Strength Profiles of Danish National Level Badminton Players

BACKGROUND

Increased age has been shown to be associated with weaker external rotators and stronger internal rotators of the shoulder in pitchers and tennis players. Whether this age-associated change is present in elite badminton players is unknown.

PURPOSE

To compare the internal and external rotation strength of the shoulder in adolescent and adult elite badminton players.

STUDY DESIGN

Cross-sectional.

METHODS

Thirty-one adolescent (12 females aged 16.8 ± 1.6 years and 19 males aged 17.1 ± 1.6 years) and 29 adult (10 females aged 25 ± 2.9 years and 19 males aged 26.2 ± 4.6 years) national level badminton players were tested pre-seasonally for external rotation (ER) and internal rotation (IR) isometric muscle strength bilaterally, using a hand-held dynamometer. Within-group ER to IR strength ratios were calculated (ER/IR×100%).

RESULTS

The adolescents had stronger shoulder ER than the adults on both sides (p < 0.05). The adult males tended to have stronger IR of the dominant shoulder than the adolescent males (p = 0.071). In the dominant shoulders, the strength ratios for adult females and males were 77% and 78%, respectively, while the same ratio for adolescent females and males were 85% and 99%, respectively. In the non-dominant shoulders, the ER/IR strength ratios for adult females and males were 90% and 87%, respectively, while the ratios for adolescent females and males were 116% and 102%, respectively.

CONCLUSION

This study is the first to demonstrate that in shoulder injury-free national team badminton players, adolescents have stronger shoulder ER than adults on both sides. Therefore, increased age appears to be associated with weaker shoulder ER muscles in elite badminton players.

LEVEL OF EVIDENCE

3b.

Upper extremity strength: Normative reference data among uninjured employees

BACKGROUND

Extremity strength testing is used to determine the ability of industrial employees to perform a physically demanding occupation safely, evaluate post-injury impairment, and monitor return to function after injury. There is an unmet clinical need for a robust and granular source of normative reference values to serve as a resource for baseline data on upper extremity isometric strength.

OBJECTIVE

Develop normative reference data for upper extremity strength among industrial employees and investigate associations between strength and physical job demands.

METHODS

Upper extremity strength data from 107,102 industrial employees were collected post-hire. In this study, isometric tests for pinch; hand grip; wrist pronation and supination; and flexion of the wrist, elbow, and shoulder strength were retrospectively analyzed in relationship to sex, age, and job level. Associations between strength scores and five levels of work, defined the United States Department of Labor's Dictionary of Occupational Titles by level of physical difficulty (1-5), were determined.

RESULTS

Higher strength scores were positively associated with more physically demanding job levels (P < 0.001), and there was a progressive increase in strength scores with increased physical job demands for both sexes (P = 0.0002). All strength scores differed significantly by decade of age (P < 0.001). All scores except for pinch strength demonstrated a moderate or high positive correlation with job level (r≥0.50).

CONCLUSIONS

The normative reference upper extremity strength data collected in this study for industrial employees may be useful for evaluating rehabilitation and recovery following injury or illness. In order to utilize normalized strength data as a post-injury reference, it is important to consider job level in addition to age and sex, as these variables are highly correlated with baseline upper extremity strength.

Effect of Holding Objects on the Occurrence of Head Impact in Falls by Older Adults: Evidence From Real-Life Falls in Long-Term Care

BACKGROUND

Falls cause approximately 80% of traumatic brain injuries in older adults, and nearly one third of falls by residents in long-term care (LTC) result in head impact. Holding objects during falls, such as mobility aids, may affect the ability of LTC residents to avoid head impact by arresting the fall with their upper limbs. We examined the prevalence of holding objects and their effect on risk for head impact during real-life falls in older adults living in LTC.

METHODS

We analyzed videos of 1105 real-life falls from standing height by 425 LTC residents, using a validated questionnaire to characterize the occurrence of head impact and whether the resident held objects during descent and impact. We classified objects as either "weight-bearing" (via contact to the fixed environment, eg, chairs and walkers) or "non-weight-bearing" (eg, cups) and tested their effect on odds for head impact with generalized estimating equations.

RESULTS

Residents held objects in more than 60% of falls. The odds for head impact were reduced for falls where weight-bearing objects were held or grasped during descent (odds ratio = 0.52; 95% confidence interval = 0.39-0.70) or maintained throughout the fall (odds ratio = 0.34; 95% confidence interval = 0.23-0.49). The most commonly held objects were chairs/wheelchairs (23% of cases), tables/counters (10% of cases), and walkers/rollators (22% of cases); all reduced the odds of head impact when held during descent. Holding non-weight-bearing objects did not affect the odds of head impact (odds ratio = 1.00; 95% confidence interval = 0.64-1.55).

CONCLUSION

Our results show that older adults in LTC use held, weight-bearing objects to reduce their risk for head impact during falls.

One versus two-handed lifting and lowering: lumbar spine loads and recommended one-handed limits protecting the lower back

The objectives of this study were to quantify loads imposed upon the lumbar spine while lifting/lowering with one versus two hands and to create guidelines for one-handed lifting/lowering that are protective of the lower back. Thirty subjects (15 male, 15 female) performed one- and two-handed exertions in a laboratory, lifting from/lowering to 18 lift origins/destinations using medicine balls of varying masses. An electromyography-assisted model predicted peak spinal loads, which were related to tissue tolerance limits to create recommended weight limits. Compared to two-handed exertions, one-handed exertions resulted in decreased spinal compression and A/P shear loading (p < 0.001) but increased lateral shear (p < 0.001). Effects were likely driven by altered moment exposures attributable to altered torso kinematics. Differences between spinal loads for one- versus two-handed exertions were influenced by asymmetry (p < 0.001) and amplified at lower lift origin/destination heights, lower object masses and larger horizontal distances between the body and the load (p < 0.001). Practitioner summary: A biomechanical model was utilised to compare spinal loading for one versus two-handed lifting/lowering. Spinal loads in compression and A/P shear were reduced for one-handed relative to two-handed exertions. As current lifting guidelines cannot appropriately be applied to one-handed scenarios, one-handed weight limits protecting the lower back are presented herein. Abbreviations: LBD: low back disorder, EMG: electromyography, A/P: anterior/posterior, MVC: maximum voluntary contraction.

Long-Term Outcomes of Pectoralis Major Transfer for the Treatment of Irreparable Subscapularis Tears: Results After a Mean Follow-up of 20 Years

BACKGROUND

One recognized salvage option in the treatment of an irreparable subscapularis tear is the pectoralis major tendon transfer (PMT). We aimed to analyze the long-term clinical and imaging outcome of PMT for irreparable subscapularis deficiency.

METHODS

Twenty-eight consecutive patients representing 30 shoulders underwent PMT at a mean age of 53.0 years (range, 35 to 67 years). At a mean of 19.7 years (range, 18 to 22 years) postoperatively, 24 shoulders (80%) were clinically examined and 21 were radiographically and sonographically assessed. The long-term results were compared with preoperative findings and previously published short-term results.

RESULTS

The mean relative Constant score (percentage of age and sex-matched normal scores; CS%) and the Subjective Shoulder Value (SSV) both improved significantly from preoperatively (CS%, 47%, and SSV, 22%) to postoperatively (CS%, 77%, and SSV, 71%; p < 0.001 for both). All patients rated their results as good or excellent. Active anterior elevation was improved from preoperatively (120°) to postoperatively (131°), but the difference was not significant. Active internal and external rotation decreased significantly from the short-term (32-month) follow-up to the time of the latest follow-up (p = 0.005 and p = 0.002, respectively); however, internal rotation remained at 6 points compared with the 8 points recorded at short-term follow-up and external rotation decreased only from a mean of 51° to 39°. Loss of active range of motion was not observed subjectively and was not subjectively limiting, represented by the high ultimate SSV and overall satisfaction. Four shoulders (19%) showed evidence of glenohumeral arthropathy (Samilson and Prieto grade 3), but clinically were mildly symptomatic to asymptomatic at the time of the latest follow-up (CS% range, 67% to 88%; SSV range, 70% to 80%). Rupture of the PMT was sonographically identified in 2 patients (10%) and was associated with radiographic evidence of advanced cuff tear arthropathy (Hamada stages ≥4). Six (20%) of the initial 30 shoulders were revised, and 1 (4%) of the 24 shoulders that were clinically examined underwent reverse total shoulder arthroplasty.

CONCLUSIONS

At long-term follow-up, PMT for isolated and combined subscapularis tears is associated with good to excellent clinical results. Although one-third of the shoulders developed mildly symptomatic or asymptomatic osteoarthritis, the need for salvage with use of reverse total shoulder arthroplasty was rare.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Characterizing the demands of backward balance loss and fall recovery during stair descent to prevent injury

Understanding the demands of balance recovery on stairs is important for developing strategies to prevent falls on stairs. This study characterized recovery strategies and whole-body movement following unexpected backward balance loss during stair descent in twelve young adults. Following balance loss, peak downward COM velocity was approximately double that experienced during non-perturbation stair descent. Participants used several balance recovery strategies: harness reliance (n = 1), no grasping reaction (n = 3), and grasping some environmental feature (n = 8). Of the five participants who used the handrail, four demonstrated grasping errors. Peak resultant handrail forces ranged from 24.2N to 238.3N. The results highlight the challenge of balance recovery during stair descent, showing that some people will use any available surface to arrest a fall. Our findings serve as a benchmark to understand the impact of stair-related interventions on fall recovery.

The influence of hand location and handle orientation on female manual arm strength

Accurate estimations of manual arm strength (MAS) are crucial in the evaluation of occupational force demands relative to population capacity. Most current strength predictions assume force application with a vertically oriented handle, but it is unknown how uni-manual force capability changes as a function of handle orientation and hand location. This study evaluated the effect of handle orientation on MAS throughout the reach envelope. Fifteen female participants exerted maximum forces in six directions (i.e. superior, inferior, anterior, posterior, medial, lateral), at five different hand locations, and MAS was measured with the handle oriented at 0° (i.e. horizontal), 45°, 90° (i.e. vertical) and 135°. Handle orientation affected MAS in all but the anterior exertion direction, with significant interactions between hand location and grip orientation existing for the superior and inferior directions. These results suggest that handle orientation is important to consider in future predictive models of manual arm strength.

Use of handrails for balance and stability: Characterizing loading profiles in younger adults

Well-designed handrails significantly enhance balance recovery, by allowing users to apply high forces to the rail and stabilize their center of mass. However, data on user-applied handrail forces during balance recovery are limited. We characterized the peak forces that 50 young adults applied to a handrail during forward and backward falling motions; quantified effects of handrail height (34, 38, 42 inches) and position prior to balance loss (standing beside the rail with or without hand contact, or facing the handrail with two-handed contact); and examined the relationship between handrail forces and individual mass. The testing environment consisted of a robotic platform that translated rapidly to destabilize participants, and a height-adjustable handrail that was mounted to the platform. Our findings support our hypotheses that starting position and handrail height significantly affect peak handrail forces in most axes. The highest handrail forces were applied when participants faced the handrail and grasped with two hands. In these cases, increased handrail height was associated with increased anterior forces and decreased downward, upward and resultant forces. As hypothesized, peak handrail forces correlated strongly with individual weight in most axes. Implications of these findings for handrail design are discussed.

Functional Connectivity During Handgrip Motor Fatigue in Older Adults Is Obesity and Sex-Specific

The prevalence of obesity in older adults, particularly in females, is increasing rapidly and is associated with declines in both the brain and physical health. Both the obese and the female populations have shown greater motor fatigue than their counterparts, however, the central neural mechanisms for fatigue are unclear. The present study measured fatigue-related functional connectivity across frontal and sensorimotor areas using functional near-infrared spectroscopy (fNIRS). Fifty-nine older adults (30 non-obese and 29 obese) performed submaximal handgrip motor fatigue until voluntary exhaustion. Functional connectivity and cerebral hemodynamics were compared across eight cortical areas during motor fatigue and across obesity and sex groups along with neuromuscular fatigue outcomes (i.e., endurance time, strength loss, and force steadiness). Both obesity- and sex-specific functional architecture and mean activation differences during motor fatigue in older adults were observed, which were accompanied by fatigue-related changes in variability of force steadiness that differed between groups. While primary indicators of fatigue, i.e., endurance and strength loss, did not differ between groups, the motor steadiness changes indicated different neural adaptation strategies between the groups. These findings indicate that obesity and sex differences exist in brain function in older adults, which may affect performance during motor fatigue.

Reference values for muscle strength: a systematic review with a descriptive meta-analysis

BACKGROUND

Muscle strength is an important component of health.

OBJECTIVE

To describe and evaluate the studies which have established the reference values for muscle strength on healthy individuals and to synthesize these values with a descriptive meta-analysis approach.

METHODS

A systematic review was performed in MEDLINE, LILACS, and SciELO databases. Studies that investigated the reference values for muscle strength of two or more appendicular/axial muscle groups of health individuals were included. Methodological quality, including risk of bias was assessed by the QUADAS-2. Data extracted included: country of the study, sample size, population characteristics, equipment/method used, and muscle groups evaluated.

RESULTS

Of the 414 studies identified, 46 were included. Most of the studies had adequate methodological quality. Included studies evaluated: appendicular (80.4%) and axial (36.9%) muscles; adults (78.3%), elderly (58.7%), adolescents (43.5%), children (23.9%); isometric (91.3%) and isokinetic (17.4%) strength. Six studies (13%) with similar procedures were synthesized with meta-analysis. Generally, the coefficient of variation values that resulted from the meta-analysis ranged from 20.1% to 30% and were similar to those reported by the original studies. The meta-analysis synthesized the reference values of isometric strength of 14 muscle groups of the dominant/non-dominant sides of the upper/lower limbs of adults/elderly from developed countries, using dynamometers/myometer.

CONCLUSIONS

Most of the included studies had adequate methodological quality. The meta-analysis provided reference values for the isometric strength of 14 appendicular muscle groups of the dominant/non-dominant sides, measured with dynamometers/myometers, of men/women, of adults/elderly. These data may be used to interpret the results of the evaluations and establish appropriate treatment goals.

Neuromuscular Control and Performance Differences Associated With Gender and Obesity in Fatiguing Tasks Performed by Older Adults

Obesity rates in the geriatric population have emerged as a serious health concern in recent decades. Yet, obesity-related differences in neuromuscular performance and motor control during fatiguing tasks, and how they are modified by gender, specifically among older adults, are still largely unexplored. The first aim of this study was to understand obesity and gender-related differences in endurance time among older adults. Motor variability has been linked with inter-individual differences in the rate of fatigue development, and as potentially revealing underlying mechanisms of neuromuscular control. Hence, the second and third aims of this study were to investigate to what extent motor variability at baseline could predict inter-individual differences in endurance time, and whether systematic obesity and gender differences exist in motor variability among older adults. Fifty-nine older adults (65 years or older) were recruited into four groups: obese male, obese female, non-obese male, and non-obese female. Participants performed submaximal intermittent isometric knee extensions until exhaustion. Knee extension force and muscle activation signals (surface electromyography) of a primary agonist muscle, the Vastus Lateralis (VL), were collected. Endurance time and metrics quantifying both the size and structure of variability were computed for the force and EMG signals, using coefficient of variation (within cycles and between cycles) and sample entropy measures. While group differences in endurance time were primarily associated with gender, adding individual motor variability measures as predictor variables explained significantly more variance in endurance time, thus highlighting the relevance of motor variability in understanding neuromotor control strategies. Males exhibited longer endurance times, higher EMG CV, lower EMG SaEn, lower force CV, and higher force SaEn than females. These findings are interpreted to indicate males as using a motor strategy involving better “distribution” of the neural efforts across synergists and antagonists to achieve better performance during the knee extension task. No obesity-related changes in endurance time were found. However, obese individuals exhibited a greater cycle-to-cycle variability in muscle activation, indicating a larger alteration in the recruitment of motor units across successive contractions and potentially increased neural costs, which may have contributed to comparable endurance time and performance as non-obese older adults.

A Prospective Look at the Link Between Frailty and Shoulder Function in Asymptomatic Elderly Individuals

Introduction:

The relationship between shoulder function and overall function in the elderly patients is not well understood. It is hypothesized that there is an increased tolerance of shoulder dysfunction in this population. The purpose of our study was to investigate and better understand the relationship between shoulder function, general musculoskeletal health, and frailty in the elderly patients.

Materials and Methods:

The dominant shoulders of 75 individuals aged ≥65 years without known dominant shoulder pathology were assessed. Demographic data were collected. Functional evaluation was conducted by administering the Constant, American Shoulder and Elbow Surgeons (ASES), and the Short Musculoskeletal Function Assessment (SMFA) questionnaires. Shoulder range of motion and strength were measured and the Fried frailty phenotype was calculated. Mean age of the patients was 73.6 years. Sixty-seven percent of the patients were female. Mean body mass index was 31.2 kg/m².

Results:

Twenty-eight percent of the patients reported the use of an assistive device for ambulation. As frailty increased among the 3 Fried frailty phenotypes (robust, prefrail, and frail), patients had statistically significant lower mean Constant scores (P < .0001), ASES scores (P < .0001), higher overall SMFA scores (P < .0001), and an increase in the use of assistive device for ambulation. Individuals who reported the use of an assistive device for ambulation had lower Constant and ASES scores (P < .0001 and P = .045, respectively) and higher overall SMFA scores (P < .0001). There was no evidence of correlation between body mass index and any of the other measures.

Conclusions:

Frailty and the use of an assistive device for ambulation correlate with poor shoulder function in patients who do not register shoulder complaints and have no known shoulder pathology.

Level of Evidence:

Level IV, Prognostic.

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.

Shoulder Strength Requirements for Upper Limb Functional Tasks: Do Age and Rotator Cuff Tear Status Matter?

Understanding upper limb strength requirements for daily tasks is imperative for early detection of strength loss that may progress to disability due to age or rotator cuff tear. We quantified shoulder strength requirements for 5 upper limb tasks performed by 3 groups: uninjured young adults and older adults, and older adults with a degenerative supraspinatus tear prior to repair. Musculoskeletal models were developed for each group representing age, sex, and tear-related strength losses. Percentage of available strength used was quantified for the subset of tasks requiring the largest amount of shoulder strength. Significant differences in strength requirements existed across tasks: upward reach 105° required the largest average strength; axilla wash required the largest peak strength. However, there were limited differences across participant groups. Older adults with and without a tear used a larger percentage of their shoulder elevation (p < .001, p < .001) and external rotation (p < .001, p = .017) strength than the young adults, respectively. Presence of a tear significantly increased percentage of internal rotation strength compared to young (p < .001) and uninjured older adults (p = .008). Marked differences in strength demand across tasks indicate the need for evaluating a diversity of functional tasks to effectively detect early strength loss, which may lead to disability.

The 'Arm Force Field' method to predict manual arm strength based on only hand location and force direction

This paper describes the development of a novel method (termed the 'Arm Force Field' or 'AFF') to predict manual arm strength (MAS) for a wide range of body orientations, hand locations and any force direction. This method used an artificial neural network (ANN) to predict the effects of hand location and force direction on MAS, and included a method to estimate the contribution of the arm's weight to the predicted strength. The AFF method predicted the MAS values very well (r² = 0.97, RMSD = 5.2 N, n = 456) and maintained good generalizability with external test data (r² = 0.842, RMSD = 13.1 N, n = 80). The AFF can be readily integrated within any DHM ergonomics software, and appears to be a more robust, reliable and valid method of estimating the strength capabilities of the arm, when compared to current approaches.

Age-related changes analyzing shoulder kinematics by means of inertial sensors

BACKGROUND

Shoulder kinematics alteration has been previously proven in subjects suffering from shoulder damage, which is also related to shoulder degenerative changes as consequence of aging. Because of that, it would be interesting to investigate age-related kinematics. The main objective was to analyze age-related differences in shoulder kinematics by means of inertial sensors in asymptomatic subjects during shoulder abduction and flexion.

METHODS

Twenty-five right arms were measured (11 from the young adult group and 14 from the older adult group). In the young group, age ranged between 20 and 34years old, while in the older group age ranged between 45 and 72years old. Angular mobility and linear acceleration from the scapula, humerus and sternum as well as mobility in the glenohumeral and scapulothoracic joints were calculated by three inertial sensors. Subjects performed shoulder abduction and flexion.

FINDINGS

Significant differences in mobility were found in humerus and scapula in mobility and acceleration during both tasks. Those differences varied depending on the plane analyzed. A moderate negative correlation between age and humerus and scapula ranged from -0.624** to -0.409*. A positive correlation was found in some planes.

INTERPRETATION

The results from the present study suggest that humerus elevation and scapular protraction-retraction as well as medio-lateral rotation are more likely to present kinematics age-related differences, i.e. older: age is associated with less mobility and acceleration in some planes and axes. Results provide inertial tridimensional measures from shoulder complex comparing groups of different age. Findings also concur with previous research supporting age-related shoulder changes.

Analysis of effects of loading and postural demands on upper limb reaching in older adults using statistical parametric mapping

Continuous time-series data are frequently distilled into single values and analyzed using discrete statistical methods, underutilizing large datasets. Statistical parametric mapping (SPM) allows hypotheses over the entire spectrum, but consistency with discrete analyses of kinematic data is unclear. We applied SPM to evaluate effect of load and postural demands during reaching on thoracohumeral kinematics in older and young adults, and examined consistency between one-dimensional SPM and discrete analyses of the same dataset. We hypothesized that older adults would choose postures that bring the humerus anterior to the frontal plane (towards flexion) even for low demand tasks, and that SPM would reveal differences persisting over larger temporal portions of the reach. Ten healthy older (72.4±3.1yrs) and 16 young (22.9±2.5yrs) adults reached upward and forward with high and low loads. SPM and discrete t-tests were used to analyze group effects for elevation plane, elevation, and axial rotation joint angles and velocity. Older adults used more positive (anterior) elevation plane and less elevated postures to initiate and terminate reaching (p<0.008), with long duration differences during termination. When reaching upward, differences in elevation persisted over longer temporal periods at midreach for high loads (32-58% of reach) compared to low load (41-45%). SPM and discrete analyses were consistent, but SPM permitted clear identification of temporal periods over which differences persisted, while discrete methods allowed analysis of extracted values, like ROM. This work highlights the utility of SPM to analyze kinematics time series data, and emphasizes importance of task selection when assessing age-related changes in movement.

Association of Strength Measurement with Rotator Cuff Tear in Patients with Shoulder Pain: The Rotator Cuff Outcomes Workgroup Study

OBJECTIVE

This study examines the association between strength measurements and supraspinatus tear in patients with shoulder pain. This study characterized determinants of abduction strength among patients with tears.

DESIGN

A total of 208 patients with shoulder pain were recruited. Strength was tested using a handheld dynamometer. Supraspinatus tears were diagnosed by combination of clinical assessment and blinded magnetic resonance imaging review. Associations of supraspinatus tear with patient characteristics and strength measurements (abduction, internal rotation and external rotation) were assessed using multivariable logistic regression models.

RESULTS

Patients with supraspinatus tear had decreased abduction strength (P = 0.02) and decreased external rotation strength (P < 0.01). When adjusted for age, sex, tear laterality, and body mass index, decreased abduction strength (odds ratio, 1.18/kg; 95% confidence interval, 1.06-1.32) and decreased external rotation strength (odds ratio, 1.29/kg; 95% confidence interval, 1.14-1.48) were associated with supraspinatus tear. In patients with tear, age 60 yrs or older, female sex, and visual analog scale pain score were significantly associated with decreased abduction strength but tear size, fatty infiltration, and atrophy were not.

CONCLUSIONS

Decreased abduction and external rotation strength were associated with supraspinatus tear in patients with shoulder pain. In this cohort, the abduction strength of patients with tears was influenced by demographic factors but not tear characteristics.

Determinants and magnitudes of manual force strengths and joint moments during two-handed standing maximal horizontal pushing and pulling

Pushing and pulling are common occupational exertions that are increasingly associated with musculoskeletal complaints. This study focuses on the sensitivity of shoulder capacity to gender, handle height, exertion type (push or pull) and handle orientation for these tasks. All factors except for handle orientation influenced unilateral and total manual force strength (p < 0.01), with exertion type being the most influential. Interaction effects also existed between handle height and exertion type. Additionally, joint moments at the shoulders and low back were influenced by all factors studied (p < 0.01), with exertion type again being most influential. Knowledge of the relative influence of multiple factors on shoulder capacity can provide guidance regarding these factors when designing or evaluating occupational pushing and pulling tasks for a diverse population. Practitioner Summary: pushing and pulling comprise nearly half of all manual materials handling tasks. Practitioners often assess, design or modify these tasks while incorporating constraints, including manual force direction and handle interface. This study provides guidance to aid design of pushing and pulling tasks in the context of shoulder physical capacity.

An Application of the Multivariate Linear Mixed Model to the Analysis of Shoulder Complexity in Breast Cancer Patients

In this study, four major muscles acting on the scapula were investigated in patients who had been treated in the last six years for unilateral carcinoma of the breast. Muscle activity was assessed by electromyography during abduction and adduction of the affected and unaffected arms. The main principal aim of the study was to compare shoulder muscle activity in the affected and unaffected shoulder during elevation of the arm. A multivariate linear mixed model was introduced and applied to address the principal aims. The result of fitting this model to the data shows a huge improvement as compared to the alternatives.

The return of subscapularis strength after shoulder arthroplasty

BACKGROUND

During shoulder arthroplasty, the subscapularis tendon is released and repaired. Whether subscapularis strength subsequently returns to normal is poorly understood. This study's purpose was to determine whether subscapularis strength returns to normal after shoulder replacement and whether any preoperative factors predict the return of strength postoperatively.

METHODS

Sixty-four patients underwent unilateral shoulder arthroplasty. Subscapularis strength was compared between the surgical and contralateral (normal) limbs at baseline (preoperatively) and follow-up. In addition, operative arm subscapularis strength recovery was compared with ipsilateral supraspinatus strength recovery. Independent variables were assessed for their effect on subscapularis strength, including sex, age, dominant-side surgery, preoperative strength, preoperative external rotation, subscapularis management technique, and fatty infiltration.

RESULTS

The mean subscapularis strength ratio at 24 months from baseline was 1.19 ± 2.23 (P = .0007). The normal side was significantly stronger than the operative side at all time points (P < .0001). The operative-side subscapularis mean strength ratio was 0.54 ± 0.28 of normal at baseline and 0.70 ± 0.24 at 24 months. Defining normal strength as ±15%, 15% of patients were normal at baseline up to 22% at 24 months. At 24 months, the mean supraspinatus strength ratio from baseline (3.13 ± 6.11) was significantly greater than the subscapularis mean strength ratio (P = .0007). Multivariable regression analysis did not demonstrate any correlation (P > .05) between the independent variables studied and final subscapularis strength.

DISCUSSION

Although significant strength improvement from baseline was observed at 2 years after shoulder arthroplasty, subscapularis strength returned to normal in only a minority of patients. Potential prognostic variables associated with final subscapularis strength remain elusive.

Effects of Age, Gender and Level of Co-contraction on Elbow and Shoulder Rotational Stiffness and Damping in the Impulsively End-Loaded Upper Extremity

Whether an arm will buckle under an impulsive end-load should partly depend on the elastic and viscous properties of the pretensed arm muscles. In measuring these properties we hypothesized that neither age, gender, nor muscle pre-contraction level would affect the bilinear elbow or shoulder lumped rotational stiffness or damping parameters in the impulsively end-loaded upper extremity of 38 healthy men and women. Subjects were instructed to preactivate triceps to either 25, 50 or 75% of maximum myoelectric activity levels. Then a standardized impulsive end-load was applied via a 6-axis load cell to the wrist of the slightly flexed arm in the prone posture. Arm kinematic responses were acquired at 280 Hz and an inverse dynamics analysis was used to estimate the bilinear rotational stiffnesses and damping parameters at the elbow and shoulder. The results show that pre-contraction level affected normalized joint rotational stiffness and damping coefficients (p < 0.02). Age affected the initial stiffness for the elbow (p < 0.05), and gender affected that of the shoulder in the sagittal plane (p < 0.006). Arm muscle strength was positively related to normalized stiffness at the elbow, but not the shoulder. We conclude that age, gender and pre-contraction level each affect the viscoelastic behavior of the end-loaded upper extremity in healthy adults.

Isometric shoulder strength reference values for physically active collegiate males and females

Background:

It is common clinical practice to assess muscle strength during examination of patients following shoulder injury or surgery. Strength comparisons are often made between the patient’s injured and uninjured shoulders, with the uninjured side used as a reference without regard to upper extremity dominance. Despite the importance of strength measurements, little is known about expected normal baselines of the uninjured shoulder. The purpose of this study was to report normative values for isometric shoulder strength for physically active college-age men and women without history of shoulder injury.

Methods:

University students—546 males (18.8 ± 1.0 years, 75.3 ± 12.2 kg) and 73 females (18.7 ± 0.9 years, 62.6 ± 7.0 kg)—underwent thorough shoulder evaluations by an orthopaedic surgeon and completed bilateral isometric strength measurements with a handheld dynamometer. Variables measured included internal rotation, external rotation, abduction, supine internal rotation and external rotation at 45°, and lower trapezius in prone flexion.

Results:

Significant differences were found between the dominant and nondominant shoulder for internal rotation, internal rotation at 45°, abduction, and prone flexion in males and in internal rotation at 45° and prone flexion for females (P ≤ 0.01).

Driver kinematic and muscle responses in braking events with standard and reversible pre-tensioned restraints: validation data for human models

The objectives of this study are to generate validation data for human models intended for simulation of occupant kinematics in a pre-crash phase, and to evaluate the effect of an integrated safety system on driver kinematics and muscle responses. Eleven male and nine female volunteers, driving a passenger car on ordinary roads, performed maximum voluntary braking; they were also subjected to autonomous braking events with both standard and reversible pre-tensioned restraints. Kinematic data was acquired through film analysis, and surface electromyography (EMG) was recorded bilaterally for muscles in the neck, the upper extremities, and lumbar region. Maximum voluntary contractions (MVCs) were carried out in a driving posture for normalization of the EMG. Seat belt positions, interaction forces, and seat indentions were measured. During normal driving, all muscle activity was below 5% of MVC for females and 9% for males. The range of activity during steady state braking for males and females was 13-44% in the cervical and lumbar extensors, while antagonistic muscles showed a co-contraction of 2.3-19%. Seat belt pre-tension affects both the kinematic and muscle responses of drivers. In autonomous braking with standard restraints, muscle activation occurred in response to the inertial load. With pre-tensioned seat belts, EMG onset occurred earlier; between 71 ms and 176 ms after belt pre-tension. The EMG onset times decreased with repeated trials and were shorter for females than for males. With the results from this study, further improvement and validation of human models that incorporate active musculature will be made possible.

Touch displays: the effects of palm rejection technology on productivity, comfort, biomechanics and positioning

Direct touch displays can improve the human-computer experience and productivity; however, the higher hand locations may increase shoulder fatigue. Palm rejection (PR) technology may reduce shoulder loads by allowing the palms to rest on the display and increase productivity by registering the touched content and fingertips through the palms rather than shoulders. The effects of PR were evaluated by having participants perform touch tasks while posture and reaction force on the display were measured. Enabling PR, during which the subjects could place the palms on the display (but were not required to), resulted in increased wrist extension, force applied to the display and productivity, and less discomfort, but had no effect on the self-selected positioning of the display. Participants did not deliberately place their palms on the display; therefore, there was no reduction in shoulder load and the increased productivity was not due to improved hand registration. The increased productivity may have been due to reduced interruptions from palm contacts or reduced motor control demands.

Normal isometric strength of rotatorcuff muscles in adults

Objectives

The most prevalent disorders of the shoulder are related to the muscles of rotator cuff. In order to develop a mechanical method for the evaluation of the rotator cuff muscles, we created a database of isometric force generation by the rotator cuff muscles in normal adult population. We hypothesised the existence of variations according to age, gender and dominancy of limb.

Methods

A total of 400 healthy adult volunteers were tested, classified into groups of 50 men and women for each decade of life. Maximal isometric force was measured at standardised positions for supraspinatus, infraspinatus and subscapularis muscles in both shoulders in every person. Torque of the force was calculated and normalised to lean body mass. The profiles of mean torque-time curves for each age and gender group were compared.

Results

Our data showed that men gradually gained maximal strength in the fifth decade, and showed decreased strength in the sixth. In women the maximal strength was gained in the fourth decade with gradual decline to the sixth decade of life. The dominant arm was stronger in most of the tested groups. The torque profiles of the rotator cuff muscles in men at all ages were significantly higher than that in women.

Conclusions

We found previously unrecognised variations of rotator cuff muscles’ isometric strength according to age, gender and dominancy in a normal population. The presented data may serve as a basis for the future studies for identification of the abnormal patterns of muscle isometric strength in patients with pathology of the rotator cuff muscles.

Cite this article: Bone Joint Res 2013;2:214–19.