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.

Assessing shoulder muscle stretch reflexes following breast cancer treatment and postmastectomy breast reconstruction.

Muscle stiffness is altered following postmastectomy breast reconstruction and breast cancer treatment. The exact mechanisms underlying these alterations are unknown; however, muscle stretch reflexes may play a role. This work examined short- (SLR) and long-latency (LLR) shoulder muscle stretch reflexes in breast cancer survivors. Forty-nine patients who had undergone postmastectomy breast reconstruction, 17 who had undergone chemoradiation, and 18 healthy, age-matched controls were enrolled. Muscle activity was recorded from the clavicular and sternocostal regions of the pectoralis major and anterior, middle, and posterior deltoids during vertical ab/adduction or horizontal flex/extension perturbations while participants maintained minimal torques. SLR and LLR were quantified for each muscle. Our major finding was that following postmastectomy breast reconstruction, SLR and LLR are impaired in the clavicular region of the pectoralis major. Individuals who had chemoradiation had impaired stretch reflexes in the clavicular and sternocostal region of the pectoralis major, anterior, middle, and posterior deltoid. These findings indicate that breast cancer treatments alter the regulation of shoulder muscle stretch reflexes and may be associated with surgical or nonsurgical damage to the pectoral fascia, muscle spindles, and/or sensory Ia afferents.NEW & NOTEWORTHY Shoulder muscle stretch reflexes may be impacted following postmastectomy breast reconstruction and chemoradiation. Here, we examined short- and long-latency shoulder muscle stretch reflexes in two experiments following common breast reconstruction procedures and chemoradiation. We show impairments in pectoralis major stretch reflexes following postmastectomy breast reconstruction and pectoralis major and deltoid muscle stretch reflexes following chemoradiation. These findings indicate that breast cancer treatments alter the regulation of shoulder muscle stretch reflexes.

Stretch reflex gain scaling at the shoulder varies with synergistic muscle activity.

The unique anatomy of the shoulder allows for expansive mobility but also sometimes precarious stability. It has long been suggested that stretch-sensitive reflexes contribute to maintaining joint stability through feedback control, but little is known about how stretch-sensitive reflexes are coordinated between the muscles of the shoulder. The purpose of this study was to investigate the coordination of stretch reflexes in shoulder muscles elicited by rotations of the glenohumeral joint. We hypothesized that stretch reflexes are sensitive to not only a given muscle's background activity but also the aggregate activity of all muscles crossing the shoulder based on the different groupings of muscles required to actuate the shoulder in three rotational degrees of freedom. We examined the relationship between a muscle's background activity and its reflex response in eight shoulder muscles by applying rotational perturbations while participants produced voluntary isometric torques. We found that this relationship, defined as gain scaling, differed at both short and long latencies based on the direction of voluntary torque generated by the participant. Therefore, gain scaling differed based on the aggregate of muscles that were active, not just the background activity in the muscle within which the reflex was measured. Across all muscles, the consideration of torque-dependent gain scaling improved model fits (ΔR2) by 0.17 ± 0.12. Modulation was most evident when volitional torques and perturbation directions were aligned along the same measurement axis, suggesting a functional role in resisting perturbations among synergists while maintaining task performance.NEW & NOTEWORTHY Careful coordination of muscles crossing the shoulder is needed to maintain the delicate balance between the joint's mobility and stability. We provide experimental evidence that stretch reflexes within shoulder muscles are modulated based on the aggregate activity of muscles crossing the joint, not just the activity of the muscle in which the reflex is elicited. Our results reflect coordination through neural coupling that may help maintain shoulder stability during encounters with environmental perturbations.

Age and sex influence the activation-dependent stiffness of the pectoralis major.

The pectoralis major fiber regions contribute uniquely to the mobility and the stability of the shoulder complex. It is unknown how age and sex influence the stiffness of these regions during volitional contractions, but this knowledge is critical to inform clinical interventions targeting the pectoralis major. The aim of the present study was to determine if the activation-dependent stiffness of the pectoralis major fiber regions differs between the sexes and if it is altered with age. Ultrasound shear wave elastography was used to acquire shear wave velocity from the clavicular and the sternocostal fiber regions of 48 healthy participants, including 24 younger (12 males, 12 females, mean ± SD age 25 ± 4.1 years) and 24 older adults (12 males, 12 females, 55 ± 3.6 years). Participants performed vertical adduction and horizontal flexion torques in neutral and 90° externally rotated shoulder positions, and one of the two shoulder abduction positions (60° and 90°) at varying torque magnitudes (passive, 15% and 30% of maximal voluntary contraction). Separate linear mixed-effects models were run for each fiber region and shoulder position to determine if the activation-dependent stiffness differed between the sexes and was altered in older adults. Age-related alterations in stiffness during volitional contractions were observed in both fiber regions and were dependent on the task. Alterations in activation-dependent stiffness due to age were more pronounced in females than males. Additionally, females had greater stiffness than males during volitional contractions in both fiber regions. The present findings provide the first line of evidence that the activation-dependent stiffness of the pectoralis major fiber regions is influenced by sex and changes with age.

Posture-dependent neuromuscular contributions to three-dimensional isometric shoulder torque generation.

The execution of activities of daily living requires the generation of three-dimensional shoulder torques through the coordinated activations of 20 shoulder muscles. Changes in shoulder posture can influence the contributions of individual shoulder muscles to shoulder torque generation, but it is unclear how the coordinated activity of shoulder muscles changes with shoulder plane of elevation and elevation angle. The purpose of this study was to characterize how neuromuscular coordination underlying three-dimensional shoulder function varies with arm posture. Muscle activations were obtained using surface electromyography from 16 shoulder muscles, as 12 healthy participants repeated eight three-dimensional isometric shoulder torques in 20 arm postures. Nonnegative matrix factorization revealed the muscle synergies underlying shoulder torque generation across each of the experimental postures, while the normalized similarity index assessed changes in overall synergy structure and linear mixed-effects models assessed changes in the weighted contributions of individual muscles to each synergy. Our analysis revealed that three distinct muscle synergies underlie healthy three-dimensional shoulder function. The overall structure of these synergies remained more similar than is expected by chance, despite changes in shoulder posture. However, the weighted contributions of five muscles composing the first synergy were influenced by changes in shoulder plane of elevation angle, and six muscles composing the second synergy were influenced by elevation angle. The weighted contributions of individual muscles composing the third synergy were unaffected by posture. These findings suggest that the neuromuscular control plan for healthy shoulder function consists of three distinct synergies, whose overall structure is fixed across shoulder posture.NEW AND NOTEWORTHY This study is the first to identify the muscle synergies underlying three-dimensional isometric shoulder torque generation. Although the overall structure of these synergies was unaffected by arm posture, the weighted contributions of several muscles composing two synergy patterns changed as a function of the elevation or plane of elevation of the shoulder. Our findings provide valuable insight for the development of targeted interventions for the restoration of shoulder function after neuromuscular or orthopedic pathologies.

The influence of reconstruction choice and inclusion of radiation therapy on functional shoulder biomechanics in women undergoing mastectomy for breast cancer.

PURPOSE: The functional implications of reconstructing the breast mound with a latissimus dorsi (LD) flap or placing an implant under the pectoralis major (PM) muscle is complicated by potential comorbidities from disinserting these muscles and adjuvant radiotherapy. We utilized novel robot-assisted measures of shoulder stiffness and strength to dissociate how breast reconstruction choice and inclusion of radiation therapy impact shoulder morbidity in post-mastectomy reconstruction patients. METHODS: Shoulder strength and stiffness were collected from 10 irradiated LD flap breast reconstruction patients, 14 two-stage subpectoral implant reconstruction patients (subpectoral), and 10 irradiated deep inferior epigastric perforator (DIEP) flap patients an average of 659 days post-reconstruction. Univariate ANOVAs examined surgical group differences in strength and stiffness. RESULTS: There were main effects of surgical group on vertical adduction, vertical abduction, and internal rotation strength. The LD flap group was significantly weaker than the subpectoral group in all measures and significantly weaker than the DIEP group during vertical adduction. There was also a main effect of surgical group on vertical adduction stiffness, where the LD group exhibited significantly reduced stiffness while producing vertical adduction torque. No significant differences between the subpectoral and DIEP groups existed for any measure of shoulder strength or stiffness. CONCLUSIONS: Disinsertion of the LD, not the disinsertion of the PM or radiotherapy, contributes to strength deficits following LD flap breast reconstructions. The combined disinsertion of the PM and LD compromises shoulder stability in the vertical plane. Shoulder function should be a focal point of the surgical decision-making process and postsurgical care.

The influence of wrist posture, grip type, and grip force on median nerve shape and cross-sectional area.

During grasping, the median nerve undergoes mechanical stress in the carpal tunnel which may contribute to carpal tunnel syndrome. This study investigated the effects of wrist posture, grip type, and grip force on the shape and cross-sectional area of the median nerve. Ultrasound examination was used to obtain cross-sectional images of the dominant wrist of 16 healthy subjects (8 male) at the proximal carpal tunnel during grasping. The cross-sectional area, circularity, and axis lengths of the median nerve were assessed in 27 different conditions (3 postures × 3 grip types × 3 force levels). There were no significant changes in median nerve cross-sectional area (P > 0.05). There were significant interactions across posture, grip type, and grip force affecting nerve circularity and axis lengths. When the wrist was flexed, increasing grip force caused the median nerve to shorten in the mediolateral direction and lengthen in the anteroposterior direction (P < 0.04), becoming more circular. These effects were significant during four finger pinch grip and chuck grip (P < 0.05) but not key grip (P > 0.07). With the wrist extended, the nerve became more flattened (less circular) as grip force increased during four finger pinch grip and chuck grip (P < 0.04) but not key grip (P > 0.3). Circularity was lower during the four finger pinch compared to chuck or key grip (P < 0.03). The findings suggest that grip type and wrist posture significantly alter the shape of the median nerve. Clin. Anat. 30:470-478, 2017. © 2017 Wiley Periodicals, Inc.

The influence of chair recline and head and neck position on upper trapezius activity and stiffness during seated computer work.

Increasing chair recline during seated computer work may reduce the load placed on the upper trapezius (UT), a common location of pain for those with idiopathic chronic neck pain. This study determined the effect of increasing chair recline on UT stiffness and muscle activity during computer work in people with and without idiopathic chronic neck pain. Surface electromyography and ultrasound shear wave elastography were collected from three subdivisions of the UT in 15 individuals with idiopathic chronic neck pain and 15 sex-matched healthy controls. Participants sat in a standardized computer-work setup while chair recline (0°, 25°, 45°) and head and neck position (self-selected, neutral, flexed) were systematically adjusted and maintained for 2.5-min intervals. Repeated-measures ANOVAs were completed for each sex, muscle, and data type, with group (chronic neck pain, control), chair recline (0°,25°,45°), head and neck position (self-selected, flexed, neutral), and side of collected data (dominant, non-dominant) as fixed factors. Men with idiopathic chronic neck pain demonstrated greater UT stiffness in the cranial subdivision when compared to healthy men. Additionally, the 25° and 45° recline levels increased the stiffness of men's dominant UT compared to men's non-dominant UT. Women's UT was more affected by head and neck position, and a neutral head and neck position resulted in lower UT activation, but higher UT stiffness for the cranial subdivision and midway between C-7 and the acromion process. Overall, our findings suggest that the commonly suggested neutral position may not be a beneficial prompt when positioning someone during seated computer work.

Pectoralis Muscle Dosimetry and Posttreatment Rehabilitation Utilization for Patients With Early-Stage Breast Cancer.

PURPOSE: Up to 50% of women treated for localized breast cancer will experience some degree of arm or shoulder morbidity. Although radiation is thought to contribute to this morbidity, the mechanism remains unclear. Prior studies have shown biologic and radiographic changes in the pectoralis muscles after radiation. This study thus aimed to investigate the relationship between radiation to the pectoralis muscles and referrals for rehabilitation services posttreatment for arm and shoulder morbidity. METHODS AND MATERIALS: A retrospective 1:1 matched case-control study was conducted for patients with breast cancer who were and were not referred for breast or shoulder rehabilitation services between 2014 and 2019 at a single academic institution. Patients were included if they had a lumpectomy and adjuvant radiation. Patients who underwent an axillary lymph node dissection were excluded. Cohorts were matched based on age, axillary surgery, and use of radiation boost. Muscle doses were converted to equivalent dose in 2 Gy fractions assuming an α:ß ratio of 2.5 and were compared between the 2 groups. RESULTS: In our cohort of 50 patients of a median age 60 years (interquartile range, 53-68 years), 36 patients (72%) underwent a sentinel lymph node biopsy in addition to a lumpectomy. Although pectoralis muscle doses were generally higher in those receiving rehabilitation services, this was not statistically significant. Pectoralis major V20-40 Gy reached borderline significance, as did pectoralis major mean dose (17.69 vs 20.89 Gy; P = .06). CONCLUSIONS: In this limited cohort of patients, we could not definitively conclude a relationship between pectoralis muscle doses and use of rehabilitation services. Given the borderline significant findings, this should be further investigated in a larger cohort.

How gender and task difficulty affect a sport-protective response in young adults.

We tested the hypotheses that gender and task difficulty affect the reaction, movement, and total response times associated with performing a head protective response. Twenty-four healthy young adults (13 females) performed a protective response by raising their hands from waist level to block a foam ball fired at their head from an air cannon. Participants initially stood 8.25 m away from the cannon ('low difficulty'), and were moved successively closer in 60 cm increments until they failed to block at least five of eight balls ('high difficulty'). Limb motion was quantified using optoelectronic markers on the participants' left wrist. Males had significantly faster total response times (P = 0.042), a trend towards faster movement times (P = 0.054), and faster peak wrist velocity (P < 0.001) and acceleration (P = 0.032) than females. Reaction time, movement time, and total response time were significantly faster under high difficulty conditions for both genders (P < 0.001). This study suggests that baseball and softball pitchers and fielders should have sufficient time to protect their head from a batted ball under optimal conditions if they are adequately prepared for the task.

The in vivo passive stretch response of the pectoralis major is region-specific.

The pectoralis major (PM) is a broad muscle commonly divided into three regions, which contribute uniquely to shoulder stability and movement. The PM muscle regions likely respond differently to stretch, but this has never been shown in vivo. We used shear wave elastography to assess the stretch response of different PM muscle regions during shoulder abduction and external rotation in 20 healthy male participants. Participants' shoulder was passively rotated through their range of motion in 5.7° increments and shear wave velocities (SWV) were obtained for each muscle region. A piece-wise model was fitted to the SWV-joint angle data, from which slack angle, slack stiffness and elasticity coefficient were determined. For shoulder abduction, we found that the sternocostal region had a significantly smaller slack angle (p = 0.049) and greater slack stiffness (p = 0.005) than the abdominal region, but there was no difference for elasticity coefficient (p = 0.074). For external rotation, only slack stiffness was greater for the sternocostal than the abdominal region (p < 0.001) with no differences found for slack angle (p = 0.18) and elasticity coefficient (p = 0.74). However, our data indicates that neither region was slack in this condition. These findings indicate that the sternocostal and abdominal regions respond differently to passive stretch, highlighting the PM's functional differentiation. This differentiation should be considered during treatment interventions such as PM muscle harvesting or treatments for breast cancer.

The impact of local therapies for breast cancer on shoulder muscle health and function.

Advances in breast cancer treatment have improved patient survival but have also created complications, such as shoulder morbidity, impacting the patient's quality of life. Local therapies for breast cancer influence shoulder muscle health through changes to the muscular microenvironment, macroscopic muscle morphology, and neuromuscular function. Our findings suggest both surgery and radiation therapy compromise the healthy functioning of shoulder musculature. Mastectomy and post-mastectomy breast reconstruction directly affect shoulder function through muscle morphology and neuromuscular function alterations. Radiation therapy damages satellite cells and myocytes, causing cell death both during treatment and years after recovery. This damage creates an environment limited in its ability to prevent atrophy. However, research to date is limited to a small number of analyses with small experimental populations and a lack of control for covariates. Future research to uncover the pathophysiological mechanisms underlying shoulder morbidity after breast cancer treatment must integrate measures of shoulder muscle health and shoulder function.

The influence of idiopathic chronic neck pain on upper trapezius and sternocleidomastoid muscle activity and elasticity during functional reaching: A cross-sectional study.

It remains unclear whether idiopathic chronic neck pain is associated with changes in muscle stiffness alongside alterations in neuromuscular control. Therefore, the purpose of this study was to determine the influence of idiopathic chronic neck pain on the muscle stiffness and muscle activity of the upper trapezius and sternocleidomastoid muscles during the maintenance of unilateral and bilateral functional reaching tasks. Surface electromyography (EMG) and ultrasound shear wave elastography were collected from the sternocleidomastoid and upper trapezius muscles in 18 individuals with idiopathic chronic neck pain and 18 matched healthy controls. Participants completed three functional reaching tasks; 1) unilateral forward reach, 2) bilateral forward reach, and 3) unilateral upward reach, and held at the top of each reaching movement for data to be collected bilaterally. A univariate ANOVA was utilized for each outcome measure (mean EMG amplitude and shear wave velocity) and each reaching task. Individuals with idiopathic chronic neck pain exhibited significantly lower upper trapezius activation during bilateral reaches without corresponding changes to stiffness during similar trials. Similarly, this cohort exhibited decreased sternocleidomastoid stiffness during forward reaching, without corresponding activation changes. Lastly, women demonstrated consistently higher sternocleidomastoid activation and stiffness when compared to men. These findings indicate individuals with idiopathic chronic neck pain may adapt their movement strategies, possibly for pain avoidance. The demonstrated changes in muscle stiffness independent of changes in muscle activity highlight the importance of evaluating both muscle stiffness and activation in individuals with idiopathic chronic neck pain prior to designing rehabilitation programs.

Neuromuscular compensation strategies adopted at the shoulder following bilateral subpectoral implant breast reconstruction.

Immediate two-stage subpectoral implant breast reconstruction after mastectomy requires the surgical disinsertion of the sternocostal fiber region of the pectoralis major (PM). The disinsertion of the PM would need increased contributions from intact shoulder musculature to generate shoulder torques. This study aimed to identify neuromuscular compensation strategies adopted by subpectoral implant breast reconstruction patients using novel muscle synergy analyses. Fourteen patients treated bilaterally with subpectoral implant breast reconstruction (>2.5 years post-reconstruction) were compared to ten healthy controls. Surface electromyography was obtained from sixteen shoulder muscles as participants generated eight three-dimensional (3D) shoulder torques in five two-dimensional arm postures bilaterally. Non-negative matrix factorization revealed the muscle synergies utilized by each experimental group on the dominant and non-dominant limbs, and the normalized similarity index assessed group differences in overall synergy structure. Bilateral subpectoral implant patients exhibited similar shoulder strength to healthy controls on the dominant and non-dominant arms. Our results suggest that 3D shoulder torque is driven by three shoulder muscle synergies in both healthy participants and subpectoral implant patients. Two out of three synergies were more similar than is expected by chance between the groups on the non-dominant arm, whereas only one synergy is more similar than is expected by chance on the dominant arm. While bilateral shoulder strength is maintained following bilateral subpectoral implant breast reconstruction, a closer analysis of the muscle synergy patterns underlying 3D shoulder torque generation reveals that subpectoral implant patients adopt compensatory neuromuscular strategies only with the dominant arm.

The Influence of Functional Shoulder Biomechanics as a Mediator of Patient-Reported Outcomes following Mastectomy and Breast Reconstruction.

BACKGROUND: Postmastectomy breast reconstruction techniques differentially influence patient-reported physical and psychosocial well-being. Objective measures of shoulder biomechanics, which are uniquely influenced by reconstruction technique, may provide insight into the influence of reconstruction technique on patient-reported outcomes. METHODS: Robot-assisted measures of shoulder strength and stiffness, and five validated patient-reported outcomes surveys were obtained from 46 women who had undergone mastectomy and a combined latissimus dorsi flap plus subpectoral implant, subpectoral implant, or DIEP flap breast reconstruction. Mediation analyses examined the role of functional shoulder biomechanics as a mediator between reconstruction technique and patient-reported outcomes. RESULTS: Reconstruction technique affected shoulder biomechanics, with latissimus dorsi flap plus subpectoral implant patients exhibiting reduced shoulder strength and stiffness compared with subpectoral implant and DIEP flap patients. Increasing external rotation strength was predictive of improved upper extremity function (p = 0.04). Increasing shoulder stiffness while at rest was predictive of worsened upper extremity function (p = 0.03). Increasing shoulder stiffness at rest and during contraction was indicative of worsened psychosocial well-being (all p ≤ 0.02). Reconstruction technique did not predict survey scores of function directly, or when mediated by functional shoulder biomechanics. CONCLUSIONS: In the current cohort, latissimus dorsi plus subpectoral implant breast reconstructions significantly reduced shoulder strength and stiffness when compared with the other techniques. In addition, objective measures of shoulder biomechanics were predictive of patient-reported physical and psychosocial well-being. The results emphasize the need for improved perioperative screening for shoulder functional deficits in patients undergoing breast reconstruction. CLINICAL QUESITON/LEVEL OF EVIDENCE: Therapeutic, II.

The relationship between muscle activation and shear elastic modulus of the sternocleidomastoid muscle during 3-D torque production.

The sternocleidomastoid (SCM) is a primary neck torque generator, but the relationship between its muscle activation and shear elastic modulus during 3-D torque production is unknown. This study examined variations in neural control and shear elastic modulus of the SCM across various 3-D isometric torques. Our primary hypothesis was that the SCM would display similar preferred directions where muscle activity and shear elastic modulus were maximal during voluntary 3-D isometric torque production. Surface electromyography (EMG) and ultrasound shear wave elastography (SWE) data were collected from the SCM in 20 participants performing 3-D isometric target-matching at two different torque amplitudes. We used spherical statistics to compare the preferred directions calculated from the SWE and EMG data at 40% and 80% torque level during 3-D isometric torque production. We demonstrated a small but significant difference between EMG and SWE preferred directions, with the SWE preferred direction oriented more towards ipsilateral bending and less towards contralateral axial rotation than the preferred direction for the EMG data. We conclude that, although small differences exist, SCM shear elastic modulus is largely driven by activation during 3-D neck torques for healthy individuals.

Identifying predictors of upper extremity muscle elasticity with healthy aging.

Ultrasound shear wave elastography (SWE) can provide accurate in vivo measurements of the effect of advanced age on muscle elasticity. Our objective was to determine whether passive muscle elasticity was influenced by posture, chronological age, sex, body mass index, and clinical measures of upper extremity function for healthy adults. The dominant arm of 33 male and 33 female participants (ranging from 20 to 89 years old) was examined using a Supersonic Imagine Aixplorer ultrasound SWE system. The mean and standard deviation of shear wave velocity (SWV) was measured from elastography maps for five upper extremity muscles examined at rest: anterior deltoid (AD), biceps brachii (BB), clavicular (CL) and sternocostal (SC) region of the pectoralis major and middle trapezius (MT). Linear mixed models for each muscle were used to assess how SWV was influenced by humeral elevation, chronological age, sex, BMI and three functional measures. All significances are reported at α = 0.05. Humeral elevation influenced shear wave velocity at a statistically significant level for AD, BB, SC and MT (all p < 0.047). Chronological age was a significant predictor of mean SWV for the sternocostal region of the pectoralis major and the middle trapezius (both p < 0.03). These same muscles were also less homogenous (based on their standard deviations) with increased age, particularly for female participants. Performance-based functional assessments of the upper extremity were predictors of mean SWV for the clavicular region of the pectoralis major (all p < 0.04). These results suggest ultrasound SWE has potential utility for assessing age-related changes to muscle elasticity, but these associations were muscle-dependent.

Quantifying the Multidimensional Impedance of the Shoulder During Volitional Contractions.

The neuromuscular control of the shoulder requires regulation of 3D joint mechanics, but it is unknown how these mechanics vary during tasks that load the shoulder in different directions. The purpose of this study was to quantify how the 3D mechanics of the shoulder change with voluntary torque production. Eleven participants produced voluntary isometric torques in one of six directions along three measurement axes. Impedance was estimated by applying small, pseudorandom angular perturbations about the shoulder as participants maintained steady state torques. The nonparametric impedance frequency response functions estimated from the data were parameterized by a collection of second-order linear systems to model the 3D inertia, viscosity, and stiffness of the shoulder. Each component of the 3D stiffness matrix scaled linearly with volitional torque production. Viscosity also increased monotonically with torque but nonlinearly. The directions of maximal stiffness and viscosity were consistently aligned towards the direction of torque production. Further, the shoulder was least stiff and least viscous in the direction of internal/external rotation, suggesting it may be more prone to injury along this axis. These experimental findings and the corresponding mathematical model summarizing our results provide novel insights into how the neuromuscular system regulates 3D shoulder mechanics in response to volitional muscle activations.

Quantifying the three-dimensional joint position sense of the shoulder.

Joint position sense is important for performing activities of daily living and recreational activities. The objective of this study was to develop new insights into the proprioceptive capabilities of the shoulder using a novel virtual reality paradigm where participants actively recreated shoulder positions in all three dimensions. This allows for better identification of changes in joint position sense across different shoulder postures. Ten males and ten female healthy adults matched a cursor controlled by shoulder rotations calculated from motion capture tracking, to a target shoulder position presented in a virtual environment with the use of a virtual reality headset. Four elevation angles, three plane of elevation angles, and three rotation angles were investigated, totaling thirty-six angles that encompassed the range of motion of the shoulder. Joint position sense was enhanced as the elevation angle was increased, and further enhanced when the arm was more externally rotated and elevated. As elevation angle increased to 90°, joint position sense significantly increased. There was also a significant interaction of external rotation on elevation angle. As elevation angle increased, participants were more accurate when the arm was externally, but exhibited greater variability. These improvements in joint position sense are likely produced by increased tension in muscles and capsuloligamentous mechanoreceptors within the shoulder. As many sports and activities of daily living require joint position sense to complete a task, the ability to elevate and externally rotate is important for adequate shoulder proprioception and control.

Postural differences in shoulder dynamics during pushing and pulling.

Assessments of shoulder dynamics (e.g. the inertial, viscous, and stiffness properties of the joint) can provide important insights into the stability of the joint at rest and during volitional contraction. The purpose of this study was to investigate how arm posture influences shoulder dynamics while generating pushing or pulling torques in the horizontal plane. Sixteen healthy participants were examined in seven postures encompassing a large workspace of the shoulder. At each posture, the participant's shoulder was rapidly perturbed while measuring the resultant change in shoulder torque about the glenohumeral axis. Participants were examined both at rest and while producing horizontal flexion and extension torques scaled to 15% of a maximum voluntary contraction. Shoulder stiffness, viscosity, and damping ratio were estimated using impedance-based matching, and changes in these outcome measures with torque level, elevation angle, and plane of elevation angle were explored with a linear mixed effects model. Shoulder stiffness was found to decrease with increasing elevation angles (p < 0.001) without subsequent changes in viscosity, leading to a greater damping ratios at higher elevation angles (p < 0.001). Shoulder stiffness, viscosity, and damping ratio (all p < 0.05) were all found to significantly increase as the plane of elevation of the arm was increased. The relationship between the viscosity, stiffness and the damping ratio of the shoulder is one that the central nervous system must regulate in order to maintain stability, protect against injury, and control the shoulder joint as the inertial and muscle contributions change across different arm postures.