Sex-Related Differences in Shoulder Complex Joint Dynamics Variability During Pediatric Manual Wheelchair Propulsion.

More than 80% of adult manual wheelchair users with spinal cord injuries will experience shoulder pain. Females and those with decreased shoulder dynamics variability are more likely to experience pain in adulthood. Sex-related differences in shoulder dynamics variability during pediatric manual wheelchair propulsion may influence the lifetime risk of pain. We evaluated the influence of sex on 3-dimensional shoulder complex joint dynamics variability in 25 (12 females and 13 males) pediatric manual wheelchair users with spinal cord injury. Within-subject variability was quantified using the coefficient of variation. Permutation tests evaluated sex-related differences in variability using an adjusted critical alpha of P = .001. No sex-related differences in sternoclavicular or acromioclavicular joint kinematics or glenohumeral joint dynamics variability were observed (all P ≥ .042). Variability in motion, forces, and moments are considered important components of healthy joint function, as reduced variability may increase the likelihood of repetitive strain injury and pain. While further work is needed to generalize our results to other manual wheelchair user populations across the life span, our findings suggest that sex does not influence joint dynamics variability in pediatric manual wheelchair users with spinal cord injury.

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.

The Influence of Age at Pediatric-Onset Spinal Cord Injury and Years of Wheelchair Use on Shoulder Complex Joint Dynamics During Manual Wheelchair Propulsion.

Objective: To assess the association of age at pediatric-onset spinal cord injury (SCI) and years of manual wheelchair use with shoulder dynamics. Design: Upper extremity kinematics and hand-rim kinetics were obtained during manual wheelchair propulsion. An inverse dynamics model computed three-dimensional acromioclavicular, sternoclavicular, and glenohumeral joint dynamics. Linear mixed effects models evaluated the association of age at injury onset and years of wheelchair use with shoulder dynamics. Setting: Motion laboratory within a children's hospital. Participants: Seventeen manual wheelchair users (N=17; 6 female, 11 male; mean age: 17.2 years, mean age at SCI onset: 11.5 years) with pediatric-onset SCI (levels: C4-T11) and International Standards for Neurological Classification of SCI grades: A (11), B (3), C (2), and N/A (2). Interventions: Not applicable. Main Outcome Measures: Acromioclavicular, sternoclavicular, and glenohumeral angles and ranges of motion, and glenohumeral forces and moments. Results: We observed a decrease in maximum acromioclavicular upward rotation (ß [95% confidence interval {CI}]=3.02 [0.15,5.89], P=.039) and an increase in acromioclavicular downward/upward rotation range of motion (ß [95% CI]=0.44 [0.08,0.80], P=.016) with increasing age at SCI onset. We found interactions between age at onset and years of use for maximum glenohumeral abduction (ß [95% CI]=0.16 [0.03,0.29], P=.017), acromioclavicular downward/upward rotation range of motion (ß [95% CI]=-0.05 [-0.09,-0.01], P=.008), minimum acromioclavicular upward rotation (ß [95% CI]=-0.34 [-0.64,-0.04], P=.026). A decrease in glenohumeral internal rotation moment (ß [95% CI]=-0.09 [-0.17,-0.009], P=.029) with increasing years of use was found. Conclusions: Age at injury and the years of wheelchair use are associated with shoulder complex biomechanics during wheelchair propulsion. These results are noteworthy, as both age at SCI onset and years of wheelchair use are considered important factors in the incidence of shoulder pain. These results suggest that investigations of biomechanical changes over the lifespan are critical.

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.

Biomechanical analysis of wheelchair athletes with paraplegia during cross-training exercises.

CONTEXT: Extreme conditioning programs (ECPs), such as CrossFit®, are a relatively new method of fitness with rapid growth in individuals with paraplegia. However, it is unknown if wheelchair users are at an additional risk of musculoskeletal injury during these exercises. Biomechanical characterization is necessary to determine the safety and efficacy of ECPs as an exercise modality for wheelchair users with paraplegia. OBJECTIVE: To characterize the three-dimensional (3-D) thorax and upper extremity joint kinematics of paraplegic wheelchair athletes during exercises commonly prescribed as part of ECPs. DESIGN: Observational study. PARTICIPANTS: Three male wheelchair athletes, average age of 37.1 ± 4.6 years, with spinal cord injury levels of T8, L2, and T10, with varying exercise experience. METHODS: 3-D movement was acquired using motion capture during the performance of four exercises: battle ropes, sled pull, overhead press, and sledgehammer swing. A custom upper extremity inverse kinematics model was applied to compute 3-D joint angles. OUTCOME MEASURES: 3-D peak thorax, glenohumeral, elbow, and wrist joint angles and ranges of motion (ROM), Visual Analog Scale (VAS), and Borg Scale of Perceived Exertion. RESULTS: Large joint motions were required for the exercises, at times demanding extreme shoulder and/or wrist flexion and extension, abduction, and external rotation, which are concerning for injury risk in wheelchair users. Participants, however, were able to perform the exercises pain free. CONCLUSION: These quantitative findings highlight that wheelchair athletes may be exposed to potentially injurious positions during common ECP exercises. These findings provide insight that may lead to improved clinical guidelines for prescription and training of exercise regimens, particularly involving ECPs, for wheelchair users.

Biological Sex-Related Differences in Glenohumeral Dynamics Variability during Pediatric Manual Wheelchair Propulsion.

Shoulder pain and pathology are extremely common in adult manual wheelchair users with spinal cord injury (SCI). Within this population, biological sex and variability in shoulder joint dynamics have been shown to be important contributors to both shoulder pain and pathology. Sex-related differences in shoulder dynamics variability during pediatric manual wheelchair propulsion may influence a user's lifetime risk of shoulder pain and pathology. The purpose of this study was to assess the influence of biological sex on variability in three-dimensional (3-D) glenohumeral joint dynamics in pediatric manual wheelchair users with SCI. An inverse dynamics model computed 3-D glenohumeral joint angles, forces, and moments of 20 pediatric manual wheelchair users. Levene's tests assessed biological sex-related differences in variability. Females exhibited less variability in glenohumeral joint kinematics and forces, but greater variability in joint moments than males. Evaluation of glenohumeral joint dynamics with consideration for biological sex and variability strengthens our interpretation of the relationships among shoulder function, pain, and pathology in pediatric manual wheelchair users.Clinical Relevance- Female pediatric manual wheelchair users may be at an increased risk of shoulder repetitive strain injuries due to decreased glenohumeral joint motion and force variability during propulsion. This work establishes quantitative methods for determining the effects of biological sex on the variability of shoulder joint dynamics.

The Influence of Sex on Upper Extremity Joint Dynamics in Pediatric Manual Wheelchair Users With Spinal Cord Injury.

INTRODUCTION: Manual wheelchair propulsion is a physically demanding task associated with upper extremity pain and pathology. Shoulder pain is reported in over 25% of pediatric manual wheelchairs users, and this number rises over the lifespan. Upper extremity biomechanics in adults has been associated with shoulder pain and pathology; however, few studies have investigated upper extremity joint dynamics in children. Furthermore, sex may be a critical factor that is currently unexplored with regard to pediatric wheelchair mobility. OBJECTIVES: To investigate differences in upper extremity joint dynamics between pediatric male and female manual wheelchair users with spinal cord injury (SCI) during wheelchair propulsion. METHODS: Novel instrumented wheelchair hand-rims synchronized with optical motion capture were used to acquire upper extremity joint dynamics of 20 pediatric manual wheelchair users with SCI (11 males, 9 females). Thorax, sternoclavicular, acromioclavicular, glenohumeral, elbow, and wrist joint kinematics and kinetics were calculated during wheelchair propulsion. Linear mixed models were used to assess differences between sexes. RESULTS: Females exhibited significantly greater peak forearm pronation (p = .007), normalized wrist lateral force (p = .03), and normalized elbow posterior force (p = .04) than males. Males exhibited significantly greater peak sternoclavicular joint retraction (p < .001) than females. No significant differences between males and females were observed for the glenohumeral joint (p > .012). CONCLUSION: This study found significant differences in upper extremity joint dynamics between sexes during manual wheelchair propulsion. Our results underscore the importance of considering sex when evaluating pediatric wheelchair mobility and developing comprehensive wheelchair training interventions for early detection and prevention of upper extremity pain and pathology.

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.

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.

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.

Mechanical properties of the shoulder and pectoralis major in breast cancer patients undergoing breast-conserving surgery with axillary surgery and radiotherapy.

Breast-conserving surgery (BCS) and radiotherapy reduce breast cancer recurrence but can cause functional deficits in breast cancer survivors. A cross-sectional study quantified the long-term pathophysiological impact of these treatments on biomechanical measures of shoulder stiffness and ultrasound shear wave elastography measures of the shear elastic modulus of the pectoralis major (PM). Nine node-positive patients treated with radiotherapy to the breast and regional nodes after BCS and axillary lymph node dissection (Group 1) were compared to nine node-negative patients treated with radiotherapy to the breast alone after BCS and sentinel node biopsy (Group 2) and nine healthy age-matched controls. The mean follow-up for Group 1 and Group 2 patients was 988 days and 754 days, respectively. Shoulder stiffness did not differ between the treatment groups and healthy controls (p = 0.23). The PM shear elastic modulus differed between groups (p = 0.002), with Group 1 patients exhibiting a stiffer PM than Group 2 patients (p < 0.001) and healthy controls (p = 0.027). The mean prescribed radiotherapy dose to the PM was significantly correlated with passive shear elastic modulus (p = 0.018). Breast cancer patients undergoing more extensive axillary surgery and nodal radiotherapy did not experience long-term functional deficits to shoulder integrity but did experience long-term mechanical changes of the PM.

Functional integrity of the shoulder joint and pectoralis major following subpectoral implant breast reconstruction.

Subpectoral implants for breast reconstruction after mastectomy requires the surgical disinsertion of the sternocostal fiber region of the pectoralis major. This technique is associated with significant shoulder strength and range of motion deficits, but it is unknown how it affects the underlying integrity of the shoulder joint or pectoralis major. The aim of this study was to characterize the long-term effects of this reconstruction approach on shoulder joint stiffness and pectoralis major material properties. Robot-assisted measures of shoulder strength and stiffness and ultrasound shear wave elastography images from the pectoralis major were acquired from 14 women an average of 549 days (range: 313-795 days) post reconstruction and 14 healthy, age-matched controls. Subpectoral implant patients were significantly weaker in shoulder adduction (p < 0.001) and exhibited lower shoulder stiffness when producing submaximal adduction torques (p = 0.004). The underlying material properties of the clavicular fiber region of the pectoralis major were altered in subpectoral implant patients, with significantly reduced shear wave velocities in the clavicular fiber region of the pectoralis major when generating adduction torques (p = 0.023). The clinical significance of these findings are that subpectoral implant patients do not fully recover shoulder strength or stability in the long-term, despite significant recovery time, and substantial shoulder musculature left intact. The impact of these procedures extends to the remaining, intact volume of the pectoralis major. Optimization of shoulder function should be a key aspect of the post-reconstruction standard of care. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1610-1619, 2019.

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.

Quadriceps-strengthening exercise and quadriceps and knee biomechanics during walking in knee osteoarthritis: A two-centre randomized controlled trial.

OBJECTIVE: To assess the effect of quadriceps strengthening on quadriceps muscle force, power, and work and tibio-femoral compressive loads during walking in adults with knee osteoarthritis. METHODS: Study design: Two-center, randomized, controlled trial. INTERVENTION: Patients with knee osteoarthritis were randomly allocated to quadriceps strengthening program (3 times weekly) or no attention control group. MAIN OUTCOME MEASURES: Primary outcome was change from baseline in peak quadriceps force during walking at 12 weeks. Secondary outcomes included quadriceps power and work, knee compression forces during walking estimated with musculoskeletal modeling, muscle strength and pain and function. Outcomes were measured at baseline and 12 weeks. RESULTS: 30 patients were randomized to receive either training (n = 15) or no attention (n = 15). At follow-up, there were no statistical differences between groups for maximum quadriceps force, quadriceps positive power, negative work, and positive work, and knee compressive force. Maximum negative quadriceps power in early stance was statistically significantly increased 36% in the training group compared to the control group which was most likely partially a response to faster walking velocity at follow-up. Muscle strength and patient reported pain and function were improved in the training group compared to the control group. CONCLUSIONS: Quadriceps strength training leads to increased muscle strength and improved symptomatic and functional outcomes but does not change quadriceps or knee joint biomechanics during walking. The biomechanical mechanism of improved health with strength training in knee osteoarthritis patients remains unknown. ClinicalTrials.gov Identifier: NCT01538407.

Identifying the mechanical and neural properties of the sternocleidomastoid muscles.

Neck muscles are preferentially activated in specific force directions, but the constraints that the central nervous system considers when programming these preferred directions of muscle activity are unknown. The current study used ultrasound shear wave elastography (SWE) to investigate whether the material properties of the sternocleidomastoid (SCM) muscles exhibit preferred directions similar to their preferred direction of muscle activity during an isometric task. Twenty-four healthy participants matched isometric forces in 16 axial directions. All force targets were scaled to 20% of a maximum voluntary contraction. Muscle activity was recorded with surface electromyography (EMG) from six muscles (the bilateral SCMs, upper trapezius, and splenius capitis muscles), and shear wave velocities (SWVs) were recorded with SWE from both SCM muscles. We observed statistically significant differences between the preferred directions of muscle activity and SWVs for both the left SCM ( P = 0.002) and the right SCM ( P < 0.001), with the SWE data exhibiting a more lateral preferred direction. Significant differences in the spatial focus ( P < 0.001) were also observed, with the dispersion of SWV data covering a greater angular range than the EMG data during isometric tasks. The preferred directions of muscle activity and material properties for the SCM muscles were closer than previous comparisons of muscle activity and moment arms, suggesting muscle mechanics could play a more important role than anatomy in how the central nervous system spatially tunes muscle activation. NEW & NOTEWORTHY Our study used a novel combination of surface electromyography and ultrasound shear wave elastography to investigate the neuromuscular control of the neck. Our work highlights differences in how the activation and material properties of the sternocleidomastoid muscles are modulated as the central nervous system stabilizes the neck during isometric force production. These findings provide normative data for future studies to investigate pathologic changes to both the activation and material properties of the sternocleidomastoid muscles.

Quantifying differences in the material properties of the fiber regions of the pectoralis major using ultrasound shear wave elastography.

The sternocostal and clavicular regions of the pectoralis major are independently harvested to provide shoulder stability, but surgical decision making does not consider the biomechanical consequences that disinsertion of one fiber region over the other has on shoulder function. Differences in material properties between the fiber regions could influence which tissue is more optimal for surgical harvesting. The current study utilized ultrasound shear wave elastography (SWE) to investigate whether the in vivo material properties differ between the fiber regions. Shear wave velocities (SWVs) were collected from the sternocostal and clavicular fiber regions of the pectoralis major from ten healthy young male participants. Participants produced isometric shoulder torques of varying magnitudes (passive, 15%, and 30% MVC) and directions (horizontal and vertical adduction). Four shoulder positions encompassing different vertical abduction and external rotation angles were examined. One-way ANOVAs tested the hypotheses that differences in SWVs during rest existed between the fiber regions asa function of shoulder position, and differences in SWVs during contraction existed asa function of shoulder position and torque direction. In all shoulder positions, the clavicular region exhibited greater SWVs during rest than the sternocostal region (P<0.001). In shoulder positions that did not include external rotation, the clavicular region exhibited greater SWVs during contraction when producing horizontal adduction torques (P<0.001), while the sternocostal region exhibited greater SWVs during contraction when producing vertical adduction torques at 30% MVC (P<0.01). Our results suggest that each fiber region of the pectoralis major provides unique contributions to passive and active shoulder function.