Chronic Effects of Pitching on Muscle Thickness and Strength of the Scapular Stabilizers in Professional Baseball Players.

BACKGROUND: The posterior scapular muscles eccentrically contract to disperse the high forces observed in the deceleration phase of pitching. Muscular adaptations often occur following chronic eccentric loading, however, no study has evaluated the adaptations of the posterior scapular muscles with regard to throwing and their relationship with humeral retroversion (HR) in professional pitchers. HYPOTHESIS: Significant chronic adaptations in muscle thickness (MT) and strength of the trapezius and rhomboids would be observed in healthy professional baseball pitchers, and there would be a significant relationship between humeral adaptations (ie, HR) and posterior scapular muscle adaptations (ie, strength and MT). STUDY DESIGN: Cross-sectional; Level 3. METHODS: A total of 28 healthy male professional baseball pitchers (age, 22 ± 2 years; mass, 95 ± 17 kg; height, 190 ± 7 cm) were included in the study. Bilateral isometric muscle strength of the upper trapezius (UT), middle trapezius, lower trapezius (LT), and rhomboids was measured during a maximum voluntary isometric contraction. Diagnostic ultrasound images of the UT, middle trapezius, LT, rhomboid major, and rhomboid minor muscles were collected bilaterally to measure MT. HR was also quantified bilaterally with ultrasound. Paired sample t tests were used to compare dominant and nondominant strength and MT. Pearson correlation coefficients were used to assess the relationship between HR, isometric strength, and MT. RESULTS: A significantly increased MT of the LT was found on the dominant arm compared with the nondominant arm (5.4 ± 1.1 mm vs 4.4 ± 1.5 mm; P = 0.00). The Pearson correlation coefficient demonstrated a significant weak negative relationship between HR and rhomboid major MT (P = 0.03; R = -0.36), and a significant weak negative correlation between HR and middle trapezius isometric strength (P = 0.03; R = -0.37). CONCLUSION: LT thickness was greater in the throwing arm compared with the nonthrowing arm of pitchers, suggesting a positive adaptation of the LT. Interestingly, there was a weak negative relationship between HR and both rhomboid major MT and middle trapezius isometric strength. This negative relationship suggests that since increased HR leads to decreased internal rotation range of motion during deceleration, the scapula may be forced into anterior tilt and protraction, which can place excessive eccentric load on the rhomboid major and middle trapezius.

Chronic adaptations of the long head of the biceps tendon and groove in professional baseball pitchers.

BACKGROUND AND HYPOTHESIS: The long head of the biceps tendon (LHBT) plays a significant shoulder stabilizing role during pitching, with the large forces and repetitions involved in overhead throwing likely contributing to LHBT pathology. Determining whether the LHBT undergoes adaptive changes in baseball pitchers and how these changes relate to bicipital groove morphology can improve our understanding of the biceps function at the glenohumeral joint. Therefore, the purpose of this study was to determine the chronic adaptations of the bicipital groove morphology and the LHBT in professional baseball pitchers, with a secondary purpose of evaluating biceps integrity as it relates to torsional changes of the bicipital groove. We hypothesized that the throwing arm of professional baseball pitchers would exhibit chronic adaptations of the LHBT compared with their nonthrowing arm, and that these adaptations would be related to the bicipital groove morphology. MATERIALS AND METHODS: Fifty-three professional baseball pitchers were enrolled at the beginning of the 2015 Major League Baseball spring training. Ultrasound was used to bilaterally measure humeral retroversion and to capture images of the bicipital groove and the LHBT. MATLAB software was used to calculate the area of the bicipital groove, and ImageJ software was used to quantify the area, echogenicity, and circularity of the LHBT. RESULTS: The dominant arm LHBT cross-sectional area was significantly smaller than the nondominant arm (9 mm2 vs. 10 mm2; P = .011), whereas the dominant arm LHBT echogenicity was significantly higher than the nondominant arm (65 optical density vs. 59 optical density; P = .002). Pitchers with more bicipital groove rotational adaptation (more retroversion) had significantly more LHBT echogenicity adaptation compared with pitchers with less bicipital groove rotational adaptation (12 vs. 2; P = .023). CONCLUSION: There are significant bilateral differences in the LHBT of professional baseball pitchers. An adaptation in bony rotation was associated with a larger bilateral difference in LHBT echogenicity but was not related to bilateral differences in LHBT area or circularity. Therefore, the bilateral difference in echogenicity is impacted by bony morphology, whereas the bilateral difference in cross-sectional area may be independent of bony morphology in this healthy population.

Master swimmers with shoulder pain and disability have altered functional and structural measures.

CONTEXT: Supraspinatus tendinopathy and shoulder pain are common in competitive youth swimmers; however, no studies have investigated clinical and structural factors contributing to shoulder pain and disability in master level swimmers. OBJECTIVE: The objectives of this study were: 1) to determine the prevalence of shoulder pain and disability in master level swimmers, 2) to identify the most provocative special tests for shoulder pain, and 3) to determine if shoulder clinical and tissue specific measures, training variables and volume vary between those with and without shoulder pain, dissatisfaction and disability. DESIGN: Cross-sectional. SETTING: Collegiate swimming facilities. PATIENTS OR OTHER PARTICIPANTS: Thirty-nine adult masters level swimmers were evaluated and included in the data analysis. MAIN OUTCOME MEASURES: A survey of demographics, training, and pain and disability ratings using the Penn Shoulder Score and Disability of Arm Shoulder Hand sports module. Swimmers underwent a clinical exam including shoulder passive range of motion (PROM), posterior shoulder endurance test (PSET), supraspinatus tendon structure and posterior capsule thickness. One-way ANOVAs were used to compare demographics, clinical and structural findings between those with significant pain, dissatisfaction and disability (+PDD) and those without (-PDD). RESULTS: Fifteen percent of subjects reported pain at rest, 28% with normal activities (eating, dressing), and 69% with strenuous activities (sports) and 50% reported disability. The +PDD group had less shoulder internal rotation (10°), less ER (8°), and completed less yardage per day and per year. There were significant differences in the supraspinatus tendon structure between the +PDD and -PDD groups. CONCLUSION: Masters swimmers with pain and disability are able to self-limit yardage and likely why they recorded less yardage. The reduced shoulder motion (IR and ER) without posterior capsule differences may be due to rotator cuff muscle/tendon restrictions and the supraspinatus tendon structure may indicate degeneration caused by previous overuse resulting in pain.

Changes in clinical measures and tissue adaptations in collegiate swimmers across a competitive season.

BACKGROUND: Competitive swimmers incur shoulder pain and injury. Physical characteristics such as shoulder range of motion (ROM) and endurance and tissue adaptations such as posterior capsule thickness (PCT) may be risk factors in addition to high training volume. HYPOTHESIS/PURPOSE: 1) To identify the most provocative special test and prevalence of positive special tests for shoulder impingement tests in a group of collegiate swimmers, (2) to assess shoulder pain and disability, internal rotation (IR) and external rotation, and horizontal adduction (HADD) ROM and posterior shoulder endurance longitudinally over a competitive collegiate season, and (3) determine if there is a relationship between swimming yardage, supraspinatus tendon organization, and PCT. METHODS: Thirty Division III swimmers were tested poolside at the beginning (T1), middle (T2), and end (T3) of their season. Dependent variables included pain and disability, shoulder ROM, Posterior Shoulder Endurance Test (PSET) value, and PCT. Analyses of variance with follow-up t tests compared measures over time, and Pearson correlation coefficients were performed. RESULTS: Despite increased swimming yardage, disability was reduced from T1 to T3 (P = .003). There was a reduction in bilateral IR and HADD ROM from T1 to T3. PSET values increased on the right from T1 to T3 (P = .014). There was a significant positive correlation between swimming yardage at T1 and T2 and PCT at T3 (P = .034, P = .028). CONCLUSION: A loss of shoulder IR and HADD was observed across the season concurrent with less swimming-related disability, which may indicate a favorable adaptation. Improved PSET scores over the season is consistent with prior research linking endurance and less pain and disability.

Movement Coordination During Humeral Elevation in Individuals With Newly Acquired Spinal Cord Injury.

Humeral elevation is a critical motion for individuals who use a manual wheelchair given that, in a typical day, wheelchair users reach overhead 5 times more often than able-bodied controls. Kinematic analyses in individuals with chronic spinal cord injury (SCI) have focused on weight-bearing tasks rather than overhead reaching. This technical report presents shoulder movement coordination during overhead reaching in individuals with newly acquired SCI. Eight volunteers with acute SCI and 8 matched, uninjured controls participated. Three-dimensional kinematics were collected during seated, humeral elevation. Scapular and thoracic rotations during humeral elevation were averaged across repetitions. The linear relationship of scapular upward rotation to humeral elevation provided movement coordination analysis. Maximal elevation was reduced in SCI with increased thoracic kyphosis. Medium to large effect sizes were found at each elevation angle, with reduced scapular external rotation, posterior tilt, and increased thoracic kyphosis for those with SCI. The linear relationship occurred later and within a significantly (P = .02) smaller range of humeral elevation in SCI. Altered movement coordination, including a diminished linear association of scapular upward rotation and humeral elevation (scapulohumeral rhythm), is found with reduced maximal elevation and increased thoracic kyphosis during overhead reaching tasks in those with acute SCI.

Tendon Biomechanics and Crimp Properties Following Fatigue Loading Are Influenced by Tendon Type and Age in Mice.

In tendon, type-I collagen assembles together into fibrils, fibers, and fascicles that exhibit a wavy or crimped pattern that uncrimps with applied tensile loading. This structural property has been observed across multiple tendons throughout aging and may play an important role in tendon viscoelasticity, response to fatigue loading, healing, and development. Previous work has shown that crimp is permanently altered with the application of fatigue loading. This opens the possibility of evaluating tendon crimp as a clinical surrogate of tissue damage. The purpose of this study was to determine how fatigue loading in tendon affects crimp and mechanical properties throughout aging and between tendon types. Mouse patellar tendons (PT) and flexor digitorum longus (FDL) tendons were fatigue loaded while an integrated plane polariscope simultaneously assessed crimp properties at P150 and P570 days of age to model mature and aged tendon phenotypes (N = 10-11/group). Tendon type, fatigue loading, and aging were found to differentially affect tendon mechanical and crimp properties. FDL tendons had higher modulus and hysteresis, whereas the PT showed more laxity and toe region strain throughout aging. Crimp frequency was consistently higher in FDL compared with PT throughout fatigue loading, whereas the crimp amplitude was cycle dependent. This differential response based on tendon type and age further suggests that the FDL and the PT respond differently to fatigue loading and that this response is age-dependent. Together, our findings suggest that the mechanical and structural effects of fatigue loading are specific to tendon type and age in mice. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:36-42, 2020.

Dynamic Loading and Tendon Healing Affect Multiscale Tendon Properties and ECM Stress Transmission.

The extracellular matrix (ECM) is the primary biomechanical environment that interacts with tendon cells (tenocytes). Stresses applied via muscle contraction during skeletal movement transfer across structural hierarchies to the tenocyte nucleus in native uninjured tendons. Alterations to ECM structural and mechanical properties due to mechanical loading and tissue healing may affect this multiscale strain transfer and stress transmission through the ECM. This study explores the interface between dynamic loading and tendon healing across multiple length scales using living tendon explants. Results show that macroscale mechanical and structural properties are inferior following high magnitude dynamic loading (fatigue) in uninjured living tendon and that these effects propagate to the microscale. Although similar macroscale mechanical effects of dynamic loading are present in healing tendon compared to uninjured tendon, the microscale properties differed greatly during early healing. Regression analysis identified several variables (collagen and nuclear disorganization, cellularity, and F-actin) that directly predict nuclear deformation under loading. Finite element modeling predicted deficits in ECM stress transmission following fatigue loading and during healing. Together, this work identifies the multiscale response of tendon to dynamic loading and healing, and provides new insight into microenvironmental features that tenocytes may experience following injury and after cell delivery therapies.

Nonsurgical treatment and early return to activity leads to improved Achilles tendon fatigue mechanics and functional outcomes during early healing in an animal model.

Achilles tendon ruptures are common and devastating injuries; however, an optimized treatment and rehabilitation protocol has yet to be defined. Therefore, the objective of this study was to investigate the effects of surgical repair and return to activity on joint function and Achilles tendon properties after 3 weeks of healing. Sprague-Dawley rats (N = 100) received unilateral blunt transection of their Achilles tendon. Animals were then randomized into repaired or non-repaired treatments, and further randomized into groups that returned to activity after 1 week (RTA1) or after 3 weeks (RTA3) of limb casting in plantarflexion. Limb function, passive joint mechanics, and tendon properties (mechanical, organizational using high frequency ultrasound, histological, and compositional) were evaluated. Results showed that both treatment and return to activity collectively affected limb function, passive joint mechanics, and tendon properties. Functionally, RTA1 animals had increased dorsiflexion ROM and weight bearing of the injured limb compared to RTA3 animals 3-weeks post-injury. Such functional improvements in RTA1 tendons were evidenced in their mechanical fatigue properties and increased cross sectional area compared to RTA3 tendons. When RTA1 was coupled with nonsurgical treatment, superior fatigue properties were achieved compared to repaired tendons. No differences in cell shape, cellularity, GAG, collagen type I, or TGF-ß staining were identified between groups, but collagen type III was elevated in RTA3 repaired tendons. The larger tissue area and increased fatigue resistance created in RTA1 tendons may prove critical for optimized outcomes in early Achilles tendon healing following complete rupture. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2172-2180, 2016.

Evaluating changes in tendon crimp with fatigue loading as an ex vivo structural assessment of tendon damage.

The complex structure of tendons relates to their mechanical properties. Previous research has associated the waviness of collagen fibers (crimp) during quasi-static tensile loading to tensile mechanical properties, but less is known about the role of fatigue loading on crimp properties. In this study (IACUC approved), mouse patellar tendons were fatigue loaded while an integrated plane polariscope simultaneously assessed crimp properties. We demonstrate a novel structural mechanism whereby tendon crimp amplitude and frequency are altered with fatigue loading. In particular, fatigue loading increased the crimp amplitude across the tendon width and length, and these structural alterations were shown to be both region and load dependent. The change in crimp amplitude was strongly correlated to mechanical tissue laxity (defined as the ratio of displacement and gauge length relative to the first cycle of fatigue loading assessed at constant load throughout testing), at all loads and regions evaluated. Together, this study highlights the role of fatigue loading on tendon crimp properties as a function of load applied and region evaluated, and offers an additional structural mechanism for mechanical alterations that may lead to ultimate tendon failure.

In situ fibril stretch and sliding is location-dependent in mouse supraspinatus tendons.

Tendons are able to transmit high loads efficiently due to their finely optimized hierarchical collagen structure. Two mechanisms by which tendons respond to load are collagen fibril sliding and deformation (stretch). Although many studies have demonstrated that regional variations in tendon structure, composition, and organization contribute to the full tendon׳s mechanical response, the location-dependent response to loading at the fibril level has not been investigated. In addition, the instantaneous response of fibrils to loading, which is clinically relevant for repetitive stretch or fatigue injuries, has also not been studied. Therefore, the purpose of this study was to quantify the instantaneous response of collagen fibrils throughout a mechanical loading protocol, both in the insertion site and in the midsubstance of the mouse supraspinatus tendon. Utilizing a novel atomic force microscopy-based imaging technique, tendons at various strain levels were directly visualized and analyzed for changes in fibril d-period with increasing tendon strain. At the insertion site, d-period significantly increased from 0% to 1% tendon strain, increased again from 3% to 5% strain, and decreased after 5% strain. At the midsubstance, d-period increased from 0% to 1% strain and then decreased after 7% strain. In addition, fibril d-period heterogeneity (fibril sliding) was present, primarily at 3% strain with a large majority occurring in the tendon midsubstance. This study builds upon previous work by adding information on the instantaneous and regional-dependent fibrillar response to mechanical loading and presents data proposing that collagen fibril sliding and stretch are directly related to tissue organization and function.

Effect of isolated hyperglycemia on native mechanical and biologic shoulder joint properties in a rat model.

Recently, diabetes has been linked to rotator cuff disease and adhesive capsulitis, conditions with increased stiffness and inflammation. Unfortunately, limited research exists examining how hyperglycemia affects the native shoulder (tendon and capsule) properties. Therefore, the objectives of this study were to compare shoulder joint mechanics, tendon properties (mechanics and immunohistochemistry), and capsule of healthy control and hyperglycemic rats 8 weeks following induction of hyperglycemia with a submaximal dose of streptozotocin (STZ). Eighteen rats were injected with STZ to induce hyperglycemia or citrate buffer (control) and underwent normal cage activity for 8 weeks. Passive joint mechanics demonstrated significantly less external rotation in the hyperglycemic group compared to controls, with no other group differences. Tendon mechanical properties (stiffness and modulus) were not significantly different between groups at both the insertion site and mid-substance. Immunohistochemistry staining of the tendon and capsule demonstrated significantly increased interleukin 1-beta (IL1-ß) and advanced glycated end-products (AGE) staining localized to the insertion and mid-substance of the tendon but not the capsule. In addition, tumor necrosis factor alpha (TNF-α) staining was significantly increased in the superior capsule but not the supraspinatus tendon. This study demonstrates that isolated hypergylcemia does not diminish shoulder mechanical properties but does induce a chronic inflammatory response.

Bioengineered stromal cell-derived factor-1α analogue delivered as an angiogenic therapy significantly restores viscoelastic material properties of infarcted cardiac muscle.

Ischemic heart disease is a major health problem worldwide, and current therapies fail to address microrevascularization. Previously, our group demonstrated that the sustained release of novel engineered stromal cell-derived factor 1-a analogue (ESA) limits infarct spreading, collagen deposition, improves cardiac function by promoting angiogenesis in the region surrounding the infarct, and restores the tensile properties of infarcted myocardium. In this study, using a well-established rat model of ischemic cardiomyopathy, we describe a novel and innovative method for analyzing the viscoelastic properties of infarcted myocardium. Our results demonstrate that, compared with a saline control group, animals treated with ESA have significantly improved myocardial relaxation rates, while reducing the transition strain, leading to restoration of left ventricular mechanics.

Disruption of the anterior-posterior rotator cuff force balance alters joint function and leads to joint damage in a rat model.

The rotator cuff assists in shoulder movement and provides dynamic stability to the glenohumeral joint. Specifically, the anterior-posterior (AP) force balance, provided by the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, is critical for joint stability and concentric rotation of the humeral head on the glenoid. However, limited understanding exists of the consequences associated with disruption of the AP force balance (due to tears of both the supraspinatus and infraspinatus tendons) on joint function and joint damage. We investigated the effect of disrupting the APforce balance on joint function and joint damage in an overuse rat model. Twenty-eight rats underwent 4 weeks of overuse to produce a tendinopathic condition and were then randomized into two surgical groups: Detachment of the supraspinatus only or detachment of the supraspinatus and infraspinatus tendons. Rats were then gradually returned to their overuse protocol. Quantitative ambulatory measures including medial/lateral, propulsion, braking, and vertical forces were significantly different between groups. Additionally, cartilage and adjacent tendon properties were significantly altered. These results identify joint imbalance as a mechanical mechanism for joint damage and demonstrate the importance of preserving rotator cuff balance when treating active cuff tear patients.

Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury.

Achilles tendon injuries affect both athletes and the general population, and their incidence is rising. In particular, the Achilles tendon is subject to dynamic loading at or near failure loads during activity, and fatigue induced damage is likely a contributing factor to ultimate tendon failure. Unfortunately, little is known about how injured Achilles tendons respond mechanically and structurally to fatigue loading during healing. Knowledge of these properties remains critical to best evaluate tendon damage induction and the ability of the tendon to maintain mechanical properties with repeated loading. Thus, this study investigated the mechanical and structural changes in healing mouse Achilles tendons during fatigue loading. Twenty four mice received bilateral full thickness, partial width excisional injuries to their Achilles tendons (IACUC approved) and twelve tendons from six uninjured mice were used as controls. Tendons were fatigue loaded to assess mechanical and structural properties simultaneously after 0, 1, 3, and 6 weeks of healing using an integrated polarized light system. Results showed that the number of cycles to failure decreased dramatically (37-fold, p<0.005) due to injury, but increased throughout healing, ultimately recovering after 6 weeks. The tangent stiffness, hysteresis, and dynamic modulus did not improve with healing (p<0.005). Linear regression analysis was used to determine relationships between mechanical and structural properties. Of tendon structural properties, the apparent birefringence was able to best predict dynamic modulus (R(2)=0.88-0.92) throughout healing and fatigue life. This study reinforces the concept that fatigue loading is a sensitive metric to assess tendon healing and demonstrates potential structural metrics to predict mechanical properties.

Biceps detachment decreases joint damage in a rotator cuff tear rat model.

BACKGROUND: Pathology in the long head of the biceps tendon often occurs in patients with rotator cuff tears. Arthroscopic tenotomy is the most common treatment. However, the role of the long head of the biceps at the shoulder and the consequences of surgical detachment on the remaining shoulder structures remain unknown. QUESTIONS/PURPOSES: We hypothesized that detachment of the long head of the biceps, in the presence of supraspinatus and infraspinatus tears, would decrease shoulder function and decrease mechanical and histologic properties of both the subscapularis tendon and the glenoid articular cartilage. METHODS: We detached the supraspinatus and infraspinatus or the supraspinatus, infraspinatus, and long head of the biceps after 4 weeks of overuse in a rat model. Animals were gradually returned to overuse activity after detachment. At 8 weeks, the subscapularis and glenoid cartilage biomechanical and histologic properties were evaluated and compared. RESULTS: The supraspinatus, infraspinatus, and long head of the biceps group had a decreased change in braking and vertical force. [corrected]. This group also had an increased upper and lower subscapularis modulus but without any differences in glenoid cartilage modulus. Finally, this group had a significantly lower cell density in both the upper and lower subscapularis tendons, although cartilage histology was not different. CONCLUSIONS: Detachment of the long head of the biceps tendon in the presence of a posterior-superior cuff tear resulted in improved shoulder function and less joint damage in this animal model. CLINICAL RELEVANCE: This study provides evidence in an animal model that supports the use of tenotomy for the management of long head of the biceps pathology in the presence of a two-tendon cuff tear. However, long-term clinical trials are required.

Exercise protocol induces muscle, tendon, and bone adaptations in the rat shoulder.

BACKGROUND: a rat model of supraspinatus overuse has suggested mechanisms governing tendon degeneration; however, delineating which changes are pathologic or simply physiologic adaptations to increased loading remains a question. The objective of this study was to develop and characterize a rat exercise model that induces systemic and local shoulder adaptations without mechanical injury to the supraspinatus tendon. METHODS: exercise rats completed a treadmill training protocol for 12 weeks. Body, fat pad, and heart weights were determined. Supraspinatus tendon collagen content, cross-sectional area, and mechanical properties were measured. Supraspinatus muscle cross-sectional area, weight, and the expression of mitochondrial oxidative phosphorylation (OXPHOS) proteins were measured. Humeri were analyzed with µCT and mechanically tested. RESULTS: exercise decreased fat pad mass. Supraspinatus muscle hypertrophied and had increased OXPHOS proteins. Humerus trabecular bone had increased anisotropic orientation, and cortical bone showed increased bone and tissue mineral density. Importantly, the supraspinatus tendon did not have diminished mechanical properties, indicating that this protocol was not injurious to the tendon. CONCLUSION: this study establishes the first rat exercise protocol that induces adaptations in the shoulder. Future research can use this as a comparison model to study how the supraspinatus tendon adapts to loading and undergoes degeneration with overuse.

Analysis of collagen organization in mouse achilles tendon using high-frequency ultrasound imaging.

Achilles tendon ruptures are traumatic injuries, and techniques for assessing repair outcomes rely on patient-based measures of pain and function, which do not directly assess tendon healing. Consequently, there is a need for a quantitative, in vivo measure of tendon properties. Therefore, the purpose of this study was to validate ultrasound imaging for evaluating collagen organization in tendons. In this study, we compared our novel, high-frequency ultrasound (HFUS) imaging and analysis method to a standard measure of collagen organization, crossed polarizer (CP) imaging. Eighteen mouse Achilles tendons were harvested and placed into a testing fixture where HFUS and CP imaging could be performed simultaneously in a controlled loading environment. Two experiments were conducted: (1) effect of loading on collagen alignment and (2) effect of an excisional injury on collagen alignment. As expected, it was found that both the HFUS and CP methods could reliably detect an increase in alignment with increasing load, as well as a decrease in alignment with injury. This HFUS method demonstrates that structural measures of collagen organization in tendon can be determined through ultrasound imaging. This experiment also provides a mechanistic evaluation of tissue structure that could potentially be used to develop a targeted approach to aid in rehabilitation or monitor return to activity after tendon injury.

The dynamics of collagen uncrimping and lateral contraction in tendon and the effect of ionic concentration.

Under tensile loading, tendon undergoes a number of unique structural changes that govern its mechanical response. For example, stretching a tendon is known to induce both the progressive "uncrimping" of wavy collagen fibrils and extensive lateral contraction mediated by fluid flow out of the tissue. However, it is not known whether these processes are interdependent. Moreover, the rate-dependence of collagen uncrimping and its contribution to tendon's viscoelastic mechanical properties are unknown. Therefore, the objective of this study was to (a) develop a methodology allowing for simultaneous measurement of crimp, stress, axial strain and lateral contraction in tendon under dynamic loading; (b) determine the interdependence of collagen uncrimping and lateral contraction by testing tendons in different swelling conditions; and (c) assess how the process of collagen uncrimping depends on loading rate. Murine flexor carpi ulnaris (FCU) tendons in varying ionic environments were dynamically stretched to a set strain level and imaged through a plane polariscope with the polarizer and analyzer at a fixed angle. Analysis of the resulting images allowed for direct measurement of the crimp frequency and indirect measurement of the tendon thickness. Our findings demonstrate that collagen uncrimping and lateral contraction can occur independently and interstitial fluid impacts tendon mechanics directly. Furthermore, tensile stress, transverse contraction and degree of collagen uncrimping were all rate-dependent, suggesting that collagen uncrimping plays a role in tendon's dynamic mechanical response. This study is the first to characterize the time-dependence of collagen uncrimping in tendon, and establishes structure-function relationships for healthy tendons that can be used to better understand and assess changes in tendon mechanics after disease or injury.

Returning to overuse activity following a supraspinatus and infraspinatus tear leads to joint damage in a rat model.

Large rotator cuff tears (supraspinatus and infraspinatus) are common in patients who perform overhead activities (laborers, athletes). In addition, following large cuff tears, these patients commonly attempt to return to pre-injury activity levels. However, there is a limited understanding of the damaging effects on the uninjured joint tissues when doing so. Therefore, the objective of this study was to investigate the effect of returning to overuse activity following a supraspinatus and infraspinatus tear on shoulder function and the structural and biological properties of the intact tendons and glenoid cartilage. Forty rats underwent 4 weeks of overuse followed by detachment of the supraspinatus and infraspinatus tendons and were then randomized into two groups: return to overuse or cage activity. Ambulatory measurements were performed over time and structural and biological properties of the adjacent tendons and cartilage were evaluated. Results demonstrated that animals returning to overuse activity did not have altered shoulder function but despite this, did have altered cartilage and tendon properties. These mechanical changes corresponded to altered transcriptional regulation of chondrogenic genes within cartilage and tendon. This study helps define the mechanical and biological mechanisms leading to joint damage and provides a framework for treating active cuff tear patients.