Impact of repetition number on muscle performance and histological response

Inhalation of polycarbonate emissions generated during 3D printing processes affects neuroendocrine function in male rats

Three-dimensional (3D) printing of manufactured goods has increased in the last 10 years. The increased use of this technology has resulted in questions regarding the influence of inhaling emissions generated during printing. The goal of this study was to determine if inhalation of particulate and/or toxic chemicals generated during printing with polycarbonate (PC) plastic affected the neuroendocrine system. Male rats were exposed to 3D-printer emissions (592 µg particulate/m3 air) or filtered air for 4 h/day (d), 4 days/week and total exposures lengths were 1, 4, 8, 15 or 30 days. The effects of these exposures on hormone concentrations, and markers of function and/or injury in the olfactory bulb, hypothalamus and testes were measured after 1, 8 and 30 days exposure. Thirty days of exposure to 3D printer emissions resulted in reductions in thyroid stimulating hormone, follicle stimulating hormone and prolactin. These changes were accompanied by (1) elevation in markers of cell injury; (2) reductions in active mitochondria in the olfactory bulb, diminished gonadotropin releasing hormone cells and fibers as well as less tyrosine hydroxylase immunolabeled fibers in the arcuate nucleus; and (3) decrease in spermatogonium. Polycarbonate plastics may contain bisphenol A, and the effects of exposure to these 3D printer-generated emissions on neuroendocrine function are similar to those noted following exposure to bisphenol A.

A systematic review of in vivo stretching regimens on inflammation and its relevance to translational yoga research

OBJECTIVE

To conduct a systematic review evaluating the impact of stretching on inflammation and its resolution using in vivo rodent models. Findings are evaluated for their potential to inform the design of clinical yoga studies to assess the impact of yogic stretching on inflammation and health.

METHODS

Studies were identified using four databases. Eligible publications included English original peer-reviewed articles between 1900-May 2020. Studies included those investigating the effect of different stretching techniques administered to a whole rodent model and evaluating at least one inflammatory outcome. Studies stretching the musculoskeletal and integumentary systems were considered. Two reviewers removed duplicates, screened abstracts, conducted full-text reviews, and assessed methodological quality.

RESULTS

Of 766 studies identified, 25 were included for synthesis. Seven (28%) studies had a high risk of bias in 3 out of 10 criteria. Experimental stretching protocols resulted in a continuum of inflammatory responses with therapeutic and injurious effects, which varied with a combination of three stretching parameters--duration, frequency, and intensity. Relative to injurious stretching, therapeutic stretching featured longer-term stretching protocols. Evidence of pro- and mixed-inflammatory effects of stretching was found in 16 muscle studies. Evidence of pro-, anti-, and mixed-inflammatory effects was found in nine longer-term stretching studies of the integumentary system.

CONCLUSION

Despite the overall high quality of these summarized studies, evaluation of stretching protocols paralleling yogic stretching is limited. Both injurious and therapeutic stretching induce aspects of inflammatory responses that varied among the different stretching protocols. Inflammatory markers, such as cytokines, are potential outcomes to consider in clinical yoga studies. Future translational research evaluating therapeutic benefits should consider in vitro studies, active vs. passive stretching, shorter-term vs. longer-term interventions, systemic vs. local effects of stretching, animal models resembling human anatomy, control and estimation of non-specific stresses, development of in vivo self-stretching paradigms targeting myofascial tissues, and in vivo models accounting for gross musculoskeletal posture.

Diverse Role of Biological Plasticity in Low Back Pain and Its Impact on Sensorimotor Control of the Spine

Pain is complex. It is no longer acceptable to consider pain solely as a peripheral phenomenon involving activation of nociceptive neurons. The contemporary understanding of pain involves consideration of different underlying pain mechanisms and an increasing awareness of plasticity in all of the biological systems. Of note, recent advances in technology and understanding have highlighted the critical importance of neuroimmune interactions, both in the peripheral and central nervous systems, and the interaction between the nervous system and body tissues in the development and maintenance of pain, including low back pain (LBP). Further, the biology of many tissues changes when challenged by pain and injury, as reported in a growing body of literature on the biology of muscle, fat, and connective tissue. These advances in understanding of the complexity of LBP have implications for our understanding of pain and its interaction with the motor system, and may change how we consider motor control in the rehabilitation of LBP. This commentary provides a state-of-the-art overview of plasticity of biology in LBP. The paper is divided into 4 parts that address (1) biology of pain mechanisms, (2) neuroimmune interaction in the central nervous system, (3) neuroimmune interaction in the periphery, and (4) brain and peripheral tissue interaction. Each section considers the implications for clinical management of LBP. J Orthop Sports Phys Ther 2019;49(6):389-401. doi:10.2519/jospt.2019.8716.

Efficacy of Age-Specific High-Intensity Stretch-Shortening Contractions in Reversing Dynapenia, Sarcopenia, and Loss of Skeletal Muscle Quality

During the aging process, skeletal muscle performance and physiology undergoes alterations leading to decrements in functional capacity, health-span, and independence. Background: The utility and implementation of age-specific exercise is a paramount research agenda focusing on ameliorating the loss of both skeletal muscle performance and physiology; yet, to date, no consensus exists as to the most appropriate mechanical loading protocol design or overall exercise prescription that best meets this need. Thus, the purpose of this review is to highlight the most optimal type of exercise presently available and provide the most current, evidence-based findings for its efficacy. The hypothesis that high-intensity, stretch-shortening contractions (SSCs)-a form of "resistance-type exercise" training-present as the preferred exercise mode for serving as an intervention-based modality to attenuate dynapenia, sarcopenia, and decreased muscle quality with aging, even restoring the overall youthful phenotype, will be demonstrated. Conclusions: Appreciating the fundamental evidence supporting the use of high-intensity SSCs in positively impacting aging skeletal muscle's responsivity and their use as a specific and sensitive countermeasure is crucial. Moreover, from an applied perspective, SSCs may improve skeletal muscle quality and rejuvenate health-span and, ultimately, lead to augmented functional capacity, independence, and quality of life concomitant with decreased morbidity.

Inflammaging and the Age-Specific Responsiveness to Stretch-Shortening Contractions

With aging, muscle injury from rapid, continuous stretch-shortening contractions (SSC) is prolonged, and maladaptation to moderate-velocity, intermittent SSC is more common. We hypothesize that high baseline levels of inflammatory signaling and oxidative stress may underlie these outcomes, whereas careful modulation of high-intensity SSC training design resets basal conditions and permits muscle adaptation to SSC.

Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training

Background

The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice.

Methods

Young (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution.

Results

The main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles.

Conclusions

The findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-017-1397-4) contains supplementary material, which is available to authorized users.

An Old Problem: Aging and Skeletal-Muscle-Strain Injury

Clinical Scenario: Even though chronological aging is an inevitable phenomenological consequence occurring in every living organism, it is biological aging that may be the most significant factor challenging our quality of life. Development of functional limitations, resulting from improper maintenance and restoration of various organ systems, ultimately leads to reduced health and independence. Skeletal muscle is an organ system that, when challenged, is often injured in response to varying stimuli. Overt muscle-strain injury can be traumatic, clinically diagnosable, properly managed, and a remarkably common event, yet our contemporary understanding of how age and environmental stressors affect the initial and subsequent induction of injury and how the biological processes resulting from this event are modifiable and, eventually, lead to functional restoration and healing of skeletal muscle and adjacent tissues is presently unclear. Even though the secondary injury response to and recovery from "contraction-induced" skeletal-muscle injury are impaired with aging, there is no scientific consensus as to the exact mechanism responsible for this event. Given the multitude of investigative approaches, particular consideration given to the appropriateness of the muscle-injury model, or research paradigm, is critical so that outcomes may be physiologically relevant and translational. In this case, methods implementing stretch-shortening contractions, the most common form of muscle movements used by all mammals during physical movement, work, and activity, are highlighted.

CLINICAL RELEVANCE

Understanding the fundamental evidence regarding how aging influences the responsivity of skeletal muscle to strain injury is vital for informing how clinicians approach and implement preventive strategies, as well as therapeutic interventions. From a practical perspective, maintaining or improving the overall health and tissue quality of skeletal muscle as one ages will positively affect skeletal muscle's safety threshold and responsivity, which may reduce incidence of injury, improve recovery time, and lessen overall fiscal burdens.

Enhancement of Skeletal Muscle in Aged Rats Following High-Intensity Stretch-Shortening Contraction Training

Exercise is the most accessible, efficacious, and multifactorial intervention to improve health and treat chronic disease. High-intensity resistance exercise, in particular, also maximizes skeletal muscle size and strength-outcomes crucial at advanced age. However, such training is capable of inducing muscle maladaptation when misapplied at old age. Therefore, characterization of parameters (e.g., mode and frequency) that foster adaptation is an active research area. To address this issue, we utilized a rodent model that allowed training at maximal intensity in terms of muscle activation and tested the hypothesis that muscles of old rats adapt to stretch-shortening contraction (SSC) training, provided the training frequency is sufficiently low. At termination of training, normalized muscle mass (i.e., muscle mass divided by tibia length) and muscle quality (isometric force divided by normalized muscle mass) were determined. For young rats, normalized muscle mass increased by ∼20% regardless of training frequency. No difference was observed for muscle quality values after 2 days versus 3 days per week training (0.65 ± 0.09 N/mg/mm vs. 0.59 ± 0.05 N/mg/mm, respectively). For old rats following 3 days per week training, normalized muscle mass was unaltered and muscle quality was 30% lower than young levels. Following 2 days per week training at old age, normalized muscle mass increased by 17% and muscle quality was restored to young levels. To investigate this enhanced response, oxidative stress was assessed by lipid peroxidation quantification. For young rats, lipid peroxidation levels were unaltered by training. With aging, baseline levels of lipid peroxidation increased by 1.5-fold. For old rats, only 2 days per week training decreased lipid peroxidation to levels indistinguishable from young values. These results imply that, appropriately scheduled high-intensity SSC training at old age is capable of restoring muscle to a younger phenotype in terms of lipid peroxidation levels and muscle quality.

Changes in the expression of calcitonin gene-related peptide after exposure to injurious stretch-shortening contractions

One of the factors that can result in musculoskeletal injuries, and time off work, is exposure to repetitive motion. The goal of this study was to determine if skeletal muscle injury induced by exposure to injurious stretch-shortening cycles (iSSCs), resulted in hyperalgesia in the hind limb and changes in calcitonin-gene related peptide (CGRP) immunolabeling in the dorsal root ganglia (DRG) in young and old male rats.

METHODS

Young (3months) and old (30months) male Fisher 344×BN F1 rats were anesthetized with isoflurane and the left hind limbs were exposed to 15 sets of 10 SSCs. Control animals were exposed to a single bout of SSCs of equal intensity. Sensitivity to mechanical stimulation was assessed using von Frey filaments prior to beginning the experiment, and on days 2 and 9 following exposure to iSSCs. Rats were euthanized one, 3 or 10days after the exposure. The ipsilateral DRG were dissected from the L4-5 region of the spine, along with the left tibialis anterior (LTA) muscle.

RESULTS

Rats exposed to iSSCs were more sensitive to mechanical stimulation than control rats 2days after the exposure, and showed a reduction in peak force 3days after exposure. Changes in sensitivity to pressure were not associated with increases in CGRP labeling in the DRG at 3days. However, 9days after exposure to iSSCs, old rats still displayed an increased sensitivity to mechanical stimulation, and this hyperalgesia was associated with an increase in CGRP immunolabeling in the DRG. Young rats exposed to iSSC did not display a change in CGRP immunolabeling and sensitivity to mechanical stimulation returned to control levels at 10days.

CONCLUSIONS

These findings suggest that hyperalgesia seen shortly after exposure to iSSC is not influenced by CGRP levels. However, in cases where recovery from injury may be slower, as it is in older rats, CGRP may contribute to the maintenance of hyperalgesia.

A method to test contractility of the supraspinatus muscle in mouse, rat, and rabbit

The rotator cuff (RTC) muscles not only generate movement but also provide important shoulder joint stability. RTC tears, particularly in the supraspinatus muscle, are a common clinical problem. Despite some biological healing after RTC repair, persistent problems include poor functional outcomes with high retear rates after surgical repair. Animal models allow further exploration of the sequela of RTC injury such as fibrosis, inflammation, and fatty infiltration, but there are few options regarding contractility for mouse, rat, and rabbit. Histological findings can provide a "direct measure" of damage, but the most comprehensive measure of the overall health of the muscle is contractile force. However, information regarding normal supraspinatus size and contractile function is scarce. Animal models provide the means to compare muscle histology, imaging, and contractility within individual muscles in various models of injury and disease, but to date, most testing of animal contractile force has been limited primarily to hindlimb muscles. Here, we describe an in vivo method to assess contractility of the supraspinatus muscle and describe differences in methods and representative outcomes for mouse, rat, and rabbit.

Desensitized morphological and cytokine response after stretch-shortening muscle contractions as a feature of aging in rats

Recovery from contraction-induced injury is impaired with aging. At a young age, the secondary response several days following contraction-induced injury consists of edema, inflammatory cell infiltration, and segmental muscle fiber degeneration to aid in the clearance of damaged tissue and repair. This morphological response has not been wholly established at advanced age. Our aim was to characterize muscle fiber morphology 3 and 10 days following stretch-shortening contractions (SSCs) varying in repetition number (i.e. 0, 30, 80, and 150) for young and old rats. For muscles of young rats, muscle fiber degeneration was overt at 3 days exclusively after 80 or 150 SSCs and returned significantly closer to control values by 10 days. For muscles of old rats, no such responses were observed. Transcriptional microarray analysis at 3 days demonstrated that muscles of young rats differentially expressed up to 2144 genes while muscles of old rats differentially expressed 47 genes. Bioinformatic analysis indicated that cellular movement was a major biological process over-represented with genes that were significantly altered by SSCs especially for young rats. Protein levels in muscle for various cytokines and chemokines, key inflammatory factors for cell movement, increased 3- to 50-fold following high-repetition SSCs for young rats with no change for old rats. This age-related differential response was insightful given that for control (i.e. 0 SSCs) conditions, protein levels of circulatory cytokines/chemokines were increased with age. The results demonstrate ongoing systemic low-grade inflammatory signaling and subsequent desensitization of the cytokine/chemokine and morphological response to contraction-induced injury with aging - features which accompany age-related impairment in muscle recovery.

Increased CCN2, substance P and tissue fibrosis are associated with sensorimotor declines in a rat model of repetitive overuse injury

Key clinical features of cumulative trauma disorders include pain, muscle weakness, and tissue fibrosis, although the etiology is still under investigation. Here, we characterized the temporal pattern of altered sensorimotor behaviors and inflammatory and fibrogenic processes occurring in forearm muscles and serum of young adult, female rats performing an operant, high repetition high force (HRHF) reaching and grasping task for 6, 12, or 18 weeks. Palmar mechanical sensitivity, cold temperature avoidance and spontaneous behavioral changes increased, while grip strength declined, in 18-week HRHF rats, compared to controls. Flexor digitorum muscles had increased MCP-1 levels after training and increased TNFalpha in 6-week HRHF rats. Serum had increased IL-1beta, IL-10 and IP-10 after training. Yet both muscle and serum inflammation resolved by week 18. In contrast, IFNγ increased at week 18 in both muscle and serum. Given the anti-fibrotic role of IFNγ, and to identify a mechanism for the continued grip strength losses and behavioral sensitivities, we evaluated the fibrogenic proteins CCN2, collagen type I and TGFB1, as well as the nociceptive/fibrogenic peptide substance P. Each increased in and around flexor digitorum muscles and extracellular matrix in the mid-forearm, and in nerves of the forepaw at 18 weeks. CCN2 was also increased in serum at week 18. At a time when inflammation had subsided, increases in fibrogenic proteins correlated with sensorimotor declines. Thus, muscle and nerve fibrosis may be critical components of chronic work-related musculoskeletal disorders. CCN2 and substance P may serve as potential targets for therapeutic intervention, and CCN2 as a serum biomarker of fibrosis progression.

Magnetic resonance imaging of graded skeletal muscle injury in live rats

INTRODUCTION

Increasing number of stretch-shortening contractions (SSCs) results in increased muscle injury.

METHODS

Fischer Hybrid rats were acutely exposed to an increasing number of SSCs in vivo using a custom-designed dynamometer. Magnetic resonance imaging (MRI) imaging was conducted 72 hours after exposure when rats were infused with Prohance and imaged using a 7T rodent MRI system (GE Epic 12.0). Images were acquired in the transverse plane with typically 60 total slices acquired covering the entire length of the hind legs. Rats were euthanized after MRI, the lower limbs removed, and tibialis anterior muscles were prepared for histology and quantified stereology.

RESULTS

Stereological analyses showed myofiber degeneration, and cellular infiltrates significantly increased following 70 and 150 SSC exposure compared to controls. MRI images revealed that the percent affected area significantly increased with exposure in all SSC groups in a graded fashion. Signal intensity also significantly increased with increasing SSC repetitions.

DISCUSSION

These results suggest that contrast-enhanced MRI has the sensitivity to differentiate specific degrees of skeletal muscle strain injury, and imaging data are specifically representative of cellular histopathology quantified via stereological analyses.

Volitional Weight-Lifting in Rats Promotes Adaptation via Performance and Muscle Morphology prior to Gains in Muscle Mass

Investigation of volitional animal models of resistance training has been instrumental in our understanding of adaptive training. However, these studies have lacked reactive force measurements, a precise performance measure, and morphological analysis at a distinct phase of training - when initial strength gains precede muscle hypertrophy. Our aim was to expose rats to one month of training (70 or 700 g load) on a custom-designed weight-lifting apparatus for analysis of reactive forces and muscle morphology prior to muscle hypertrophy. Exclusively following 700 g load training, forces increased by 21% whereas muscle masses remained unaltered. For soleus (SOL) and tibialis anterior (TA) muscles, 700 g load training increased muscle fiber number per unit area by ∼20% and decreased muscle fiber area by ∼20%. Additionally, number of muscle fibers per section increased by 18% for SOL muscles. These results establish that distinct morphological alterations accompany early strength gains in a volitional animal model of load-dependent adaptive resistance training.

The interaction of force and repetition on musculoskeletal and neural tissue responses and sensorimotor behavior in a rat model of work-related musculoskeletal disorders

Background

We examined the relationship of musculoskeletal risk factors underlying force and repetition on tissue responses in an operant rat model of repetitive reaching and pulling, and if force x repetition interactions were present, indicative of a fatigue failure process. We examined exposure-dependent changes in biochemical, morphological and sensorimotor responses occurring with repeated performance of a handle-pulling task for 12 weeks at one of four repetition and force levels: 1) low repetition with low force, 2) high repetition with low force, 3) low repetition with high force, and 4) high repetition with high force (HRHF).

Methods

Rats underwent initial training for 4–6 weeks, and then performed one of the tasks for 12 weeks, 2 hours/day, 3 days/week. Reflexive grip strength and sensitivity to touch were assayed as functional outcomes. Flexor digitorum muscles and tendons, forelimb bones, and serum were assayed using ELISA for indicators of inflammation, tissue stress and repair, and bone turnover. Histomorphometry was used to assay macrophage infiltration of tissues, spinal cord substance P changes, and tissue adaptative or degradative changes. MicroCT was used to assay bones for changes in bone quality.

Results

Several force x repetition interactions were observed for: muscle IL-1alpha and bone IL-1beta; serum TNFalpha, IL-1alpha, and IL-1beta; muscle HSP72, a tissue stress and repair protein; histomorphological evidence of tendon and cartilage degradation; serum biomarkers of bone degradation (CTXI) and bone formation (osteocalcin); and morphological evidence of bone adaptation versus resorption. In most cases, performance of the HRHF task induced the greatest tissue degenerative changes, while performance of moderate level tasks induced bone adaptation and a suggestion of muscle adaptation. Both high force tasks induced median nerve macrophage infiltration, spinal cord sensitization (increased substance P), grip strength declines and forepaw mechanical allodynia by task week 12.

Conclusions

Although not consistent in all tissues, we found several significant interactions between the critical musculoskeletal risk factors of force and repetition, consistent with a fatigue failure process in musculoskeletal tissues. Prolonged performance of HRHF tasks exhibited significantly increased risk for musculoskeletal disorders, while performance of moderate level tasks exhibited adaptation to task demands.

Increased serum and musculotendinous fibrogenic proteins following persistent low-grade inflammation in a rat model of long-term upper extremity overuse

We examined the relationship between grip strength declines and muscle-tendon responses induced by long-term performance of a high-repetition, low-force (HRLF) reaching task in rats. We hypothesized that grip strength declines would correlate with inflammation, fibrosis and degradation in flexor digitorum muscles and tendons. Grip strength declined after training, and further in weeks 18 and 24, in reach limbs of HRLF rats. Flexor digitorum tissues of reach limbs showed low-grade increases in inflammatory cytokines: IL-1β after training and in week 18, IL-1α in week 18, TNF-α and IL-6 after training and in week 24, and IL-10 in week 24, with greater increases in tendons than muscles. Similar cytokine increases were detected in serum with HRLF: IL-1α and IL-10 in week 18, and TNF-α and IL-6 in week 24. Grip strength correlated inversely with IL-6 in muscles, tendons and serum, and TNF-α in muscles and serum. Four fibrogenic proteins, TGFB1, CTGF, PDGFab and PDGFbb, and hydroxyproline, a marker of collagen synthesis, increased in serum in HRLF weeks 18 or 24, concomitant with epitendon thickening, increased muscle and tendon TGFB1 and CTGF. A collagenolytic gelatinase, MMP2, increased by week 18 in serum, tendons and muscles of HRLF rats. Grip strength correlated inversely with TGFB1 in muscles, tendons and serum; with CTGF-immunoreactive fibroblasts in tendons; and with MMP2 in tendons and serum. Thus, motor declines correlated with low-grade systemic and musculotendinous inflammation throughout task performance, and increased fibrogenic and degradative proteins with prolonged task performance. Serum TNF-α, IL-6, TGFB1, CTGF and MMP2 may serve as serum biomarkers of work-related musculoskeletal disorders, although further studies in humans are needed.

Performance of repetitive tasks induces decreased grip strength and increased fibrogenic proteins in skeletal muscle: role of force and inflammation

BACKGROUND

This study elucidates exposure-response relationships between performance of repetitive tasks, grip strength declines, and fibrogenic-related protein changes in muscles, and their link to inflammation. Specifically, we examined forearm flexor digitorum muscles for changes in connective tissue growth factor (CTGF; a matrix protein associated with fibrosis), collagen type I (Col1; a matrix component), and transforming growth factor beta 1 (TGFB1; an upstream modulator of CTGF and collagen), in rats performing one of two repetitive tasks, with or without anti-inflammatory drugs.

METHODOLOGY/RESULTS

To examine the roles of force versus repetition, rats performed either a high repetition negligible force food retrieval task (HRNF), or a high repetition high force handle-pulling task (HRHF), for up to 9 weeks, with results compared to trained only (TR-NF or TR-HF) and normal control rats. Grip strength declined with both tasks, with the greatest declines in 9-week HRHF rats. Quantitative PCR (qPCR) analyses of HRNF muscles showed increased expression of Col1 in weeks 3-9, and CTGF in weeks 6 and 9. Immunohistochemistry confirmed PCR results, and also showed greater increases of CTGF and collagen matrix in 9-week HRHF rats than 9-week HRNF rats. ELISA, and immunohistochemistry revealed greater increases of TGFB1 in TR-HF and 6-week HRHF, compared to 6-week HRNF rats. To examine the role of inflammation, results from 6-week HRHF rats were compared to rats receiving ibuprofen or anti-TNF-α treatment in HRHF weeks 4-6. Both treatments attenuated HRHF-induced increases in CTGF and fibrosis by 6 weeks of task performance. Ibuprofen attenuated TGFB1 increases and grip strength declines, matching our prior results with anti-TNFα.

CONCLUSIONS/SIGNIFICANCE

Performance of highly repetitive tasks was associated with force-dependent declines in grip strength and increased fibrogenic-related proteins in flexor digitorum muscles. These changes were attenuated, at least short-term, by anti-inflammatory treatments.

Resistance loading and signaling assays for oxidative stress in rodent skeletal muscle

Resistance loading provides an important tool for understanding skeletal muscle responses and adaptations to various perturbations. A model using anesthetized rodents provides the means to control the input parameters carefully, and to measure the output parameters of each muscle contraction. Unilateral models of anesthetized loading also provide the advantage of comparing an unloaded and loaded muscle from the same animal. Voluntary models for resistance loading arguably provide a more "physiological response" but it also introduces more variability in the input parameters, which can be affected by the stimulus used to motivate the animal to exercise. After either acute or chronic periods of muscle loading, the loaded muscles can be removed and various signaling proteins can be determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) or enzyme assays. Several assays are described, which provide an indication of downstream markers for oxidative stress.

Skeletal muscle injury versus adaptation with aging: novel insights on perplexing paradigms

A growing body of data supports a view that skeletal muscle's response after mechanical loading does not always result in the classically reported "injury response." Furthermore, current evidence supports a model of muscle adaptation and/or maladaptation, distinct from overt injury, in which myofiber degeneration and inflammation do not contribute as significantly as once reported even in aged populations.

Location of myofiber damage in skeletal muscle after lengthening contractions

High-force lengthening contractions are associated with muscle damage and pain, and the muscle-tendon junction is commonly cited as the primary area where myofiber damage occurs. We induced injury in the rat tibialis anterior muscle and acquired magnetic resonance imaging (MRI) images postinjury. We also assayed membrane damage and quantified the number of centrally nucleated myofibers throughout the injured muscles. Results suggest that myofiber injury occurs primarily in the middle portion of the muscle, with interstitial edema in the middle and distal portions.

Investigations into pulsed high-intensity focused ultrasound-enhanced delivery: preliminary evidence for a novel mechanism

Pulsed high-intensity focused ultrasound (HIFU) exposures without ultrasound contrast agents have been used for noninvasively enhancing the delivery of various agents to improve their therapeutic efficacy in a variety of tissue models in a nondestructive manner. Despite the versatility of these exposures, little is known about the mechanisms by which their effects are produced. In this study, pulsed-HIFU exposures were given in the calf muscle of mice, followed by the administration of a variety of fluorophores, both soluble and particulate, by local or systemic injection. In vivo imaging (whole animal and microscopic) was used to quantify observations of increased extravasation and interstitial transport of the fluorophores as a result of the exposures. Histological analysis indicated that the exposures caused some structural alterations such as enlarged gaps between muscle fiber bundles. These effects were consistent with increasing the permeability of the tissues; however, they were found to be transient and reversed themselves gradually within 72 h. Simulations of radiation force-induced displacements and the resulting local shear strain they produced were carried out to potentially explain the manner by which these effects occurred. A better understanding of the mechanisms involved with pulsed HIFU exposures for noninvasively enhancing delivery will facilitate the process for optimizing their use.

Response of tibialis anterior tendon to a chronic exposure of stretch-shortening cycles: age effects

BACKGROUND

The purpose of the current study was to investigate the effects of aging on tendon response to repetitive exposures of stretch-shortening cycles (SSC's).

METHODS

The left hind limb from young (3 mo, N = 4) and old (30 mo, N = 9) male Fisher 344 x Brown Norway rats were exposed to 80 maximal SSCs (60 deg/s, 50 deg range of motion) 3 x/week for 4.5 weeks in vivo. After the last exposure, tendons from the tibialis anterior muscle were isolated, stored at -80 degrees C, and then tested using a micro-mechanical testing machine. Deformation of each tendon was evaluated using both relative grip-to-grip displacements and reference marks via a video system.

RESULTS

At failure, the young control tendons had higher strain magnitude than the young exposed (p < 0.01) and the old control tendons (p < .0001). Total load at inflection was affected by age only (p < 0.01). Old exposed and control tendons exhibited significantly higher loads at the inflection point than their young counterparts (p < 0.05 for both comparisons). At failure, the old exposed tendons carried higher loads than the young exposed tendons (p < 0.05). Stiffness was affected by age only at failure where the old tendons exhibited higher stiffness in both exposed and control tendons than their young counterparts (p < 0.05 and p < 0.01, respectively).

CONCLUSION

The chronic protocol enhanced the elastic stiffness of young tendon and the loads in both the young and old tendons. The old exposed tendons were found to exhibit higher load capacity than their younger counterparts, which differed from our initial hypothesis.

High force reaching task induces widespread inflammation, increased spinal cord neurochemicals and neuropathic pain

Repetitive strain injuries (RSI), which include several musculoskeletal disorders and nerve compression injuries, are associated with performance of repetitive and forceful tasks. In this study, we examined in young, adult Sprague-Dawley rats, the effects of performing a voluntary, moderate repetition, high force (MRHF; nine reaches/min; 60% maximum pulling force) task for 12 weeks on motor behavior and nerve function, inflammatory responses in forearm musculoskeletal and nerve tissues and serum, and neurochemical immunoexpression in cervical spinal cord dorsal horns. We observed no change in reach rate, but reduced voluntary participation and grip strength in week 12, and increased cutaneous sensitivity in weeks 6 and 12, the latter indicative of mechanical allodynia. Nerve conduction velocity (NCV) decreased 15% in the median nerve in week 12, indicative of low-grade nerve compression. ED-1 cells increased in distal radius and ulna in week 12, and in the median nerve and forearm muscles and tendons in weeks 6 and 12. Cytokines IL-1alpha, IL-1beta, TNF-alpha, and IL-10 increased in distal forearm bones in week 12, while IL-6 increased in tendon in week 12. However, serum analysis revealed only increased TNF-alpha in week 6 and macrophage inflammatory protein 3a (MIP3a) in weeks 6 and 12. Lastly, Substance P and neurokinin-1 were both increased in weeks 6 and 12 in the dorsal horns of cervical spinal cord segments. These results show that a high force, but moderate repetition task, induced declines in motor and nerve function as well as peripheral and systemic inflammatory responses (albeit the latter was mild). The peripheral inflammatory responses were associated with signs of central sensitization (mechanical allodynia and increased neurochemicals in spinal cord dorsal horns).

Spinal substance P and neurokinin-1 increase with high repetition reaching

Musculoskeletal injury and inflammation is associated with performance of repetitive and forceful tasks. In this study, we examined the effects of performing a voluntary, highly repetitive, negligible force (HRNF) reaching task on spinal cord neurochemicals involved in nociception. To our knowledge, no other laboratories are examining spinal cord nociceptive neurochemicals in response to repetitive motion-induced injury and inflammation. The purpose of this study was to extend our earlier findings related to central neurotransmitters from a low demand task to a higher demand task. Specifically, this study determined immunoreactivity of a peptidergic pro-nociceptive transmitter (substance P) and one of its receptors, neurokinin-1 (NK-1) receptor, in spinal cord dorsal horns in rats performing a HRNF reaching task for 6-10 weeks. The relationship of these spinal cord changes with the number of TNFalpha immunopositive cells in flexor forelimb muscles and with previously observed forearm grip strength changes from these same rats were examined. Performance of the HRNF task resulted in significantly increased substance P and NK-1 receptor immunoreactivity in the superficial lamina of spinal cord dorsal horns at 6 and 10 weeks compared to trained controls (p<0.01). The increased substance P and NK-1 receptor immmunoreactivity were positively correlated with declines in forearm grip strength, an assay of movement-related hyperalgesia (r=0.70, p<0.01 and r=0.64, p<0.05, respectively). The increased substance P and NK-1 receptor immmunoreactivity were also positively correlated with increased TNF immunopositive cells in forelimb flexor muscles (r=0.85, p<0.001 and r=0.88, p<0.001, respectively). Thus, our highly repetitive task leads to increased spinal cord pro-nociceptive neurochemicals that are most likely directed by forelimb muscle inflammation and pain.

National occupational research agenda (NORA) future directions in occupational musculoskeletal disorder health research

Musculoskeletal disorders are among the most costly health care problems facing society today. The scientific literature has indicated that psychosocial factors, individual factors, workplace physical requirements, and workplace organizational factors have been associated with risk. Since musculoskeletal risk is multi-dimensional, the magnitude of risk attributable to various factors can be of importance to scientists and policy makers in designing countermeasures to reduce injury incidence. Traditionally, the disciplines of biomechanics, physiology, and psychophysics have dominated the body of knowledge that has defined exposure limitations to work. However, recent research has explored the association of psychosocial and work organization factors with musculoskeletal problems. Advances have been made to better quantify the levels of occupational exposure by improved exposure metrics, quantification of three-dimensional loads experienced by certain joints (e.g. the spine), identification of tissue tolerance limits and tissue response to mechanical stresses, and the impact of psychosocial stresses. However, efforts to quantitatively link epidemiological, biomechanical loading, soft tissue tolerance, and psychosocial studies should be pursued to establish a better understanding of the pathways of injury and resultant preventive strategies. Although we are beginning to understand how the major risk factors influence the load-tolerance relationship of human tissue, how these risk factors interact is virtually unexplored. Since the impact of the interactions may be far greater than that of any individual factor, the impact of the interactions between risk factors must be delineated so that work-related risk can be better quantified. Efforts to quantitatively link epidemiological, biomechanical loading, soft tissue tolerance, and psychosocial studies should be pursued to establish a better understanding of the pathways of injury and resultant preventive strategies.

Adaptive stretch-shortening contractions: diminished regenerative capacity with aging

This study determined the age-related changes in acute events responsible for initiating skeletal muscle remodeling and (or) regeneration in the tibialis anterior muscle following a bout of stretch-shortening contractions (SSCs). Changes in muscle performance and morphology were quantified in young and old rats, following an acute exposure to adaptive SSCs at 6, 24, 48, 72, and 120 h postexposure (n = 6 for each age at each recovery period). Following SSC exposure, all performance measures were decreased in old rats throughout the 120 h acute phase. Estimates of edema were increased in the old vs. young exposed muscle at 120 h recovery. Both young and old rats displayed an increase in developmental myosin heavy chain (MHCdev+) labeling in the exposed muscle, indicating muscle regeneration. However, old rats displayed diminished MHCdev+ labeling, compared with young rats, suggesting limited remodeling and (or) regenerative capacity. Based on these data, diminished local muscle remodeling and (or) regeneration with aging may limit skeletal muscle adaptation following mechanical loading.

Peripheral neuritis and increased spinal cord neurochemicals are induced in a model of repetitive motion injury with low force and repetition exposure

Performance of high repetition tasks with or without force is associated with peripheral tissue inflammation, decreased nerve function and motor dysfunction. Here, we examined whether a low repetition task with negligible force (LRNF) produces fewer tissue and behavioral pathologies than previously observed with high repetition tasks using our rat model of repetitive motion injury (RMI). Thirty-seven rats were randomized into control or LRNF groups, the latter reaching and grasping a 45 mg food pellet at a rate of 3 reaches/min. This task was performed in 4, 0.5 5 h sessions with 1.5 5 h rest periods for 3 days/week for up to 12 weeks. Examination of distal median nerve, forelimb flexor tendons and bones for ED1-positive cells (macrophages and osteoclasts) revealed increases in nerve and bone in week 12. The nerve also contained increased TNF-alpha expressing cells in week 12. Examination of spinal cord dorsal horns revealed increased immunoexpression of Substance P in week 8 and neurokinin-1 in weeks 8 and 12 in the superficial lamina. Motor behavioral analyses showed no changes in reach rate across weeks, slightly reduced task duration (a measurement of voluntary task participation) in week 12, but significantly increased extra arm movement reversals during reaching in week 8. These extra movement reversals were corrections for missed food pellets during a reach. Thus, performance of even a low repetition, negligible force upper extremity task for 3 months can induce mild peripheral tissue inflammation, neurochemical increases in spinal cord dorsal horns, and declines in fine motor control.