Repetitive, negligible force reaching in rats induces pathological overloading of upper extremity bones

Let's Focus on the Fibrosis in Dupuytren Disease: Cell Communication Network Factor 2 as a Novel Target

Dupuytren disease is a progressive, benign fibroproliferative disorder of the hands that can lead to debilitating hand contractures. Once symptomatic, treatment involves either surgical intervention, specifically fasciectomy or percutaneous needle aponeurotomy, or enzymatic degradation with clostridial collagenase. Currently, collagenase is the only pharmacotherapy that has been approved for the treatment of Dupuytren contracture. There is a need for a pharmacotherapeutic that can be administered to limit disease progression and prevent recurrence after treatment. Targeting the underlying fibrotic pathophysiology is critical. We propose a novel target to be considered in Dupuytren disease-cell communication network factor 2/connective tissue growth factor-an established mediator of musculoskeletal tissue fibrosis.

Blocking CCN2 Reduces Established Bone Loss Induced by Prolonged Intense Loading by Increasing Osteoblast Activity in Rats

We have an operant model of reaching and grasping in which detrimental bone remodeling is observed rather than beneficial adaptation when rats perform a high-repetition, high-force (HRHF) task long term. Here, adult female Sprague-Dawley rats performed an intense HRHF task for 18 weeks, which we have shown induces radial trabecular bone osteopenia. One cohort was euthanized at this point (to assay the bone changes post task; HRHF-Untreated). Two other cohorts were placed on 6 weeks of rest while being simultaneously treated with either an anti-CCN2 (FG-3019, 40 mg/kg body weight, ip; twice per week; HRHF-Rest/anti-CCN2), or a control IgG (HRHF-Rest/IgG), with the purpose of determining which might improve the trabecular bone decline. Results were compared with food-restricted control rats (FRC). MicroCT analysis of distal metaphysis of radii showed decreased trabecular bone volume fraction (BV/TV) and thickness in HRHF-Untreated rats compared with FRCs; responses improved with HRHF-Rest/anti-CCN2. Rest/IgG also improved trabecular thickness but not BV/TV. Histomorphometry showed that rest with either treatment improved osteoid volume and task-induced increases in osteoclasts. Only the HRHF-Rest/anti-CCN2 treatment improved osteoblast numbers, osteoid width, mineralization, and bone formation rate compared with HRHF-Untreated rats (as well as the latter three attributes compared with HRHF-Rest/IgG rats). Serum ELISA results were in support, showing increased osteocalcin and decreased CTX-1 in HRHF-Rest/anti-CCN2 rats compared with both HRHF-Untreated and HRHF-Rest/IgG rats. These results are highly encouraging for use of anti-CCN2 for therapeutic treatment of bone loss, such as that induced by chronic overuse. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Manual Therapy Facilitates Homeostatic Adaptation to Bone Microstructural Declines Induced by a Rat Model of Repetitive Forceful Task

The effectiveness of manual therapy in reducing the catabolic effects of performing repetitive intensive force tasks on bones has not been reported. We examined if manual therapy could reduce radial bone microstructural declines in adult female Sprague–Dawley rats performing a 12-week high-repetition and high-force task, with or without simultaneous manual therapy to forelimbs. Additional rats were provided 6 weeks of rest after task cessation, with or without manual therapy. The control rats were untreated or received manual therapy for 12 weeks. The untreated TASK rats showed increased catabolic indices in the radius (decreased trabecular bone volume and numbers, increased osteoclasts in these trabeculae, and mid-diaphyseal cortical bone thinning) and increased serum CTX-1, TNF-α, and muscle macrophages. In contrast, the TASK rats receiving manual therapy showed increased radial bone anabolism (increased trabecular bone volume and osteoblast numbers, decreased osteoclast numbers, and increased mid-diaphyseal total area and periosteal perimeter) and increased serum TNF-α and muscle macrophages. Rest, with or without manual therapy, improved the trabecular thickness and mid-diaphyseal cortical bone attributes but not the mineral density. Thus, preventive manual therapy reduced the net radial bone catabolism by increasing osteogenesis, while rest, with or without manual therapy, was less effective.

When Spinal Neuromodulation Meets Sensorimotor Rehabilitation: Lessons Learned From Animal Models to Regain Manual Dexterity After a Spinal Cord Injury

Electrical neuromodulation has strongly hit the foundations of spinal cord injury and repair. Clinical and experimental studies have demonstrated the ability to neuromodulate and engage spinal cord circuits to recover volitional motor functions lost after the injury. Although the science and technology behind electrical neuromodulation has attracted much of the attention, it cannot be obviated that electrical stimulation must be applied concomitantly to sensorimotor rehabilitation, and one would be very difficult to understand without the other, as both need to be finely tuned to efficiently execute movements. The present review explores the difficulties faced by experimental and clinical neuroscientists when attempting to neuromodulate and rehabilitate manual dexterity in spinal cord injured subjects. From a translational point of view, we will describe the major rehabilitation interventions employed in animal research to promote recovery of forelimb motor function. On the other hand, we will outline some of the state-of-the-art findings when applying electrical neuromodulation to the spinal cord in animal models and human patients, highlighting how evidences from lumbar stimulation are paving the path to cervical neuromodulation.

Physical work conditions and disparities in later life functioning: Potential pathways

Research in the US on the social determinants of reduced physical functioning at older ages has typically not considered physical work conditions as contributors to disparities. We briefly describe a model of occupational stratification and segregation, review and synthesize the occupational health literature, and outline the physiological pathways through which physical work exposures may be tied to long-term declines in physical functioning. The literature suggests that posture, force, vibration, and repetition are the primary occupational risk factors implicated in the development of musculoskeletal disorders, through either acute injuries or longer-term wear and tear. Personal risk factors and environmental and structural work characteristics can modify this association. In the long-term, these musculoskeletal disorders can become chronic and ultimately lead to functional limitations and disabilities that interfere with one's quality of life and ability to remain independent. We then use data on occupational characteristics from the Occupational Information Network (O*NET) linked to the 2019 American Community Survey (ACS) to examine disparities among sociodemographic groups in exposure to these risk factors. Occupations with high levels of these physical demands are not limited to those traditionally thought of as manual or blue-collar jobs and include many positions in the service sector. We document a steep education gradient with less educated workers experiencing far greater physical demands at work than more educated workers. There are pronounced racial and ethnic differences in these exposures with Hispanic, Black, and Native American workers experiencing higher risks than White and Asian workers. Occupations with high exposures to these physical risk factors provide lower compensation and are less likely to provide employer-sponsored health insurance, making it more difficult for workers to address injuries or conditions that arise from their jobs. In sum, we argue that physical work exposures are likely an important pathway through which disparities in physical functioning arise.

Blocking CCN2 preferentially inhibits osteoclastogenesis induced by repetitive high force bone loading

Purpose/Aim: We recently found that blocking CCN2 signaling using a monoclonal antibody (FG-3019) may be a novel therapeutic strategy for reducing overuse-induced tissue fibrosis. Since CCN2 plays roles in osteoclastogenesis, and persistent performance of a high repetition high force (HRHF) lever pulling task results in a loss in trabecular bone volume in the radius, we examined here whether blocking CCN2 signaling would reduce the early catabolic effects of performing a HRHF task for 3 weeks. Materials and Methods: Young adult, female, Sprague-Dawley rats were operantly shaped to learn to pull at high force levels, before performing the HRHF task for 3 weeks. HRHF task rats were then left untreated (HRHF Untreated), treated in task weeks 2 and 3 with a monoclonal antibody that antagonizes CCN2 (HRHF+FG-3019), or treated with an IgG (HRHF+IgG), while continuing to perform the task. Non-task control rats were left untreated. Results: In metaphyseal trabeculae of the distal radius, HRHF Untreated and HRHF-IgG rats showed increased osteoblast numbers and other indices of bone formation, compared to controls, yet decreased trabecular bone volume, increased osteoclast numbers, and increased serum CTX-1 (a serum biomarker of bone resorption). HRHF+FG-3019 rats also showed increased osteoblast numbers and bone formation, but in contrast to HRHF Untreated and HRHF-IgG rats, showed higher trabecular bone volume, and reduced osteoclast numbers and serum CTX-1 levels (and statistically similar to Control levels). Conclusions: HRHF loading increased bone formation in each task group, yet blocking CCN2 dampened trabecular bone catabolism by reducing osteoclast numbers and activity.

Blocking CTGF/CCN2 reverses neural fibrosis and sensorimotor declines in a rat model of overuse-induced median mononeuropathy

Encapsulation of median nerves is a hallmark of overuse-induced median mononeuropathy and contributes to functional declines. We tested if an antibody against CTGF/CCN2 (termed FG-3019 or Pamrevlumab) reduces established neural fibrosis and sensorimotor declines in a clinically relevant rodent model of overuse in which median mononeuropathy develops. Young adult female rats performed a high repetition high force (HRHF) lever-pulling task for 18 weeks. Rats were then euthanised at 18 weeks (HRHF untreated), or rested and systemically treated for 6 weeks with either an anti-CCN2 monoclonal antibody (HRHF-Rest/FG-3019) or IgG (HRHF-Rest/IgG), with results compared with nontask control rats. Neuropathology was evident in HRHF-untreated and HRHF-Rest/IgG rats as increased perineural collagen deposition and degraded myelin basic protein (dMBP) in median nerves, and increased substance P in lower cervical dorsal root ganglia (DRG), compared with controls. Both groups showed functional declines, specifically, decreased sensory conduction velocity in median nerves, noxious cold temperature hypersensitivity, and grip strength declines, compared with controls. There were also increases of ATF3-immunopositive nuclei in ventral horn neurons in HRHF-untreated rats, compared with controls (which showed none). FG-3019-treated rats showed no increase above control levels of perineural collagen or dMBP in median nerves, Substance P in lower cervical DRGs, or ATF3-immunopositive nuclei in ventral horns, and similar median nerve conduction velocities and thermal sensitivity, compared with controls. We hypothesize that neural fibrotic processes underpin the sensorimotor declines by compressing or impeding median nerves during movement, and that inhibiting fibrosis using an anti-CCN2 treatment reverses these effects.

The Pelvic Girdle Pain deadlock: 2. Topics that, so far, have remained out of focus

INTRODUCTION

In our preceding paper, we concluded that Pelvic Girdle Pain (PGP) should be taken seriously. Still, we do not know its causes. Literature reviews on treatment fail to reveal a consistent pattern, and there are patients who do not respond well to treatment. We designated the lack of progress in research and in the clinic as 'deadlock', and proposed a 'deconstruction' of PGP, that is to say, taking PGP apart into its relevant dimensions.

PURPOSE

We examine the proposition that PGP may emerge as local inflammation. Inflammation would be a new dimension to be taken into account, between biomechanics and psychology. To explore the consequences of this idea, we present four different topics that, so far, have remained out of focus. One: The importance of microtrauma. Two: Ways to counteract chronification. Three: The importance of sickness behaviour when systemic inflammation turns into neuroinflammation of the brain. And Four: The mainly emotional and cognitive nature of chronic pain, and how aberrant neuroinflammation may render chronic pain intractable. For intractable pain, sleep and stress management are promising treatment options.

IMPLICATIONS

The authors hope that the present paper helps to stimulate the flexible creativity that is required to deal with the biological and psychological impact of PGP. Measuring inflammatory mediators in PGP should be a research priority. It should be understood that the boundaries between biology and psychology are becoming blurred. Clinicians must frequently monitor pain, disability, and mood, and be ready to switch treatment whenever the patient does not improve.

Rehabilitative Training in Animal Models of Spinal Cord Injury

Rehabilitative motor training is currently one of the most widely used approaches to promote moderate recovery following injuries of the central nervous system. Such training is generally applied in the clinical setting, whereas it is not standard in preclinical research. This is a concern as it is becoming increasingly apparent that neuroplasticity enhancing treatments require training or some form of activity as a co-therapy to promote functional recovery. Despite the importance of training and the many open questions regarding its mechanistic consequences, its use in preclinical animal models is rather limited. Here we review approaches, findings and challenges when training is applied in animal models of spinal cord injury, and we suggest recommendations to facilitate the integration of training using an appropriate study design, into pre-clinical studies.

Prolonged high force high repetition pulling induces osteocyte apoptosis and trabecular bone loss in distal radius, while low force high repetition pulling induces bone anabolism

We have an operant rat model of upper extremity reaching and grasping in which we examined the impact of performing a high force high repetition (High-ForceHR) versus a low force low repetition (Low-ForceHR) task for 18weeks on the radius and ulna, compared to age-matched controls. High-ForceHR rats performed at 4 reaches/min and 50% of their maximum voluntary pulling force for 2h/day, 3days/week. Low-ForceHR rats performed at 6% maximum voluntary pulling force. High-ForceHR rats showed decreased trabecular bone volume in the distal metaphyseal radius, decreased anabolic indices in this same bone region (e.g., decreased osteoblasts and bone formation rate), and increased catabolic indices (e.g., microcracks, increased osteocyte apoptosis, secreted sclerostin, RANKL, and osteoclast numbers), compared to controls. Distal metaphyseal trabeculae in the ulna of High-ForceHR rats showed a non-significant decrease in bone volume, some catabolic indices (e.g., decreased trabecular numbers) yet also some anabolic indices (e.g., increased osteoblasts and trabecular thickness). In contrast, the mid-diaphyseal region of High-ForceHR rats' radial and ulnar bones showed few to no microarchitecture differences and no changes in apoptosis, sclerostin or RANKL levels, compared to controls. In further contrast, Low-ForceHR rats showed increased trabecular bone volume in the radius in the distal metaphysis and increased cortical bone area its mid-diaphysis. These changes were accompanied by increased anabolic indices, no microcracks or osteocyte apoptosis, and decreased RANKL in each region, compared to controls. Ulnar bones of Low-ForceHR rats also showed increased anabolic indices, although fewer than in the adjacent radius. Thus, prolonged performance of an upper extremity reaching and grasping task is loading-, region-, and bone-dependent, with high force loads at high repetition rates inducing region-specific increases in bone degradative changes that were most prominent in distal radial trabeculae, while low force task loads at high repetition rates induced adaptive bone responses.

Prolonged performance of a high repetition low force task induces bone adaptation in young adult rats, but loss in mature rats

We have shown that prolonged repetitive reaching and grasping tasks lead to exposure-dependent changes in bone microarchitecture and inflammatory cytokines in young adult rats. Since aging mammals show increased tissue inflammatory cytokines, we sought here to determine if aging, combined with prolonged performance of a repetitive upper extremity task, enhances bone loss. We examined the radius, forearm flexor muscles, and serum from 16 mature (14-18 months of age) and 14 young adult (2.5-6.5 months of age) female rats after performance of a high repetition low force (HRLF) reaching and grasping task for 12 weeks. Young adult HRLF rats showed enhanced radial bone growth (e.g., increased trabecular bone volume, osteoblast numbers, bone formation rate, and mid-diaphyseal periosteal perimeter), compared to age-matched controls. Mature HRLF rats showed several indices of radial bone loss (e.g., decreased trabecular bone volume, and increased cortical bone thinning, porosity, resorptive spaces and woven bone formation), increased osteoclast numbers and inflammatory cytokines, compared to age-matched controls and young adult HRLF rats. Mature rats weighed more yet had lower maximum reflexive grip strength, than young adult rats, although each age group was able to pull at the required reach rate (4 reaches/min) and required submaximal pulling force (30 force-grams) for a food reward. Serum estrogen levels and flexor digitorum muscle size were similar in each age group. Thus, mature rats had increased bone degradative changes than in young adult rats performing the same repetitive task for 12 weeks, with increased inflammatory cytokine responses and osteoclast activity as possible causes.

Ergonomic task reduction prevents bone osteopenia in a rat model of upper extremity overuse

We evaluated the effectiveness of ergonomic workload reduction of switching rats from a high repetition high force (HRHF) lever pulling task to a reduced force and reach rate task for preventing task-induced osteopenic changes in distal forelimb bones. Distal radius and ulna trabecular structure was examined in young adult rats performing one of three handle-pulling tasks for 12 wk: (1) HRHF, (2) low repetition low force (LRLF); or (3) HRHF for 4 wk and than LRLF thereafter (HRHF-to-LRLF). Results were compared to age-matched controls rats. Distal forelimb bones of 12-wk HRHF rats showed increased trabecular resorption and decreased volume, as control rats. HRHF-to-LRLF rats had similar trabecular bone quality as control rats; and decreased bone resorption (decreased trabecular bone volume and serum CTX1), increased bone formation (increased mineral apposition, bone formation rate, and serum osteocalcin), and decreased osteoclasts and inflammatory cytokines, than HRHF rats. Thus, an ergonomic intervention of HRHF-to-LRLF prevented loss of trabecular bone volume occurring with prolonged performance of a repetitive upper extremity task. These findings support the idea of reduced workload as an effective approach to management of work-related musculoskeletal disorders, and begin to define reach rate and load level boundaries for such interventions.

Role of inflammation in the aging bones

Chronic inflammation in aging is characterized by increased inflammatory cytokines, bone loss, decreased adaptation, and defective tissue repair in response to injury. Aging leads to inherent changes in mesenchymal stem cell (MSC) differentiation, resulting in impaired osteoblastogenesis. Also, the pro-inflammatory cytokines increase with aging, leading to enhanced myelopoiesis and osteoclastogenesis. Bone marrow macrophages (BMMs) play pivotal roles in osteoblast differentiation, the maintenance of hematopoietic stem cells (HSCs), and subsequent bone repair. However, during aging, little is known about the role of macrophages in the differentiation and function of MSC and HSC. Aged mammals have higher circulating pro-inflammatory cytokines than young adults, supporting the hypothesis of increased inflammation with aging. This review will aid in the understanding of the potential role(s) of pro-inflammatory (M1) and anti-inflammatory (M2) macrophages in differentiation and function of osteoblasts and osteoclasts in relation to aging.

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.

Aging contributes to inflammation in upper extremity tendons and declines in forelimb agility in a rat model of upper extremity overuse

We sought to determine if tendon inflammatory and histopathological responses increase in aged rats compared to young rats performing a voluntary upper extremity repetitive task, and if these changes are associated with motor declines. Ninety-six female Sprague-Dawley rats were used in the rat model of upper extremity overuse: 67 aged and 29 young adult rats. After a training period of 4 weeks, task rats performed a voluntary high repetition low force (HRLF) handle-pulling task for 2 hrs/day, 3 days/wk for up to 12 weeks. Upper extremity motor function was assessed, as were inflammatory and histomorphological changes in flexor digitorum and supraspinatus tendons. The percentage of successful reaches improved in young adult HRLF rats, but not in aged HRLF rats. Forelimb agility decreased transiently in young adult HRLF rats, but persistently in aged HRLF rats. HRLF task performance for 12 weeks lead to increased IL-1beta and IL-6 in flexor digitorum tendons of aged HRLF rats, compared to aged normal control (NC) as well as young adult HRLF rats. In contrast, TNF-alpha increased more in flexor digitorum tendons of young adult 12-week HRLF rats than in aged HRLF rats. Vascularity and collagen fibril organization were not affected by task performance in flexor digitorum tendons of either age group, although cellularity increased in both. By week 12 of HRLF task performance, vascularity and cellularity increased in the supraspinatus tendons of only aged rats. The increased cellularity was due to increased macrophages and connective tissue growth factor (CTGF)-immunoreactive fibroblasts in the peritendon. In conclusion, aged rat tendons were overall more affected by the HRLF task than young adult tendons, particularly supraspinatus tendons. Greater inflammatory changes in aged HRLF rat tendons were observed, increases associated temporally with decreased forelimb agility and lack of improvement in task success.

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.

Aging enhances serum cytokine response but not task-induced grip strength declines in a rat model of work-related musculoskeletal disorders

BACKGROUND

We previously reported early tissue injury, increased serum and tissue inflammatory cytokines and decreased grip in young rats performing a moderate demand repetitive task. The tissue cytokine response was transient, the serum response and decreased grip were still evident by 8 weeks. Thus, here, we examined their levels at 12 weeks in young rats. Since aging is known to enhance serum cytokine levels, we also examined aged rats.

METHODS

Aged and young rats, 14 mo and 2.5 mo of age at onset, respectfully, were trained 15 min/day for 4 weeks, and then performed a high repetition, low force (HRLF) reaching and grasping task for 2 hours/day, for 12 weeks. Serum was assayed for 6 cytokines: IL-1alpha, IL-6, IFN-gamma, TNF-alpha, MIP2, IL-10. Grip strength was assayed, since we have previously shown an inverse correlation between grip strength and serum inflammatory cytokines. Results were compared to naïve (grip), and normal, food-restricted and trained-only controls.

RESULTS

Serum cytokines were higher overall in aged than young rats, with increases in IL-1alpha, IFN-gamma and IL-6 in aged Trained and 12-week HRLF rats, compared to young Trained and HRLF rats (p < 0.05 and p < 0.001, respectively, each). IL-6 was also increased in aged 12-week HRLF versus aged normal controls (p < 0.05). Serum IFN-gamma and MIP2 levels were also increased in young 6-week HRLF rats, but no cytokines were above baseline levels in young 12-week HRLF rats. Grip strength declined in both young and aged 12-week HRLF rats, compared to naïve and normal controls (p < 0.05 each), but these declines correlated only with IL-6 levels in aged rats (r = -0.39).

CONCLUSION

Aging enhanced a serum cytokine response in general, a response that was even greater with repetitive task performance. Grip strength was adversely affected by task performance in both age groups, but was apparently influenced by factors other than serum cytokine levels in young rats.

Joint inflammation and early degeneration induced by high-force reaching are attenuated by ibuprofen in an animal model of work-related musculoskeletal disorder

We used our voluntary rat model of reaching and grasping to study the effect of performing a high-repetition and high-force (HRHF) task for 12 weeks on wrist joints. We also studied the effectiveness of ibuprofen, administered in the last 8 weeks, in attenuating HRHF-induced changes in these joints. With HRHF task performance, ED1+ and COX2+ cells were present in subchondral radius, carpal bones and synovium; IL-1alpha and TNF-alpha increased in distal radius/ulna/carpal bones; chondrocytes stained with Terminal deoxynucleotidyl Transferase- (TDT-) mediated dUTP-biotin nick end-labeling (TUNEL) increased in wrist articular cartilages; superficial structural changes (e.g., pannus) and reduced proteoglycan staining were observed in wrist articular cartilages. These changes were not present in normal controls or ibuprofen treated rats, although IL-1alpha was increased in reach limbs of trained controls. HRHF-induced increases in serum C1,2C (a biomarker of collagen I and II degradation), and the ratio of collagen degradation to synthesis (C1,2C/CPII; the latter a biomarker of collage type II synthesis) were also attenuated by ibuprofen. Thus, ibuprofen treatment was effective in attenuating HRHF-induced inflammation and early articular cartilage degeneration.

Role of TNF alpha and PLF in bone remodeling in a rat model of repetitive reaching and grasping

We have previously developed a voluntary rat model of highly repetitive reaching that provides an opportunity to study effects of non-weight bearing muscular loads on bone and mechanisms of naturally occurring inflammation on upper limb tissues in vivo. In this study, we investigated the relationship between inflammatory cytokines and matricellular proteins (Periostin-like-factor, PLF, and connective tissue growth factor, CTGF) using our model. We also examined the relationship between inflammatory cytokines, PLF and bone formation processes. Rats underwent initial training for 5 weeks, and then performed a high repetition high force (HRHF) task (12 reaches/min, 60% maximum grip force, 2 h/day, 3 days/week) for 6 weeks. We then examined the effect of training or task performance with or without treatment with a rat specific TNFalpha antibody on inflammatory cytokines, osteocalcin (a bone formation marker), PLF, CTGF, and behavioral indicators of pain or discomfort. The HRHF task decreased grip strength and induced forepaw mechanical hypersensitivity in both trained control and 6-week HRHF animals. Two weeks of anti-TNFalpha treatment improved grip strength in both groups, but did not ameliorate forepaw hypersensitivity. Moreover, anti-TNFalpha treatment attenuated task-induced increases in inflammatory cytokines (TNFalpha, IL-1alpha, and MIP2 in serum; TNFalpha in forelimb bone and muscles) and serum osteocalcin in 6-week HRHF animals. PLF levels in forelimb bones and flexor digitorum muscles increased significantly in 6-week HRHF animals, increases attenuated by anti-TNFalpha treatment. CTGF levels were unaffected by task performance or anti-TNFalpha treatment in 6-week HRHF muscles. In primary osteoblast cultures, TNFalpha, MIP2 and MIP3a treatment increased PLF levels in a dose dependent manner. Also in primary osteoblast cultures, increased PLF promoted proliferation and differentiation, the latter assessed by measuring Runx2, alkaline phosphatase (ALP) and osteocalcin mRNA levels; ALP activity; as well as calcium deposition and mineralization. Increased PLF also promoted cell adhesion in MC3T3-E1 osteoblast-like cell cultures. Thus, tissue loading in vivo resulted in increased TNFalpha, which increased PLF, which then induced anabolic bone formation, the latter results confirmed in vitro.

Serum biomarkers as signals for risk and severity of work-related musculoskeletal injury

Work-related musculoskeletal disorders (MSDs) have accounted for a significant proportion of work injuries and workers' compensation claims in industrialized nations since the late 1980s. Despite epidemiological evidence for the role of repetition and force in the onset and progression of work-related MSDs, complete understanding of these important occupational health problems requires further elucidation of the underlying pathogenesis. Results from several clinical and experimental studies indicate that pathological and/or adaptive tissue changes occur as a consequence of performing repetitive and/or forceful tasks. Here, we review evidence of these tissue changes as revealed by the testing of serum biomarkers. Biomarkers of inflammation (inflammatory cytokines and C-reactive protein), cell stress or injury (malondialdehyde and creatine kinase), and collagen synthesis and degradation (collagen I carboxy-terminal propeptide and type-I collagen cross-linked C-telopeptide, respectively) and their association with MSDs will be reviewed.

Peripheral and central changes combine to induce motor behavioral deficits in a moderate repetition task

Repetitive motion disorders, such as carpal tunnel syndrome and focal hand dystonia, can be associated with tasks that require prolonged, repetitive behaviors. Previous studies using animal models of repetitive motion have correlated cortical neuroplastic changes or peripheral tissue inflammation with fine motor performance. However, the possibility that both peripheral and central mechanisms coexist with altered motor performance has not been studied. In this study, we investigated the relationship between motor behavior changes associated with repetitive behaviors and both peripheral tissue inflammation and cortical neuroplasticity. A rat model of reaching and grasping involving moderate repetitive reaching with negligible force (MRNF) was used. Rats performed the MRNF task for 2 h/day, 3 days/week for 8 weeks. Reach performance was monitored by measuring reach rate/success, daily exposure, reach movement reversals/patterns, reach/grasp phase times, grip strength and grooming function. With cumulative task exposure, reach performance, grip strength and agility declined while an inefficient food retrieval pattern increased. In S1 of MRNF rats, a dramatic disorganization of the topographic forepaw representation was observed, including the emergence of large receptive fields located on both the wrist/forearm and forepaw with alterations of neuronal properties. In M1, there was a drastic enlargement of the overall forepaw map area, and of the cortex devoted to digit, arm-digits and elbow-wrist responses. In addition, unusually low current amplitude evoked digit movements. IL-1 beta and TNF-alpha increased in forearm flexor muscles and tendons of MRNF animals. The increases in IL-1 beta and TNF-alpha negatively correlated with grip strength and amount of current needed to evoke forelimb movements. This study provides strong evidence that both peripheral inflammation and cortical neuroplasticity jointly contribute to the development of chronic repetitive motion disorders.

Periostin-like-factor and Periostin in an animal model of work-related musculoskeletal disorder

Work-related musculoskeletal disorders (WMSDs), also known as overuse injuries, account for a substantial proportion of work injuries and workers' compensation claims in the United States. However, the pathophysiological mechanisms underlying WMSDs are not well understood, especially the early events in their development. In this study we used an animal model of upper extremity WMSD, in which rats perform a voluntary repetitive reaching and pulling task for a food reward. This innovative model provides us an opportunity to investigate the role of molecules which may be used either as markers of early diagnosis of these disorders, and/or could be targeted for therapeutic purposes in the future. Periostin-like-factor (PLF), and Periostin were examined in this study. Both belong to a family of vitamin K-dependent gamma carboxylated proteins characterized by the presence of conserved Fasciclin domains and not detected in adult tissues except under conditions of chronic overload, injury, stress or pathology. The spatial and temporal pattern of PLF and Periostin localization was examined by immunohistochemistry and western blot analysis in the radius and ulna of animals performing a high repetition, high force task for up to 12 weeks and in controls. We found that PLF was present primarily in the cellular periosteum, articular cartilage, osteoblasts, osteocytes and osteoclasts at weeks 3 and 6 in all distal bone sites examined. This increase coincided with a transient increase in serum osteocalcin in week 6, indicative of adaptive bone formation at this time point. PLF immunoexpression decreased in the distal periosteum and metaphysis by week 12, coincided temporally with an increase in serum Trap5b, thinning of the growth plate and reduced cortical thickness. In contrast to PLF, once Periostin was induced by task performance, it continued to be present at a uniformly high level between 3 and 12 weeks in the trabeculae, fibrous and cellular periosteum, osteoblasts and osteocytes. In general, the data suggest that PLF is located in tissues during the early adaptive stage of remodeling but not during the pathological phase and therefore might be a marker of early adaptive remodeling.

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).

Serum and tissue cytokines and chemokines increase with repetitive upper extremity tasks

We investigated inflammation in rats performing a low repetition, negligible force (LRNF) or high repetition, negligible force (HRNF) task of reaching and retrieving food pellets at target rates of two or four reaches/min for 2 h/day, for 6-8 weeks. Serum was assayed for 11 cytokines and chemokines; forelimb tissues for four cytokines. Macrophages were counted in forelimb tissues of LRNF rats to add to results from our previous studies of HRNF rats. In HRNF rats, serum IL-1 alpha, IL-1 beta, TNFalpha, MIP2, MIP3a, and RANTES were elevated in weeks 6 and 8. In contrast, only MIP2 and MIP3a increased in serum of LRNF rats. In 8 week HRNF reach limb tissues, IL-1 alpha, IL-1beta, TNFalpha, and IL-10 increased in distal bones, IL-1 alpha and -beta in muscles, and TNFalpha in tendons. Only IL-10 increased in LRNF reach limb muscles in week 8. Serum IL-1 alpha and MIP2 correlated with macrophages in LRNF loose connective tissues, serum MIP3a and MIP2 correlated negatively with grip strength, while serum TNFalpha, MIP3a, and MIP2 correlated positively with total number of reaches. Thus, several tissue and circulating cytokines/chemokines increase in an exposure dependent manner following short-term performance of repetitive reaching tasks and correlate with macrophage infiltration and decreasing grip strength.

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.

ISSLS prize winner: repeated disc injury causes persistent inflammation

STUDY DESIGN

An in vivo rat model of disc degeneration with emphasis on characterizing acute and chronic cytokine production.

OBJECTIVE

To compare the morphologic and proinflammatory response between a single and triple-stab injury in attempts to establish mechanisms of chronic disc inflammation.

SUMMARY OF BACKGROUND DATA

The features that distinguish physiologic (asymptomatic) from pathologic (symptomatic) degeneration are unclear. Epidemiologic evidence suggests that cumulative damage and elevated disc cytokine levels may be linked to increased low back pain rates. Although acute injury stimulates a healing response that includes transient cytokine production, repetitive damage may be necessary to trigger the persistent inflammation suspected to underlie chronic pain.

METHODS

Tail discs were exposed surgically and stabbed with a number 11 blade. During the subsequent acute healing phase, triple-stab discs were percutaneously injured with a 23-gauge needle at day 3 and then again at day 6 after the initial blade incision. Cytokine (IL-1 beta, IL-6, IL-8, and TNF-alpha) production was quantified using enzyme linked immunosorbent assay, and, in addition to MAPK signaling pathways (phosphorylated forms of ERK, JNK, and p38), was localized by immunohistochemistry. Disc architecture was evaluated using histology.

RESULTS

Both single-stab and triple-stab discs degenerated with time, yet degeneration was more severe with repeated injury where nuclear proteoglycan was replaced by disorganized collagen. Four days after single-stab, there was a transient peak in IL-1 beta and IL-8 production that was localized to the wound track and associated granulation tissue. By contrast, triple-stab induced an activated annular fibroblast phenotype (p38 positive) that caused a prolonged, diffuse inflammatory response with elevated levels of TNF-alpha, IL-1 beta, and IL-8 up to 28 days after injury. Disc inflammation was accompanied by reactive changes in the adjacent vertebral marrow spaces that was initially lytic at day 4, becoming sclerotic by day 56.

CONCLUSION

Our results demonstrate that repeated injury during active healing leads to persistent inflammation and enhanced disc degeneration. These data support the premise that damage accumulation and its associated inflammation may distinguish pathologic from physiologic disc degeneration. In the future, this triple-stab model may be useful to evaluate the efficacy of anti-inflammatory low back pain treatments.

Amelioration of inflammation, angiogenesis and CTGF expression in an arthritis model by a TSP1-derived peptide treatment

OBJECTIVE

To evaluate the effect of a thrombospondin 1 (TSP1)-derived peptide on inflammation and angiogenesis in an animal model of erosive arthritis and to assess the relationship between TSP1 and connective tissue growth factor (CTGF) in the pathophysiology of rheumatoid arthritis.

METHODS

Erosive arthritis in Lewis rats was induced by peptidoglycan-polysaccharide (PG-PS). Animals were divided into four groups: (1) negative control and groups receiving, (2) no treatment, (3) treatment with a TSP1-derived peptide, and (4) treatment with a scrambled peptide. Samples obtained from ankle joint, spleen and liver were studied using histology, histomorphometry, immunohistochemistry and RT-PCR.

RESULTS

Histological data indicated that the TSP1-derived peptide treatment decreased neovascularization, leukocyte infiltration and thickening of the synovial lining of the joint, and reduced granuloma formation in the spleen and liver when compared to control groups. Higher concentrations of CTGF and TSP1 proteins were observed in the affected areas of animals which did not receive TSP1-derived peptide treatment. Also, immunofluorescence and RT-PCR analyses showed an increase in CTGF protein expression and regulation, respectively, in the tissues of untreated animals when compared to the TSP1-derived peptide treated animals. By immunofluorescence, TSP1 expression was decreased in the TSP1-derived peptide treated animals. Moreover, macrophage/monocyte-specific staining revealed a decrease in cell infiltration in the articular tissue of the TSP1-derived peptide treated animals.

CONCLUSION

Both inflammation and angiogenesis were decreased after TSP1-derived peptide treatment indicating a potential pathway by which TSP1 interaction with neutrophils induces CTGF in RA affected tissues.

Connective tissue growth factor (CTGF) acts as a downstream mediator of TGF-beta1 to induce mesenchymal cell condensation

Mesenchymal cell (MC) condensation or the aggregation of MCs precedes chondrocyte differentiation and is required for subsequent cartilage formation during endochondral ossification. In this study, we used micromass cultures of C3H10T1/2 cells as an in vitro model system for studying MC condensation and the events important for this process. Transforming growth factor beta1 (TGF-beta1) served as the initiator of MC condensation in our model system and we were interested in determining whether CTGF functions as a downstream mediator of TGF-beta1. CTGF is a matricellular protein that has been found to be expressed in MC condensations and in the perichondrium. Micromass cultures of C3H10T1/2 cells condensed under TGF-beta1 stimulation concomitant with dramatic up-regulation of CTGF mRNA and protein levels. CTGF silencing by either CTGF siRNA or CTGF antisense oligonucleotide approaches showed that TGF-beta1-induced condensation was CTGF dependent. Furthermore, silencing of CTGF expression resulted in significant reductions in cell proliferation and migration, events that are crucial during MC condensation. In addition, up-regulation of Fibronectin (FN) and suppression of Sox9 expression by TGF-beta1 was also found to be mediated by CTGF. Immunofluorescence of developing mouse vertebrae showed that CTGF, TGF-beta1 and FN were co-expressed in condensations of MCs, while Sox9 expression was low at this stage. During subsequent chondrogenesis, Sox9 expression was high in chondrocytes while CTGF expression was limited to the perichondrium. Thus, CTGF is an essential downstream mediator of TGF-beta1-induced MC condensation through its effects on cell proliferation and migration. CTGF is also involved in up-regulating FN and suppressing Sox9 expression during TGF-beta1 induced MC condensation.

Increase in inflammatory cytokines in median nerves in a rat model of repetitive motion injury

We examined cytokines in rat median nerves following performance of a high repetition reaching and grasping task at a rate of 8 reaches/min for up to 8 weeks. IL-1alpha, IL-1beta, TNF-alpha, IL-6 and IL-10 were analyzed by immunohistochemistry. Double-labeling immunohistochemistry for ED1, a marker of phagocytic macrophages, was also performed. We found increased immunoexpression of IL-6 by week 3, increases in all 5 cytokines by week 5. This response was transient as all cytokines returned to control levels by 8 weeks of performance of a high repetition negligible force task. Cytokine sources included Schwann cells, fibroblasts and phagocytic macrophages (ED1-immunopositive). These findings suggest that cytokines are involved in the pathophysiology of repetitive motion injuries in peripheral nerves.

Work-related musculoskeletal disorders of the hand and wrist: epidemiology, pathophysiology, and sensorimotor changes

The purpose of this commentary is to present recent epidemiological findings regarding work-related musculoskeletal disorders (WMSDs) of the hand and wrist, and to summarize experimental evidence of underlying tissue pathophysiology and sensorimotor changes in WMSDs. Sixty-five percent of the 333 800 newly reported cases of occupational illness in 2001 were attributed to repeated trauma. WMSDs of the hand and wrist are associated with the longest absences from work and are, therefore, associated with greater lost productivity and wages than those of other anatomical regions. Selected epidemiological studies of hand/wrist WMSDs published since 1998 are reviewed and summarized. Results from selected animal studies concerning underlying tissue pathophysiology in response to repetitive movement or tissue loading are reviewed and summarized. To the extent possible, corroborating evidence in human studies for various tissue pathomechanisms suggested in animal models is presented. Repetitive, hand-intensive movements, alone or in combination with other physical, nonphysical, and nonoccupational risk factors, contribute to the development of hand/wrist WMSDs. Possible pathophysiological mechanisms of tissue injury include inflammation followed by repair and/or fibrotic scarring, peripheral nerve injury, and central nervous system reorganization. Clinicians should consider all of these pathomechanisms when examining and treating patients with hand/wrist WMSDs.

Performance of a high-repetition, high-force task induces carpal tunnel syndrome in rats

STUDY DESIGN

A randomized controlled prospective experimental study with some repeated measures.

OBJECTIVES

To characterize behavioral, sensory, motor, and nerve conduction decrements, and histological changes in the median nerve in rats trained to perform a high-force repetitive task.

BACKGROUND

Understanding of work-related carpal tunnel syndrome is hampered by the lack of experimental studies of the causes and mechanisms of nerve compression induced by repetitive motion. Most animal models of nerve compression have not employed voluntary repetitive motion as the stimulus for pathophysiological changes.

METHODS AND MEASURES

Thirty Sprague-Dawley rats served as controls for 1 or more studies. Ten rats were trained to pull on a bar with 60% maximum force 4 times per minute, 2 h/d, 3 d/wk for 12 weeks. Motor behavior and limb withdrawal threshold force were characterized weekly. Grip strength and median nerve conduction velocity were measured after 12 weeks. Median nerves were examined immunohistochemically for ED1-positive macrophages, collagen, and connective tissue growth factor.

RESULTS

Reach rate and duration of task performance declined over 12 weeks. Grip strength and nerve conduction velocity were significantly lower after 12 weeks than in controls. Limb withdrawal threshold increased between weeks 6 and 12. Median nerves at the level of the wrist showed increases in macrophages, collagen, and connective-tissue growth-factor-positive cells. These effects were seen in both the reach and nonreach limbs.

CONCLUSIONS

This animal model exhibits all the features of human carpal tunnel syndrome, including impaired sensation, motor weakness, and decreased median nerve conduction velocity. It establishes a causal relationship between performance of a repetitive task and development of carpal tunnel syndrome.

Inflammation reduces physiological tissue tolerance in the development of work-related musculoskeletal disorders

Work-related musculoskeletal disorders (MSDs) cause substantial worker discomfort, disability and loss of productivity. Due to the difficulty in analyzing the tissues of patients in the early stages of work-related MSD, there is controversy concerning the pathomechanisms of the development of these disorders. The pathophysiology of work-related MSD can be studied more easily in animal models. The purpose of this review is to relate theories of the development of tissue injury due to repeated motion to findings of recent investigations in animals that address the role of the inflammatory response in propagating tissue injury and contributing to chronic or recurring tissue injury. These tissue effects are related to behavioral indicators of discomfort and movement dysfunction with the aim of clarifying key time points for specific intervention approaches. The results from animal models of MSD are discussed in the light of findings in patients, whose tissues are examined at a much later phase of MSD development. Finally, a conceptual model of the potentially negative impact of inflammation on tissue tolerance is proposed along with suggestions for future research directions.