RESUMEN
The commitment of mesenchymal stem cells (MSCs) to preadipocytes and the termination of differentiation to adipocytes are critical for maintaining systemic energy homeostasis. However, our knowledge of the molecular mechanisms governing the commitment of MSCs to preadipocytes and the subsequent termination of their differentiation into adipocytes remain limited. Additionally, the role of Sox6 sex-determining region Y (SRY)-box6 (Sox6), a transcription factor that regulates gene transcription, is reportedly involved in various cellular processes, including adipogenesis; however, its function in regulating preadipocyte development and the factors involved in the termination of adipogenic differentiation remain unexplored. Therefore, we investigated the role of Sox6 in regulating the differentiation of adipocytes by monitoring the effects of its overexpression in C3H10T1/2 cells (in vitro) and C57BL/6J mouse (in vivo) models of adipogenesis. We observed lower Sox6 expression in the adipose tissue of obese mice than that in control mice. Sox6 overexpression inhibited the differentiation of MSC by directly binding to the lysyl oxidase (Lox) and preadipocyte factor 1 (Pref1) promoters, which was potentiated by histone deacetylase-1(HDAC1). Our findings suggest that Sox6 is a key regulator of MSC commitment to adipocytes; therefore, targeting the Sox6-mediated regulation of this process could offer potential therapeutic avenues for addressing obesity and related metabolic disorders.
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Adipogénesis , Células Madre Mesenquimatosas , Animales , Ratones , Adipogénesis/genética , Diferenciación Celular/genética , Ratones Endogámicos C57BL , Proteína-Lisina 6-Oxidasa/genética , Proteína-Lisina 6-Oxidasa/metabolismo , Factores de Transcripción SOXD/genética , Factores de Transcripción SOXD/metabolismoRESUMEN
PURPOSE: Repetitive Lumbar Injury (RLI) is common in individuals engaged in long term performance of repetitive occupational/sports activities with the spine. The triggering source of the disorder, tissues involved in the failure and biomechanical, neuromuscular, and biological processes active in the initiation and development of the disorder, are not known. The purpose is, therefore, to test, using in-vivo feline model and healthy human subjects, the hypothesis that RLI due to prolonged exposure to repetitive lumbar flexion-extension is triggered by an acute inflammation in the viscoelastic tissues and is characterized by lingering residual creep, pronounced changes in neuromuscular control and transient changes in lumbar stability. This report, therefore, is a summary of a lengthy research program consisting of multiple projects. METHODS: A series of experimental data was obtained from in-vivo feline groups and normal humans subjected to prolonged cyclic lumbar flexion-extension at high and low loads, high and low velocities, few and many repetitions, as well as short and long in-between rest periods, while recording lumbar displacement and multifidi EMG. Neutrophil and cytokines expression analysis were performed on the dissected feline supraspinous ligaments before loading (control) and 7 h post-loading. A comprehensive, time based model was designed to represent the creep, motor control, tissue biology and stability derived from the experimental data. RESULTS: Prolonged cyclic loading induced creep in the spine, reduced muscular activity, triggered spasms and reduced stability followed, several hours later, by acute inflammation/tissue degradation, muscular hyperexcitability and hyperstability. Fast movement, high loads, many repetitions and short rest periods, triggered the full disorder, whereas low velocities, low loads, long rest and few repetitions, triggered only minor but statistically significant pro-inflammatory tissue degradation and significantly reduced stability. CONCLUSION: Viscoelastic tissue failure via inflammation is the source of RLI and is also the process which governs the mechanical and neuromuscular characteristic symptoms of the disorder. The experimental data validates the hypothesis and provides insights into the development of potential treatments and prevention.
Asunto(s)
Trastornos de Traumas Acumulados/fisiopatología , Vértebras Lumbares/fisiopatología , Espasmo/fisiopatología , Enfermedades de la Columna Vertebral/fisiopatología , Enfermedad Aguda , Adulto , Animales , Gatos , Citocinas/fisiología , Modelos Animales de Enfermedad , Electromiografía , Humanos , Ligamentos/diagnóstico por imagen , Ligamentos/fisiopatología , Dolor de la Región Lumbar/fisiopatología , Modelos Biológicos , Músculo Esquelético/inervación , Músculo Esquelético/fisiopatología , Cintigrafía , Descanso/fisiología , Estrés Mecánico , Soporte de Peso/fisiología , Adulto JovenRESUMEN
BACKGROUND CONTEXT: Cumulative (repetitive) lumbar disorder is common in the workforce, and the associated epidemiology points out high risk for lifting heavy loads, performing many repetitions, and performing movements at high velocity. Experimental verification of viscoelastic tissue degradation and a neuromuscular disorder exist for cyclic work under heavy loads. Experimental validation for a disorder because of cyclic loads under high-velocity movement is missing. PURPOSE: Obtain experimental verification that high-velocity lumbar flexion-extension results in significant increase of proinflammatory cytokines in the viscoelastic tissues. STUDY DESIGN: Laboratory experiments using two in vivo feline model groups subjected to cyclic flexion-extension at low and high velocity. METHODS: Seven hours after cumulative 60 minutes of cyclic flexion-extension at moderate load of 40 N and 0.25 Hz for first group and 0.5 Hz for the second group, the supraspinous ligaments of L3-L4 to L5-L6 were harvested and subjected to cytokines (interleukin [IL]-1ß, IL-6, IL-8, tumor necrosis factor-α, and transforming growth factor-ß1) analysis. Two-way mixed model analysis of variance with a post hoc analysis were used to assess any significant differences (p<.05) in cytokines expression level between the two groups as well as main effect and interaction with lumbar levels. RESULTS: Expression levels of the five cytokines were significantly increased in the group subjected to the high-frequency loading. CONCLUSIONS: Exposure of the lumbar spine to high-velocity flexion-extension triggers a significant increase in proinflammatory cytokines, indicating pronounced changes consistent with an acute inflammation. Further exposure to activity over prolonged periods may trigger chronic inflammation and tissue degeneration as the source of cumulative lumbar disorder.
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Trastornos de Traumas Acumulados/etiología , Trastornos de Traumas Acumulados/metabolismo , Citocinas/metabolismo , Inflamación/metabolismo , Ligamentos/metabolismo , Vértebras Lumbares/metabolismo , Soporte de Peso , Análisis de Varianza , Animales , Gatos , Trastornos de Traumas Acumulados/fisiopatología , Inflamación/fisiopatología , Ligamentos/fisiopatología , Vértebras Lumbares/fisiopatología , Región Lumbosacra/fisiopatología , Reacción en Cadena de la Polimerasa de Transcriptasa InversaRESUMEN
Repetitive or overuse disorders of the lumbar spine affect the lives of workers and athletes. We hypothesize that repetitive anterior lumbar flexion-extension under low or high load will result in significantly elevated pro-inflammatory cytokines expression several hours post-activity. High loads will exhibit significantly higher expression than low loads. Lumbar spine of in vivo feline was subjected to cyclic loading at 0.25 Hz for six 10-min periods with 10 min of rest in between. One group was subjected to a low peak load of 20 N, whereas the second group to a high peak load of 60 N. Following a 7-h post-loading rest, the supraspinous ligaments of L-3/4, L-4/5 and L-5/6 and the unstimulated T-10/11 were excised for mRNA analysis and IL-1beta, IL-6, IL-8, TNFalpha and TGFbeta1 pro-inflammatory cytokines expression. Creep (laxity) developed in the lumbar spine during the loading and the subsequent 7 h of rest was calculated. A two-way mixed model ANOVA was used to assess difference in each cytokines expression between the two groups and control. Tukey HSD post hoc analysis delineated specific significant effects. Significance was set at 0.05. Low and high-load groups exhibited development of creep throughout the cyclic loading period and gradual recovery throughout the 7-h rest period. Residual creep of 24.8 and 30.2% were present in the low and high-load groups, respectively, 7-h post-loading. Significant increases in expression of all cytokines measured relative to control were obtained for supraspinous ligaments from both low and high-load magnitudes. IL-6, IL-8 and TGFbeta1 expression in the high-load group were significantly higher relative to the low-load group. Significant increases in cytokines expression indicating tissue inflammation are observed several hours post-repetitive lumbar flexion-extension regardless of the load magnitude applied. Repetitive occupational and athletic activity, regardless of the load applied, may be associated with the potential of developing acute inflammatory conditions that may convert to chronic inflammation if the viscoelastic tissues are further exposed to repetitive activity over long periods. Appropriate rest periods are a relevant preventive measure.
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Interleucina-6/metabolismo , Interleucina-8/metabolismo , Ligamentos/metabolismo , Vértebras Lumbares/fisiología , Factor de Crecimiento Transformador beta/metabolismo , Soporte de Peso/fisiología , Análisis de Varianza , Animales , Gatos , Inflamación/metabolismo , Ligamentos/lesiones , Vértebras Lumbares/lesiones , Reacción en Cadena de la Polimerasa de Transcriptasa InversaRESUMEN
BACKGROUND: Cumulative trauma disorder is commonly reported by workers engaged in prolonged repetitive/cyclic occupational activities. Recent experimental evidence confirms that relatively short periods of cyclic lumbar flexion at high loads result in substantial creep of viscoelastic tissues, prolonged periods of its recovery to baseline together with a neuromuscular disorder and exposure to instability. The biochemical process associated with the creep and neuromuscular disorder are not well explored. The purpose of the study is to identify the ligaments as one of the organs of failure and an acute inflammation as the result of failure as a preliminary step in the development of chronic inflammation that might lead to cumulative trauma disorder elicited by high magnitude cyclic loads. METHODS: The lumbar spine of anaesthetized cats was subjected to cyclic flexion loading at high magnitudes for six periods of 10 min each with 10 min rest in between followed by 7h rest. Lumbar displacement was monitored throughout. Supraspinous ligaments from L-3/4, L-4/5, L-5/6 and unloaded T-10/11 were removed at the end of testing and assessed using mRNA expression for cytokine (IL-1beta, IL-6, IL-8, TNFalpha, TGFbeta). Cytokines expression in the lumbar ligaments were statistically compared to their self control in the unloaded thoracic ligament. The creep developed during the loading and its recovery during the 7h rest was calculated. FINDINGS: The mean creep developed during the loading period reached 57.3% recovering to a residual value of 25.5% at the end of the 7h rest. Increase in cytokine expression was seen in all lumbar ligaments with statistical significance in the L-4/5 and L-5/6 levels. INTERPRETATION: The results confirm that prolonged high magnitude cyclic loading of the lumbar spine in flexion-extension elicits substantial residual creep together with significant increases in cytokines expression, consistent with an acute inflammation, several hours post loading. Further exposure to cyclic loading over time may result in conversion to chronic inflammation.
Asunto(s)
Vértebras Lumbares/patología , Animales , Fenómenos Biomecánicos , Gatos , Trastornos de Traumas Acumulados , Citocinas/metabolismo , Elasticidad , Electromiografía , Inflamación , Ligamentos/lesiones , Región Lumbosacra/lesiones , Estrés Mecánico , Factores de Tiempo , Soporte de PesoRESUMEN
The motor control system may compensate for lumbar instability following cyclic work with differential response to load magnitude. In vivo felines were exposed to a cumulative 1 h of cyclic work at 0.25 Hz. One group exposed to light whereas the second to heavy load while recording lumbar displacement and multifidus EMG during work and in single test cycles over 7 h rest post-work. Significant laxity and reduced reflexive EMG activity were evident immediately post-work in both groups. EMG and laxity recovered over 7 h rest in the group exposed to light load whereas in the group exposed to heavy load, motor control compensation was triggered within 1-2 h post-work. The compensation was expressed by earlier and stronger muscular activation than in baseline. It is concluded that cyclic work is deleterious to spine stability immediately after work. Work with heavy loads elicits delayed motor control compensation whereas work with light loads leaves the spine unstable and exposed to injury for several hours. Overall, prolonged cyclic or repetitive work elicits a transient instability disorder, regardless of the load handled, exposing the individual to potential injury.
Asunto(s)
Vértebras Lumbares/fisiología , Movimiento/fisiología , Contracción Muscular/fisiología , Músculo Esquelético/fisiología , Columna Vertebral/fisiología , Animales , Fenómenos Biomecánicos , Gatos , Electromiografía , Vértebras Lumbares/anatomía & histología , Músculo Esquelético/inervación , Periodicidad , Columna Vertebral/anatomía & histología , Soporte de Peso/fisiologíaRESUMEN
Neuromuscular control of lumbar stability following exposure to prolonged static work, under low and high loads, was assessed in the in vivo feline model. Six sessions of 10 min work at 20N with 10 min between rest was compared to a group subjected to the same protocol but carrying high loads of 60N. Displacement and tension developed in the spine at the instant the multifidus muscles applied stabilizing contractions, and their amplitudes were obtained from their electromyogram (EMG). Significant (P < 0.001) laxity developed in the various viscoelastic tissues of the lumbar spine that did not recover during and up to 7 h of rest postwork. Simultaneously, there was a significant (P < 0.001) decrease in muscular activity in the 3-4 h immediately postwork under low load but only during the first hour in the high load group. After that period the musculature compensated for the laxity of the viscoelastic tissues by a significant (P < 0.001) increase in activity in the high-load group and a nonsignificant increase in the low group. It was concluded that during 1-3 h immediately poststatic work a significant decrease in the stabilizing function of viscoelastic tissues together with a significant decrease in muscular activity is present, and they render the spine unstable and exposed to high risk of injury. Performance of prolonged static work under low loads, while not harmful during the work, cannot be designated as a "no-risk" condition, as it may result in injury postwork.
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Vértebras Lumbares/fisiología , Contracción Muscular/fisiología , Músculo Esquelético/fisiología , Unión Neuromuscular/fisiología , Equilibrio Postural/fisiología , Adaptación Fisiológica/fisiología , Animales , Fenómenos Biomecánicos , Gatos , Elasticidad , Electromiografía , Tolerancia al Ejercicio/fisiología , Fuerza Muscular/fisiología , Aptitud Física/fisiología , Viscosidad , Soporte de Peso/fisiologíaRESUMEN
The aim of the study was to track changes in the spectrum of electromyographic (EMG) signals recorded from the feline multifidus muscles during stretching of the supraspinous ligament. The ligaments were exposed to external 40 N-tension for six consecutive trials of 10-min duration. Two experimental groups were formed, according to the rest periods supplied (10 or 20 min). EMG signals were recorded intramuscularly from the right multifidus muscles. The EMG signals from each trial were split into segments. For each segment, a representative averaged motor unit potential (AvMUP) was determined. The number of single MUPs averaged to obtain AvMUP was defined as muscular activity. The relative changes in a muscle fatigue index, median frequency as well as activity were analyzed. It was concluded that faster and powerful motor units are recruited firstly, in order to quickly provide stronger support to the spine and/or as a reaction to a minor damage of tissue. Depending on the applied rest periods, peripheral fatigue might be accumulated.
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Electromiografía , Análisis de Fourier , Ligamentos/inervación , Músculo Esquelético/inervación , Reclutamiento Neurofisiológico , Reflejo de Estiramiento , Procesamiento de Señales Asistido por Computador , Animales , Gatos , Contracción Muscular , Fatiga Muscular , Recuperación de la Función , Factores de TiempoRESUMEN
The in vivo lumbar spine of the anaesthetized feline was subjected to passive cyclic anterior flexion-extension at 0.25 Hz and 40 N peak load for cumulative 60 min duration. Displacement (or displacement neuromuscular neutral zones-DNNZ) and tension (or tension neuromuscular neutral zones-TNNZ) at which reflexive EMG activity from the multifidi muscles was initiated and terminated were recorded, for single-test cycles, before and for 7h after cyclic loading. Displacement and tension NNZs increased significantly after loading. The displacement NNZs decreased exponentially to near baseline by the 7th hour of rest. The tension NNZs, however, decreased to below the baseline by the 2nd to 3rd hour after loading and continued decreasing into the 7th hour. Peak EMG significantly decreased (49-57%) to below the baseline immediately after loading and then exponentially increased, exceeding the baseline by the 2nd to 3rd hour and reaching 33-59% above baseline by the 7th hour. EMG median frequency decreased after loading and then exceeded the baseline after the 3rd hour, indicating initial de-recruitment, followed by recruitment of new motor units. These findings suggest that the lumbar spine was exposed to instability for 2-3h after cyclic loading, due to concurrent laxity of the viscoelastic tissues and deficient muscular activity. A delayed neuromuscular compensation mechanism was found to exist, triggering the musculature significantly earlier and at higher magnitude than baseline, while the viscoelastic tissues were still lax. Thus, it is suggested that prolonged cyclic loading may compromise lumbar stability during the immediate 2-3h post-loading, increasing the risk of injury.
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Vértebras Lumbares/fisiología , Plexo Lumbosacro/fisiología , Músculo Esquelético/fisiología , Anestesia General , Animales , Gatos , Electromiografía , Humanos , Vértebras Lumbares/inervación , Contracción Muscular/fisiología , Relajación Muscular/fisiología , Músculo Esquelético/inervación , Estrés Mecánico , Soporte de PesoRESUMEN
Epidemiologic studies indicate that repetitive (cyclic) occupational activities lead to a cumulative trauma disorder (CTD), and the frequency or velocity of the movement is one of the risk factors. Experimental neurophysiological evidence to confirm the epidemiology is not available. The response of the multifidus muscles to cyclic loading in anterior lumbar flexion-extension was assessed to test the hypothesis that high-frequency loading may induce an acute neuromuscular disorder leading to CTD. Two groups of feline preparations were subjected to cyclic loading with a peak of 20 N: one at 0.25 HZ and the second at 0.5 HZ, with an equal number of cycles. Electromyogram (EMG), lumbar displacement and load were recorded throughout the loading periods and during single-cycle tests over a 7-hour rest period following the load-rest sessions. A model was developed to quantify the creep and neuromuscular responses, and analysis of variance (ANOVA) was applied to assess significance of the results. The group exposed to 0.5 HZ exhibited spontaneous spasms followed by sustained spasms during the loading periods. During the 7-hour recovery period, a significant (P < 0.001) delayed hyperexcitability as well as sustained spasms of the multifidi were present in the last 5 hours, confirming a significant (P < 0.024 to P < 0.042) acute neuromuscular disorder. High-frequency cyclic loading of the lumbar spine may trigger a severe acute neuromuscular disorder, as evidenced by the sustained spasms and delayed hyperexcitability, and should be considered as a risk factor. We suggest that workers avoid high-frequency exposure to cyclic activity in order to prevent the development of cumulative trauma disorder.
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Trastornos de Traumas Acumulados/etiología , Región Lumbosacra/lesiones , Soporte de Peso/fisiología , Algoritmos , Análisis de Varianza , Animales , Gatos , Trastornos de Traumas Acumulados/fisiopatología , Electromiografía , Modelos Neurológicos , Contracción Muscular/fisiología , Músculo Esquelético/fisiología , Factores de Riesgo , Espasmo/fisiopatologíaRESUMEN
BACKGROUND: The impact of six sequential static loading and rest of the lumbar spine on the changes in the neuromuscular neutral zones and thereby on spine stability was assessed. METHODS: Six 10 min sessions of static load of a moderate level each spaced by 10 min rest were applied to the in vivo feline model. Test cycles of 0.25 Hz and at the same moderate peak load were applied before and every hour after the static loading sequence up to 7h. Load, displacement and electromyographic activity of the lumbar multifidi muscles were recorded throughout. FINDINGS: Displacement and tension neuromuscular neutral zones were defined as the displacement or tension, in the increase and decrease phases of each cycle, when the electromyogram initiated and ceased activity, respectively. Displacement neuromuscular neutral zones demonstrated significant (P<0.001) increase immediately post-static loading, followed by an exponential decrease to pre-loading baseline by the 7th hour. Tension neuromuscular neutral zones, however, demonstrated significant (P<0.001) increase in the 2h immediately after the static loading and a significant decrease (P<0.001) thereafter. Peak electromyogram decreased in the first 3h post-loading, but significantly (P<0.001) increased thereafter to the 7th hour. INTERPRETATION: It was concluded that the first 2-3h post-static loading finds the spine with significant laxity in the viscoelastic tissues concurrently with deficient muscular activation and therefore exposed to the risk of instability. It is also evident that a neural control compensation mechanism exists where it enhances the activation of the musculature to earlier and at higher activation magnitude, 2-3h post-loading, increasing lumbar stability while the viscoelastic tissues are still lax.
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Vértebras Lumbares/fisiología , Modelos Biológicos , Neuronas Motoras/fisiología , Movimiento/fisiología , Contracción Muscular/fisiología , Músculo Esquelético/fisiología , Equilibrio Postural/fisiología , Postura/fisiología , Potenciales de Acción/fisiología , Animales , Gatos , Simulación por ComputadorRESUMEN
OBJECTIVE: To study the influence of 10 min of cyclic twisting motion on abdominal and back muscle activities. BACKGROUND: Repetitive (cyclic) occupational activity was identified by many epidemiological reports to be a risk factor for the development of work-related musculoskeletal disorders. Biomechanical and physiological confirmation, however, is lacking. METHODS: Trunk muscle electromyography (EMG) was recorded while participants performed a continuous 10-min maximum lumbar cyclic twisting to the left, and maximum isometric twist to the left and right sides was measured before and after the exercise. RESULTS: Abdominal muscles contracted symmetrically, independent of twisting direction. The left posterior muscles' integrated EMG (IEMG) decreased during the exercise, whereas the IEMG of the right posterior muscle increased. Simultaneously with increased antagonist coactivity level of the right posterior muscles after the exercise, decrease in maximal isometric left twisting torque was observed. The abdominal muscles did not exhibit any significant changes during the exercise. After the exercise, the right abdominals demonstrated a significant increase in effort, which was independent of the direction of the maximal effort isometric test. CONCLUSIONS: The change in muscle activity is attributed to neuromuscular compensation for the development of laxity and microdamage in the soft tissue (ligaments, discs, facet capsules, etc.) of the lumbar spine. APPLICATION: The results of this study increase understanding of the risk factors associated with low back disorder induced by labor-intensive occupations that involve cyclic lateral twisting.
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Fenómenos Biomecánicos , Trastornos de Traumas Acumulados/etiología , Contracción Isométrica/fisiología , Músculo Esquelético/fisiología , Anomalía Torsional/fisiopatología , Adulto , Dorso/fisiología , Trastornos de Traumas Acumulados/prevención & control , Electromiografía , Femenino , Humanos , Dolor de la Región Lumbar/etiología , Dolor de la Región Lumbar/prevención & control , Región Lumbosacra , Masculino , Músculo Esquelético/fisiopatología , Enfermedades Profesionales/etiología , Enfermedades Profesionales/prevención & control , Factores de Riesgo , TorqueRESUMEN
BACKGROUND: The epidemiology identifies cyclic lumbar loading as a risk factor for cumulative trauma disorder. Experimental biomechanical and physiological confirmation is lacking. The objective of this study was to asses the impact of different rest durations applied between periods of cyclic loading on the development of an acute lumbar disorder which, if continued to be subjected to loading, may develop into a cumulative disorder. METHODS: Three groups of in vivo feline preparations were subjected to six sequential 10 min loading periods of cyclic lumbar flexion at 40 N with a frequency of 0.25 Hz applied to the L-4/5 level. The rest durations varied from 5 min in the first group, to 10 min in the second and to 20 min in the third. Reflexive EMG from the multifidi and lumbar displacement were used to identify significant (P<0.001) effects of time and rest duration for post-load EMG and displacement. Single-cycle test were performed hourly for 7 h post-loading to assess recovery. A model developed earlier was applied to represent the experimental data. FINDINGS: The groups allowed 5 and 10 min rest exhibited an acute neuromuscular disorder expressed by a significant (P<0.001) delayed hyperexcitability 2-3 h into the 7 h recovery period with the intensity of the hyperexcitability significantly higher (P<0.001) for the group allowed only 5 min rest. The group allowed 20 min rest had a slow, uneventful recovery, free of delayed hyperexcitability. INTERPRETATIONS: Occupational and sports activities requiring repetitive (cyclic) loading of the lumbar spine may be a risk factor for the development of a cumulative lumbar disorder and may require sufficient rest, as much as twice as long as the loading period, for prevention. Comparison to similar data for static lumbar loading shows that cyclic loading is more deleterious than static loading, requiring more rest to offset the negative effect of the repeated acts of stretch.
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Trastornos de Traumas Acumulados/etiología , Trastornos de Traumas Acumulados/fisiopatología , Estimulación Física/efectos adversos , Medición de Riesgo/métodos , Compresión de la Médula Espinal/etiología , Compresión de la Médula Espinal/fisiopatología , Soporte de Peso , Animales , Gatos , Simulación por Computador , Vértebras Lumbares/lesiones , Vértebras Lumbares/fisiopatología , Modelos Biológicos , Contracción Muscular , Oscilometría/métodos , Periodicidad , Esfuerzo Físico , Estimulación Física/métodos , Descanso , Factores de RiesgoRESUMEN
OBJECTIVE: Epidemiological data suggest that high loads lifted by workers engaged in static and cyclic daily activities may be a risk factor for low back disorder. Our previous research provided physiological and biomechanical validation of the epidemiological data for static load conditions. The objective of this report was to provide physiological and biomechanical experimental validation to the epidemiological data in cyclic (repetitive) load conditions. METHODS: Three groups of in vivo feline models were subjected to 3 cyclic load levels in a series of 6 periods of 10 minutes of work spaced by 10 minutes of rest followed by 7 hours of rest. Multifidus electromyography (EMG) and lumbar displacement were statistically analyzed after processing. RESULTS: Delayed muscular hyperexcitability was observed only in moderate (40 N) and high (60 N) loads (P<0.0001) but was absent in low (20 N) loads. The magnitude of the delayed hyperexcitability was found to be higher (P<0.0001) in the high (60 N) loads compared with the moderate (40 N) loads. CONCLUSIONS: Exposure to moderate and high loads in cyclic (repetitive) work results in an acute neuromuscular disorder indicative of soft tissue inflammation that may become chronic with further exposure.
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Trastornos de Traumas Acumulados/fisiopatología , Dolor de la Región Lumbar/fisiopatología , Soporte de Peso/fisiología , Animales , Fenómenos Biomecánicos , Gatos , Trastornos de Traumas Acumulados/etiología , Electromiografía , Dolor de la Región Lumbar/etiología , Vértebras Lumbares/fisiopatología , Modelos Animales , Factores de Riesgo , Estrés MecánicoRESUMEN
Low back disorders are prominent among the work force engaged in static anterior flexion during the workday. As a continuing part of a long-term research aimed to identify the biomechanical and physiological processes and corresponding risk factors leading to such cumulative trauma disorder (CTD), we ventured to assess the effect of rest and the work-to-rest duration ratios that may prevent CTD. Three groups of the feline model were subjected to three load/rest paradigms: two 30 min loading periods spaced by 10 min rest in Group I, two 30 min loading period spaced by 30 min rest in Group II and one 60 min loading period for Group III. The cumulative loading duration in the three groups was 60 min. Each of the groups were allowed 7h of rest while monitoring EMG and lumbar viscoelastic tissue creep each hour. The results demonstrate that for two 30 min load periods with a 30 min in between rest, an acute neuromuscular disorder was not present whereas for two 30 min loading with a 10 min rest it was. Similarly, for a 60 min loading with long-term rest, the disorder was present. Post hoc Fisher analysis demonstrated significant differences in the delayed hyperexcitability between the first and second group (P<0.0001) and the third and second (P<0.0001) group. Statistical difference in the displacement data of the three groups was not present. ANOVA showed a significant effect of time post-loading (P<0.0001 and different rest durations (P<0.0001) on the EMG data during the 7h recovery. The new data allow us to conclude that a work-to-rest duration ratio of 1:1 can prevent the development of CTD as long as the work periods are not too long (<60 min). Longer static flexion durations do not respond favorably to rest even if it is of equal or longer duration. It is suggested that appropriate durations of rest may be a viable tool to avert CTD in a certain range whereas long static flexion durations should be avoided at all cost.
Asunto(s)
Dolor de la Región Lumbar/fisiopatología , Descanso/fisiología , Trabajo/fisiología , Animales , Gatos , Electromiografía , Modelos Animales , Músculo Esquelético/fisiología , Factores de Riesgo , Factores de Tiempo , Soporte de Peso/fisiologíaRESUMEN
Epidemiological data suggest that prolonged exposure to cyclic lumbar flexion elicits a chronic neuromuscular disorder and disability in workers. This study provides a physiological and biomechanical assessment of various repetitions of cyclic lumbar flexion sessions as a risk factor for development of an acute neuromuscular disorder. An in vivo feline model was subjected to 10 minutes of cyclic (0.25-HZ) loading, followed by a 10-minute rest period, repeated three times in one experimental group, six times in a second group, and nine times in the third group, followed by rest for 7 hours. Displacement of the lumbar viscoelastic tissue and reflex electromyographic (EMG) activity from the lumbar multifidus muscle were monitored. Creep developed and accumulated during each load/rest period and partially recovered during the subsequent rest. Loading periods were characterized by a decrease in reflex EMG activity with superimposed spasms. In the 7-hour recovery period, initial hyperexcitability was present in all groups, whereas only the six- and nine-repetition groups displayed significant delayed hyperexcitability, indicating the presence of acute inflammation. The mathematical model developed fit the data reasonably well, as the R2 values were generally near 0.90. It was concluded that the resulting delayed muscular hyperexcitability constitutes an acute neuromuscular disorder associated with exposure to many repetitions of cyclic lumbar flexion. The acute disorder can become chronic if not allowed sufficient rest to resolve itself. Workers engaged in cyclic lumbar flexion (e.g., loading/unloading, assembly workers) should avoid long-term exposure in order to prevent the development of a chronic neuromuscular condition known as cumulative trauma disorder.
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Traumatismos de la Espalda/fisiopatología , Trastornos de Traumas Acumulados/fisiopatología , Región Lumbosacra/lesiones , Región Lumbosacra/fisiopatología , Músculo Esquelético/fisiopatología , Enfermedades Musculoesqueléticas/fisiopatología , Animales , Traumatismos de la Espalda/etiología , Traumatismos de la Espalda/prevención & control , Fenómenos Biomecánicos , Gatos , Trastornos de Traumas Acumulados/etiología , Trastornos de Traumas Acumulados/prevención & control , Modelos Animales de Enfermedad , Electromiografía , Tolerancia al Ejercicio/fisiología , Ligamentos/lesiones , Ligamentos/fisiopatología , Contracción Muscular/fisiología , Músculo Esquelético/lesiones , Enfermedades Musculoesqueléticas/etiología , Enfermedades Profesionales/etiología , Enfermedades Profesionales/fisiopatología , Enfermedades Profesionales/prevención & control , Descanso/fisiología , Factores de Riesgo , Traumatismos de los Tendones/etiología , Traumatismos de los Tendones/fisiopatología , Traumatismos de los Tendones/prevención & control , Tendones/fisiopatología , Soporte de Peso/fisiologíaRESUMEN
BACKGROUND: Cyclic load applied to various joints during occupational and sports activities is epidemiologically linked to higher risk of neuromuscular disorder development. HYPOTHESIS: Passive cyclic loading of the knee will develop laxity and creep in the anterior cruciate ligament, and these may elicit a neuromuscular disorder in the quadriceps and hamstrings. Women may be more susceptible to the disorder. STUDY DESIGN: Controlled laboratory study. METHODS: Male and female groups were subjected to 10 minutes of passive cyclic loading (0.1 Hz) of the knee at a mild load (150-200 N) and at 35 degrees and 90 degrees flexion. Anterior tibial displacement and electromyogram from the quadriceps and hamstrings were monitored during cyclic loading. Maximal voluntary contraction of knee extension and flexion was assessed before and after cyclic loading. The effect of gender and angle on maximal voluntary contraction and quadriceps/hamstrings electromyogram was tested by a 2-way analysis of variance. Differences between the preload and postload data were tested by a paired t test. RESULTS: At a knee angle of 90 degrees, after cyclic loading, a decrease in maximal voluntary contraction during extension was present in men and women, with an associated decrease in quadriceps electromyogram activity. At 35 degrees, a decrease in maximal voluntary contraction in extension was noted in women and men. Electromyogram spasms were present in the quadriceps and hamstrings during the 10-minute cyclic loading in 51.7% of subjects. Analysis of variance demonstrated that ligament creep was significantly greater in women than in men at both knee angles. CONCLUSIONS: Even a mild cyclic loading of the anterior cruciate ligament, for a relatively short period, can elicit substantial creep, laxity, and a neuromuscular disorder. The disorder is composed of spasms and attenuated muscular function that may together create a condition that exposes the knee to injury. Women seem to be more susceptible than men. CLINICAL RELEVANCE: Cyclic actions performed at high frequencies and high-load magnitudes may lead to the occurrence of increased knee laxity and changes in neuromuscular function that, together with fatigue and changes in proprioception, may increase the risk of injury.
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Ligamento Cruzado Anterior/fisiología , Articulación de la Rodilla/fisiología , Enfermedades Neuromusculares/etiología , Adulto , Análisis de Varianza , Fenómenos Biomecánicos , Electromiografía , Femenino , Humanos , Masculino , Factores Sexuales , Espasmo/etiología , Soporte de PesoRESUMEN
The objective of this work was to study the effect of rest periods of various durations applied between six 10-min sessions of static flexion on the development of cumulative low back disorder (CLBD). Three experimental groups of a feline model were used, and the rest duration between sequential static load periods was set to 5, 10, and 20 min, with a corresponding load-to-rest ratio of 2:1, 1:1 and 1:2, respectively. The reflex electromyographic (EMG) activity from the multifidus muscles and supraspinous ligament displacement (creep) were recorded during the flexion periods and over 7 h of rest following the load-rest cycles. It was found that a minor disorder developed in all the groups whereas a severe neuromuscular disorder including a delayed hyperexcitability was observed only in the group subjected to 5 min rest. The two-way ANOVA showed a significant effect of time post loading (p<0.001) and rest duration (p<0.001) on the Normalized Integrated EMG (NIEMG) recovery data; a significant effect of time post loading on the Displacement data (p<0.001) was observed as well. The post hoc Fisher test performed on the NIEMG data during the recovery phase showed a significant difference between the group subjected to 5 min rest and the other two groups (p<0.001). These results suggest that while a short rest period of 2:1 load-to-rest ratio leads to CLBD, longer rest at 1:1 and 1:2 load-to-rest ratio are more favorable for preventing or attenuating the development of CLBD. Short rest periods between sessions of static lumbar flexion, therefore, are a risk factor for the development of CLBD.
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Dolor de la Región Lumbar/fisiopatología , Vértebras Lumbares/fisiología , Contracción Muscular/fisiología , Relajación Muscular/fisiología , Músculo Esquelético/fisiopatología , Animales , Gatos , Electromiografía , Dolor de la Región Lumbar/etiología , Modelos Animales , Recuperación de la Función/fisiología , Factores de Riesgo , Soporte de Peso/fisiologíaRESUMEN
STUDY DESIGN: In vivo feline model subjected to variable number of repetitions of a short static lumbar flexion followed by an equally long rest period. OBJECTIVES: The purpose of this study was to determine the influence of the number of repetitions as a risk factor in promoting a cumulative low back disorder in the feline model. SUMMARY OF BACKGROUND DATA: Epidemiologic data point out that the increased number of repetitions of static lumbar loading is a major risk factor in the development of cumulative low back disorder. Biomechanical and physiologic confirmation of the epidemiology is lacking. Recent work demonstrated that repetitive static loading results in accumulation of creep in the lumbar viscoelastic tissues, resulting in a neuromuscular disorder consisting of spasms during loading and hyperexcitability of lumbar muscles during following rest. It was also shown that the load magnitude is a major risk factor. It is hypothesized that increased number of repetitions of static load periods result in increased severity of the resulting neuromuscular disorder. METHODS: Static lumbar flexion of 10 minutes duration followed by 10 minutes rest was repeated three times in one experimental group, six times in the second, and nine times in the third group. In all groups, the creep developing in the lumbar viscoelastic tissues as well as the reflexive EMG from the multifidus were monitored during the flexion/rest periods and throughout a 7-hour recovery period after the repetitions. RESULTS: Creep developed and accumulated during each of the flexion/rest periods in the three experimental protocols, with larger residual creep at the end of the nine repetitions. A residual creep was still present at the end of the 7 hours of recovery allowed in each of the three groups. During the flexion/rest sessions, EMG spasms were present, and the presence of an initial hyperexcitability was detected during the 7 hours of recovery in all the preparations. The presence of a delayed hyperexcitability was revealed only in the group subjected to nine flexion/rest periods, while it was not observed in the groups subjected to three and six flexion/rest repetitions. The statistical analysis (post hoc Fisher test) performed on the normalized integrated EMG and displacement data during the recovery phase showed a significant difference between the nine repetitions group and the other two groups (P < 0.0001). The two-way ANOVA analysis revealed a significant effect of time (P < 0.005) and number of repetitions (P < 0.0001) on all considered parameters. CONCLUSIONS: It was concluded that a cumulative neuromuscular disorder develops because of repetition of static lumbar flexion, and the severity of the disorder provoked is magnified by the number of repetitions. Despite the highly favorable 1:1 work-to-rest ratio and the 7-hour post loading rest period, a full recovery of creep was not obtained in this study.
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Trastornos de Traumas Acumulados/etiología , Dolor de la Región Lumbar/etiología , Región Lumbosacra , Músculo Esquelético/fisiopatología , Soporte de Peso , Animales , Gatos , Trastornos de Traumas Acumulados/fisiopatología , Modelos Animales de Enfermedad , Tejido Elástico/fisiopatología , Electromiografía , Dolor de la Región Lumbar/fisiopatología , Vértebras Lumbares/fisiología , Modelos Biológicos , Contracción Muscular/fisiología , Relajación Muscular/fisiología , Factores de Riesgo , Espasmo/etiología , Espasmo/fisiopatologíaRESUMEN
Occupations requiring frequent periods of static lumbar flexion are known epidemiologically to be risk factors for the development of cumulative low back disorder. The impact of the load magnitude sustained during a series of short static lumbar flexions followed by an equally long rest period on the development of a cumulative low back disorder was addressed in an in vivo feline model. Static loads of 20, 40, and 60 N were applied over 10 min of flexion followed by 10-min rest sessions that were repeated six times (for a total of 2 h) while monitoring lumbar viscoelastic creep (laxity) and reflex electromyographic (EMG) activity from the multifidus muscles. Creep and EMG were also monitored over 7 h of rest following the six flexion-rest sessions. It was found that the creep developed in the 10-min flexion periods did not recover completely during the following 10 min of rest, giving rise to a large cumulative creep at the end of the work-rest session. Muscle activity demonstrated spasms during the static flexion periods as well as initial and delayed hyperexcitability during the 7-h rest period. Loads of 20 and 40 N did not result in delayed hyperexcitability, whereas loads of 60 N resulted in delayed hyperexcitability. Statistical analysis demonstrated that increased load significantly intensified the magnitude of the hyperexcitabilities (P < 0.05). Thus, repeated periods of static lumbar flexion were found to result in a transient neuromuscular disorder with an intensity directly related to the load magnitude, which should be considered a compounding risk factor.