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INTRODUCTION: The relationship between output force and motor command depends on the intrinsic dynamic responses of motor units (MUs), which can be characterized by evoking accurate sinusoidal force responses at different frequencies. In this study we sought to determine whether sinusoidal modulation of the stimulation rate of single MUs results in reliable sinusoidal force changes. METHODS: Single axons of rat ventral roots were stimulated electrically by changing the pulse rate sinusoidally at different frequency modulation (0.4-1.0-2.0-4.0 Hz for slow, 1.0-2.0-4.0-7.0 Hz for fast MUs). The twitching sinusoidal force signal was interpolated. We calculated harmonic distortion (HD) and the correlation coefficient (r) between theoretical sines and interpolated signals. RESULTS: HD was always <5%, and r was always >0.97. CONCLUSIONS: The HD and r-values obtained indicate highly reliable sinusoidal responses, which supports the potential use of this method to further characterize the dynamic behavior of single MUs.

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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.

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Spine stability of the behaving human in health or disorder is a complex, multi-factorial and time variable index. The major components of stability are the properties of the external load, passive viscoelastic tissues (ligaments, discs, facet capsules and dorso-lumbar fascia) combined with the properties of the active tissues (muscles and their sensory-motor control, co-activation and associated intra-abdominal pressure) as well as the pro-inflammatory status of the tissues. Each of the many components' contribution is time variable with dependence on the dose-duration of the work stimulus and the associated rest. Interaction of many variables is also dominant. A call is issued to focus research efforts on the development of a multi-factorial description of spine stability while representing the contribution of all relevant components as time variable functions. An example is provided, using models of four components described as functions of time derived from real data obtained simultaneously from the in-vivo feline. The example elucidates the potential errors made when using single components to assess stability and the insight gained when stability is introduced as a time dependent index. It is expected that such time variable stability assessment will provide uniform and insightful estimation that will have valuable applications in occupational, sports and rehabilitation fields as well as the leisure lifestyles required for maintaining a healthy spine.

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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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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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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.

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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.

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The mechanical and neurological properties of ligaments are reviewed and updated with recent development from the perspective which evaluates their role as a source of neuromusculoskeletal disorders resulting from exposure to sports and occupational activities. Creep, tension-relaxation, hysteresis, sensitivity to strain rate and strain/load frequency were shown to result not only in mechanical functional degradation but also in the development of sensory-motor disorders with short- and long-term implication on function and disability. The recently exposed relationships between collagen fibers, applied mechanical stimuli, tissue micro-damage, acute and chronic inflammation and neuromuscular disorders are delineated with special reference to sports and occupational stressors such as load duration, rest duration, work/rest ratio, number of repetitions of activity and velocity of movement.

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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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Human and animal models using electromyography (EMG) based methods have hypothesized that viscoelastic tissue properties becomes compromised by prolonged repetitive cyclic trunk flexion-extension which in turn influences muscular activation including the flexion-relaxation phenomenon. Empirical evidence to support this hypothesis, especially the development of viscoelastic tension-relaxation and its associated muscular response in passive cyclic activity in humans, is incomplete. The objective of this study was to examine the response of lumbar muscles to tension-relaxation development of the viscoelastic tissue during prolonged passive cyclic trunk flexion-extension. Activity of the lumbar muscles remained low and steady during the passive exercise session. Tension supplied by the posterior viscoelastic tissues decreased over time without corresponding changes in muscular activity. Active flexion, following the passive flexion session, elicited significant increase in paraspinal muscles EMG together with increase in the median frequency. It was concluded that reduction of tension in the lumbar viscoelastic tissues of humans occurs during cyclic flexion-extension and is compensated by increased activity of the musculature in order to maintain stability. It was also concluded that the ligamento-muscular reflex is inhibited during passive activities but becomes hyperactive following active cyclic flexion, indicating that moment requirements are the controlling variable. It is conceived that prolonged routine exposure to cyclic flexion minimizes the function of the viscoelastic tissues and places increasing demands on the neuromuscular system which over time may lead to a disorder and possible exposure to injury.

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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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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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UNLABELLED: Torque and laser detected surface mechanomyogram (MMG) analysis after electrical stimulation of human tibialis anterior (TA) of 14 male subjects was aimed to: (a) obtain the dynamic responses of TA muscle-joint unit from a long (LP, about 1h) and short (SP, 12.5s) stimulation protocol; (b) compare the resulting transfer function parameters from the two signals. The sinusoidal amplitude modulation of a 30 Hz stimulation train (SST) changed the number of the recruited motor units, and hence the isometric torque and the TA surface position in the same fashion. Subject instrumentation and SST amplitude range definition took about 25 min. SP: seven consecutive modulation frequencies (0.4, 6.0, 1.0, 4.5, 1.8, 3.0, and 2.5 Hz). LP: fourteen 5s long isolated frequencies (0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.5, 3.0, 4.0, 5.0, and 6.0 Hz), 5 min rest in between. Poles position (Hz) and added delay (ms) for phase correction with respect to the input sine (parameters of a critically damped II order system) were: torque 2.44+/-0.27 Hz (SP) or 2.32+/-0.33 Hz (LP) and 18.3+/-2.2 ms (SP) or 17.2+/-4.5 ms (LP); MMG 2.28+/-0.30 Hz (SP) or 2.30+/-0.44 Hz (LP) and 17.4+/-5.6 ms (SP) or 17.4+/-6.4 ms (LP). Differences were never statistically significant. CONCLUSION: it is possible to characterise the in vivo mechanics of muscle-joint unit with a short (few seconds) stimulation protocol affordable in clinical environment using both torque and MMG signals.

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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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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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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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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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