RESUMEN
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.
Asunto(s)
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 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.
Asunto(s)
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
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.