| Abstract |
The role of spinal ligaments subjected to cyclic (repetitive) and static (prolonged) occupational activities as a source of work related musculoskeletal disorders was the objective of this investigation. It was hypothesized that ligaments subjected to cyclic and static activities develop, over time, creep (stretch) or tension-relaxation (loss of tension) and reduce the mechanical stability of the intervertebral joints. It was further hypothesized that with creep or tension-relaxation, the sensory receptors in the ligaments become de-sensitized and that in turn reduces the reflexive activation of lumbar muscles which further reduces mechanical stability and increases exposure to injury and disability. It was further anticipated that many hours of rest will be required to restore normal function. The importance of the research to occupational safety and health is vested in the fact that epidemiologic data points out that workers exposed to daily activities requiring repetitive lumbar flexion (loading-unloading cargo, manufacturing/assembly, farm workers, etc.) and to static lumbar flexion (mechanics, brick/concrete workers, welders, roofers, etc.) report up to 10 times more episodes of low back disorders relative to other groups. The biomechanical and physiological processes active in the development of such disorders are not fully known nor are they scientifically experimentally validated. Identification of the biomechanical/physiological processes active in the development of such low back disorders will produce significant improvements in their prevention, treatment, restructuring work/rest schedules and designing optimal work conditions. Substantial financial savings could be materialized by the government and private/public sectors in reduction of medical expenses, disability payments, lost manpower and cost of manufacturing. The approach employed in this research utilizes the "in-vivo" feline model subjected to cyclic and static loads applied to the lumbar spine to induce anterior flexion while recording creep/tension-relaxation of the ligaments (and other viscoelastic tissues) simultaneously with EMG from the multifidus muscles. The results of the three years investigation provided new and significant insights into the biomechanical and physiological principles active in the development of a common idiopathic low back disorder. The data obtained to date also provide experimental validation of epidemiological data pointing out that repetitive and static work activities are indeed risk factors for musculoskeletal disorders. The major finding consist of the fact that twenty minutes (20 min) exposure to cyclic or static lumbar flexion resulted in a complex neuromuscular disorder consisting of seven components: Substantial development of creep/tension-relaxation in the lumbar ligaments as well as other viscoelastic tissues (discs, capsule, dorsolumbar fascia) during the static or cyclic loading periods, (i.e., work period). Sharp decrease in the reflexive activation of the posterior muscles over the same work period. Routinely observed spasms from the posterior muscles superimposed on the decreasing EMG over the work period. During a rest period immediately following the work period, the creep/tension-relaxation set in the ligaments did not fully recover over seven hours. The model developed predicts full recovery in 24-48 hours. During the first hour of the rest period, muscular hyperexcitability was present in the muscles. A delayed, often severe and prolonged muscular hyperexcitability was present in the 3rd -7th hours of rest. A slow recovery to normal reflexive muscular activity was present throughout the seven hours of rest. A mathematical model was developed and fitted to the experimental data. The model describes the various components of the neuromuscular disorder as a function of work and rest time and could be used to assess, predict and prevent low back disorders by optimizing work/rest schedules. Further refinement of the model can adapt its use to prediction and prevention of Cumulative Trauma Disorders (CTD). Overall, the research performed to date confirm experimentally that static and cyclic work activities are risk factors and delineates important biomechanical and physiological infrastructure active in the development of a transient common idiopathic low back disorder associated with repetitive and static occupational activities. Preliminary results also provide pathological and biomechanical evidence that show the same principles may be active in the development of a cumulative trauma disorder for the same work activities. All the Specific Aims (# 1 to #4) were fully met by the work accomplished over the three year period (see publication list and their relationship to the specific aims below) |
