Frequency of cyclic lumbar loading is a risk factor for cumulative trauma disorder

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.