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.