Short rest periods after static lumbar flexion are a risk factor for cumulative low back disorder

The objective of this work was to study the effect of rest periods of various durations applied between six 10-min sessions of static flexion on the development of cumulative low back disorder (CLBD). Three experimental groups of a feline model were used, and the rest duration between sequential static load periods was set to 5, 10, and 20 min, with a corresponding load-to-rest ratio of 2:1, 1:1 and 1:2, respectively. The reflex electromyographic (EMG) activity from the multifidus muscles and supraspinous ligament displacement (creep) were recorded during the flexion periods and over 7 h of rest following the load-rest cycles. It was found that a minor disorder developed in all the groups whereas a severe neuromuscular disorder including a delayed hyperexcitability was observed only in the group subjected to 5 min rest. The two-way ANOVA showed a significant effect of time post loading (p<0.001) and rest duration (p<0.001) on the Normalized Integrated EMG (NIEMG) recovery data; a significant effect of time post loading on the Displacement data (p<0.001) was observed as well. The post hoc Fisher test performed on the NIEMG data during the recovery phase showed a significant difference between the group subjected to 5 min rest and the other two groups (p<0.001). These results suggest that while a short rest period of 2:1 load-to-rest ratio leads to CLBD, longer rest at 1:1 and 1:2 load-to-rest ratio are more favorable for preventing or attenuating the development of CLBD. Short rest periods between sessions of static lumbar flexion, therefore, are a risk factor for the development of CLBD.