No changes in muscle fibre type composition in rat multifidus muscle following lesion of the lumbar intervertebral disc

The multifidus muscle has been proposed to play an important role in the development and recurrence of low-back pain (LBP). In line with this, fibre type composition has been found to be altered in humans with LBP. This study aims to investigate the changes in muscle fibre type composition of the multifidus muscle after stab disc lesion in the rat. Data were obtained from 24 male Wistar rats randomly assigned to the intervention group, in which the L4/L5 intervertebral disc was stabbed, or the control group, in which no intervention was applied. At 7, 14 and 28 days post-intervention, two fascicles of the multifidus muscle between L3 and S1 were removed bilaterally for analysis of fibre type composition. The rats' multifidus muscle consisted for the largest part of type IIB fibres in both the intervention (53 +/- 10% across all time points) and the control group (53 +/- 9% across all time points). We found no effects of disc lesion on the proportion of type I, IIA, IIX and IIB fibres. These results indicate that the fibre type composition of the multifidus muscle is not affected by disc lesion within the time period (28 days) studied. A different functional role of the multifidus muscle in the rat compared to humans, reflected in the high proportion of fast muscle fibres in the rat's multifidus muscle, may explain our findings. Differences between species in fibre type composition should be taken into account when using rats as a model to investigate the mechanisms causing (chronic) LBP in humans.