Investigating the active contractile function of the rat paraspinal muscles reveals unique cross-bridge kinetics in the multifidus

Purpose Various aspects of paraspinal muscle anatomy, biology, and histology have been studied; however, information on paraspinal muscle contractile function is almost nonexistent, thus hindering functional interpretation of these muscles in healthy individuals and those with low back disorders. The aim of this study was to measure and compare the contractile function and force-sarcomere length properties of muscle fibers from the multifidus (MULT) and erector spinae (ES) as well as a commonly studied lower limb muscle (Extensor digitorum longus (EDL)) in the rat. Methods Single muscle fibers ( n = 77 total from 6 animals) were isolated from each of the muscles and tested to determine their active contractile function; all fibers used in the analyses were type IIB. Results There were no significant differences between muscles for specific force (sF o ) ( p = 0.11), active modulus ( p = 0.63), average optimal sarcomere length ( p = 0.27) or unloaded shortening velocity ( V o ) ( p = 0.69). However, there was a significant difference in the rate of force redevelopment ( k tr ) between muscles ( p = < 0.0001), with MULT being significantly faster than both the EDL ( p = < 0.0001) and ES ( p = 0.0001) and no difference between the EDL and ES ( p = 0.41). Conclusions This finding suggests that multifidus has faster cross-bridge turnover kinetics when compared to other muscles (ES and EDL) when matched for fiber type. Whether the faster cross-bridge kinetics translate to a functionally significant difference in whole muscle performance needs to be studied further.