INTRODUCTION: The paraspinal muscles are responsible for moving and stabilizing the spinal column. There have been debates regarding the functional roles of the different paraspinal muscles, in particular the multifidus and erector spinae (combined longissimus and iliocostalis). While various aspects of paraspinal muscle anatomy, biology, and histology have been studied, information on paraspinal muscle contractile function is almost non-existent, 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 fibres 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 fibres (n = 77 total from 6 animals) were isolated from each of the muscles, chemically permeabilized, activated by immersing them in a chamber containing a high-Ca²⁺ activating solution (pCa 4.2) to elicit maximal force and tested to determine their active contractile function; all fibres 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 (stiffness) (p = 0.63), average optimal sarcomere length (p = 0.27) or unloaded shortening velocity (V_o) (p = 0.69) (Figure 1A-E). However, there was a significant difference in the rate of force redevelopment (ktr) 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) (Figure 1F).

DISCUSSION: Rate of force redevelopment (ktr) is a proxy measure of the transition from non-force bearing to force bearing cross-bridge states (i.e., weakly to strongly bound states). The present finding suggests that multifidus has faster crossbridge turnover kinetics when compared to other muscles (ES and EDL) when matched for fibre type. Since Ca²⁺ is the driver of muscle contraction, changes in the way the contractile apparatus handles Ca²⁺ could affect muscle contractile performance at the single fibre level and is therefore one possible mechanism that could explain the difference in ktr between muscles; future efforts will aim to determine if the MULT handles calcium in a “unique” way, compared to other muscles. Whether the faster crossbridge kinetics translate to a functionally significant difference in whole muscle performance needs to be studied further.

Figure Caption: Multifidus (MULT), extensor digitorum longus (EDL) and erector spinae (ES) data, shown in yellow, blue and red, respectively, for A) specific force, B) active modulus (stiffness), C) optimal sarcomere length, D) force-sarcomere length relationship, E) unloaded shortening velocity, F) rate of force redevelopment (ktr). MULT had significantly faster ktr than both EDL and ES.