Introduction.  Although the stabilizing role of the lumbar musculature is appreciated, the muscles ability to recover ligamentous injury or instability are less understood.  The purpose of this experimental study was to quantify the effects of ligament injury and muscle activation on the in vivo dynamic dorsoventral (DV) lumbar spine stiffness.

Methods.  Two pairs of electrical stimulating electrodes were placed in the multifidus at L3-L4 in fifteen anesthetized prone laying Merino sheep.   Dynamic DV spine stiffness was assessed at L3 using a computer-controlled testing apparatus oscillating from 0.5 to 20 Hz.  Five randomized trials were conducted at rest and during four 20 Hz supramaximal stimulation voltages (6, 8, 15, and 20 volts).   Five randomized trials were repeated following progressive injury to the interspinous/supraspinous ligament (ISL/SSL), and again following facetectomy.  The secant stiffness (k_y = DV force/L3 displacement, N/mm) were determined across 44 mechanical testing frequencies and normal and injured spine states.  Descriptive statistics were computed and within group comparisons evaluated with a series of repeated measures ANOVAs with post-hoc Bonferroni correction, and condition differences were assessed using a paired-observations t-tests (two-tailed POTT).

Results.  The dynamic stiffness varied nearly 4-fold over the 0.5 to 20 Hz frequency range consistent with the viscoelastic nature of the ovine spine. In the intact spine, muscle stimulation provided significant up to two-fold increases in k_y at 43 of 44 mechanical testing frequencies examined (p<0.034).  Likewise, a significant graded increase in k_y was observed for submaximal contractions for most mechanical excitation frequencies when compared to rest (p<0.048). Injury to the ISL/SSL caused a significant loss in spinal stiffness (p<0.05) that was able to be recovered by submaximal (8V, 15V) and maximal (20V) muscle stimulations at most mechanical excitation frequencies (p<0.05).  Facetectomy, however, caused a significant loss (P<0.05) of dynamic spinal stiffness at all mechanical excitation frequencies which was only able to be recovered with maximal (20 V) muscle stimulation. 

Discussion.  Dynamic lumbar spine stiffness is dependent upon mechanical testing frequency and is appreciably decreased with ligament and joint injuries.  Certain levels of muscle stimulation can recover stiffness in ISL/SSL injury but only maximal multifidus stimulation can recover stability in conditions of facetectomy.  Quantifying the passive and active contributions of the lumbar spine musculature to lumbar spine stiffness contributes to biomechanical modeling, surgical, and rehabilitative considerations in instability.