METHODS: To investigate muscle stimulation, four needle EMG recording electrodes and two stimulating electrodes were placed into the multifidus muscles bilaterally at the levels of L3 and L4 in ten adolescent Merino sheep. Constant voltage (4 V), electrical stimulations with varying pulse durations (2.5, 5, 10, and 20 Hz) were randomly administered. At rest and during muscle stimulation, a computer-controlled voice coil actuator equipped with a load cell was used to deliver an oscillatory PA force directly to the L3 spinous process over a 2 to 20 Hz range (20 N preload to 200 N peak). In each trial L3 displacement and EMG responses were recorded. The PA secant stiffness (force/displacement, kN/m) was determined and the effect of muscle excitation on the PA stiffness was assessed at each mechanical excitation frequency using a paired-observations t-test (p<.05).
To investigate mechanical stimulus, the apparatus delivered a PA mechanical force to the L3 spinous process of fifteen adolescent Merino sheep. PA forces (48 N peak, ~20% body weight) were randomly applied at periodic excitation frequencies of 2 Hz, 6 Hz, 12 Hz and 2 to 20 Hz sweep. Force and displacement were recorded over a 13 to 22 second interval.
RESULTS: During maximum muscle stimulation the PA stiffness increased 3- to 9-fold and the increase was statistically significant (p<.05) for 22 of the 28 mechanical stimulation frequencies. For sub-maximum muscle stimulations, the PA stiffness was significantly increased for 7/28 mechanical excitation frequencies (10 Hz stimulus), 6/28 mechanical excitation frequencies (5 Hz stimulus), and 1/28 mechanical excitation frequencies (2.5 Hz stimulus) when compared to rest.
During the application og mechanical stimulation, the in vivo posteroanterior stiffness of the ovine spine was frequency dependent and varied 3.7-fold over the 0.5 Hz to 19.7 Hz mechanical excitation frequency range. Minimum and maximum PA stiffness were 3.86 ± 0.38 N/mm and 14.1 ± 9.95 N/mm at 4.0 Hz and 19.7 Hz, respectively. Stiffness values based on the swept-sine measurements were not significantly different from corresponding periodic oscillations (2, 6 and 10.6 Hz). The mean variance in the swept-sine PA dynamic stiffness assessment method was 15%, which was similar to the variance associated with the periodic oscillation method (10% to 16%).
DISCUSSION: Sub-maximal and maximal neuromuscular excitation causes PA spine stiffness to increase, which may be important biomechanical considerations in the pathomechanisms and diagnosis of low back pain. Dynamic stiffness assessments could be used to provide important information concerning the mechanical status of the normal and dysfunctional or pathologic spine.
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