Stiffness and neuromuscular reflex response of the human spine to posteroanterior manipulative thrusts in patients with low back pain

Background: Studies investigating posteroanterior (PA) forces in spinal sti ffness assessment have shown relationships to spinal level, body type, and lumbar extensor muscle activity. Such measures may be important determinant s in discriminating between patients who are asymptomatic and those who hav e low back pain. However, little objective evidence is available concerning , variations in PA stiffness their clinical significance. Moreover, althoug h several studies have assessed only load input in relation to stiffness, a more complete assessment based on dynamic stiffness measurements (force/ve locity) and concomitant neuromuscular response may offer more information c oncerning mechanical properties of the low back. Objective: To determine the stiffness and neuromuscular characteristics of the symptomatic low back. Study Design: This study is a prospective clinical study investigating the in vivo mechanical and muscular behavior of human lumbar spinal segments to high loading rate PA manipulative thrusts in research subjects with low ba ck pain (LBP). Methods: Twelve men and 10 women, aged 15 to 73 years (mean age of 42.8 +/- 17.5 years) underwent physical examination and completed outcome assessmen t instruments, including Visual Analog Scale, Oswestry Low Back Disability Index, and SF-36 health status questionnaires. Clinical categorization was made on the basis of symptom frequency and LBP history. A hand-held spinal manipulation device, equipped with a preload control frame and impedance he ad, was used to deliver high-rate (<0.1 millisecond) PA manipulative thrust s (190 N) to several common spinal landmarks, including the ilium, sacral b ase, and L5, L4, L2, T12, and T8 spinous. and transverse processes. Surface , linear-enveloped, electromyographic (sEMG) recordings were obtained from electrodes (8 leads) located over the L3 and L5 paraspinal musculature to m onitor the bilateral neuromuscular activity of the erector spinae group dur ing the PA thrusts. Maximal-effort isometric trunk extensions were performe d by the, research subjects before and immediately after the testing protoc ol to normalize sEMG data. The accelerance or stiffness index (peak acceler ation/peak force, kg-l) and composite sEMG neuromuscular reflex response we re calculated for each of the thrusts. Results: Posteroanterior stiffness obtained at the sacroiliac joints, trans verse processes, or spinous processes was not different for subjects groupe d according to LBP chronicity. However, in those with frequent or constant LBP symptoms, there was a significantly increased spinous process (SP) stif fness index (7.0 kg-1) (P < .05) in comparison with SP stiffness index (6.5 kg-1) of subjects with only occasional or no LBP symptoms. Subjects with f requent or constant LBP symptoms also reported significantly greater scores on the visual analog scale (P = .001), Oswestry (P = .001), and perceived health status (P = .03) assessments. The average SP stiffness index was 6.6 % greater (P < .05) and 19.1% greater (P < .001) than the average sacroilia c stiffness index and average transverse process stiffness index, respectiv ely. Conclusions: This study is the first to assess erector spinae neuromuscular reflex responses simultaneously during spinal stiffness examination. This study demonstrated increased spinal stiffness index and positive neuromuscu lar reflex responses in subjects with frequent or constant LBP as compared with those reporting intermittent or no LBP.