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 stiffness assessment have shown relationships to spinal level, body type, and lumbar extensor muscle activity. Such measures may be important determinants in discriminating between patients who are asymptomatic and those who have low back pain. However, little objective evidence is available concerning variations in PA stiffness and their clinical significance. Moreover, although several studies have assessed only load input in relation to stiffness, a more complete assessment based on dynamic stiffness measurements (force/velocity) and concomitant neuromuscular response may offer more information concerning 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 back 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 assessment 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 head, was used to deliver high-rate (<0.1 millisecond) PA manipulative thrusts (190 N) to several common spinal landmarks, including the ilium, sacral base, 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 monitor the bilateral neuromuscular activity of the erector spinae group during the PA thrusts. Maximal-effort isometric trunk extensions were performed by the research subjects before and immediately after the testing protocol to normalize sEMG data. The accelerance or stiffness index (peak acceleration/peak force, kg-1) and composite sEMG neuromuscular reflex response were calculated for each of the thrusts.

Results: Posteroanterior stiffness obtained at the sacroiliac joints, transverse processes, or spinous processes was not different for subjects grouped according to LBP chronicity. However, in those with frequent or constant LBP symptoms, there was a significantly increased spinous process (SP) stiffness 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 frequent 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 sacroiliac stiffness index and average transverse process stiffness index, respectively.

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 neuromuscular reflex responses in subjects with frequent or constant LBP as compared with those reporting intermittent or no LBP.