Muscle response pattern to sudden trunk loading in healthy individuals andin patients with chronic low back pain

Study Design. A quick-release method in four directions of isometric trunk exertions was used to study the muscle response patterns in 17 patients wit h chronic low back pain and 17 matched control subjects, Objectives. It was hypothesized that patients with low back pain would reac t to sudden load release with a delayed muscle response and would exhibit a ltered muscle-recruitment patterns. Summary of Background Data. A delay in erector spinae reaction time after s udden loading has been observed in patients with low back pain. Muscle recr uitment and timing pattern play an important role in maintaining;; lumbar s pine stability. Methods. Subjects were placed in a semiseated position in an apparatus that provided stable fixation of the pelvis. They exerted isometric contraction s in trunk flexion, extension, and lateral bending. Each subject performed three trials at two constant force levels. The resisted force was suddenly released with an electromagnet and electromyogram signals from 12 trunk mus cles were recorded. The time delay between the magnet release and the shut- off or switch-on of muscle activity (reaction time) was compared between tw o groups of subjects using two-factor analysis of variance. Results. The number of reacting muscles and reaction times averaged over al l trials and directions showed the following results: For healthy control s ubjects a sh ut-off of agonistic muscles (with a reaction time of 53 msec) occurred before the switch-on of antagonistic muscles (with a reaction time of 70 msec). Patients exhibited a pattern of co-contraction, with agonists remaining active (3.4 out of 6 muscles switched off) while antagonists swi tched on (5.3 out of 6 muscles). Patients also had longer muscle reaction t imes for muscles shutting off (70 msec) and switching on (83 msec) and furt hermore, their individual muscle reaction times showed greater variability. Conclusions. Patients with low back pain, in contrast to healthy control su bjects, demonstrated a significantly different muscle response pattern in r esponse to sudden load release. These differences may either constitute a p redisposing factor to low back injuries or a compensation mechanism to stab ilize the lumbar spine.