FLEXION-RELAXATION PHENOMENON IN THE BACK MUSCLES - A COMPARATIVE-STUDY BETWEEN HEALTHY-SUBJECTS AND PATIENTS WITH CHRONIC LOW-BACK-PAIN

At a certain position of trunk flexion, there is a sudden onset of ele ctrical silence in back muscles. This is called ''flexion-relaxation ( F-R) phenomenon.'' The goals of this study were (1) to evaluate the re lationship between flexion angle and activity of back muscles during f lexion movement and (2) to determine what the difference is between he althy subjects and patients with chronic low back pain (CLBP). Twenty- five healthy subjects (13 males and 12 females; average age, 28.3 yr) and 20 patients with CLBP (12 males and 8 females; average age, 34.1 y r) volunteered for this study. The subjects were asked to flex forward maximally from the erect position and to maintain full flexion, follo wed by returning to the initial upright position. Flexion angle of tru nk and hip was measured during the examination. Electromyographic acti vity of erector spinae was also monitored simultaneously. F-R phenomen on was observed in all healthy subjects before reaching the maximum fl exion. Electrical silence continued even after extending the trunk beg an, In contrast, no patients with CLBP demonstrated F-R phenomenon. A significant difference in muscular activities of erector spinae betwee n the groups was obtained when returning to the erect position from th e maximum flexion. Moreover, time lag between trunk and hip movement w as much greater in patients than in healthy subjects, This study demon strated that neuromuscular coordination between trunk and hip could be abnormal in patients with CLBP.