BACK AND HIP EXTENSOR FATIGABILITY IN CHRONIC LOW-BACK-PAIN PATIENTS AND CONTROLS

Objective: To compare the lumbar paraspinal and gluteus maximus muscle fatigability between chronic law back pain patients and healthy contr ols by using electromyographic (EMG) spectral analysis during a maxima l isometric endurance task. Design: A cross-sectional comparative stud y between chronic low back pain patients and healthy control subjects. Setting: Physical medicine and rehabilitation clinic in Finland. Subj ects: Twenty women with nonspecific chronic low back pain (longer than 3 months) and 15 healthy controls. Intervention: Subjects performed m aximal voluntary isometric back extensions (MVC) at 30 degrees forward flexion in a specialty designed measurement unit. A 50% MVC load was used in isometric endurance test. Outcome Measures: Low back pain inte nsity was assessed by using the visual analogue scale and functional d isability by the Oswestry disability index. Time to endurance was meas ured. Bipolar surface EMG recordings were made over the lumbar paraspi nal muscles (L3-L4, L5-S1) and over the gluteus maximus muscles. Avera ge EMG (aEMG%), and initial spectral median frequency (MFinit) and cha nge over time (MFslope) were computed. Results: Pain intensity and fun ctional disability were higher and MVC and time to endurance were lowe r in the chronic low back pain group (p < .05). Similar muscle activit y levels (aEMG%) and MFinit indicated similar muscle loading in both g roups at the beginning of the endurance test, EMG spectral decreases ( MFslope) indicated that lumbar paraspinal muscle fatigability was simi lar in both groups. In the chronic low back pain group, the gluteus ma ximus fatigued faster than in the control group (greater MFslope, p < .05). However, the shorter endurance time indicated greater fatigabili ty in the chronic low back pain group in general (p < .05). Conclusion s: The chronic low back pain patients were weaker and fatigued faster than the healthy controls. The EMG fatigue analysis results suggest th at the gluteus maximus muscles are more fatigable in chronic low back pain patients than in healthy control subjects during a sustained back extension endurance test. (C) 1998 by the American Congress of Rehabi litation Medicine and the American Academy of Physical Medicine and Re habilitation.