Lumbar muscle fatigue : Analysis of electromyography, endurance time and subjective factors in patients with lumbar disc herniation and healthy subjects

Abstract: The overall aim of the thesis was to investigate the relation between objectively and subjectively assessed lumbar muscle fatigue and subjective factors in patients with lumbar disc herniation and in healthy subjects. Eighty healthy subjects and 43 patients undergoing surgery for lumbar disc herniation and selected consecutively for the study participated. Muscle fatigue in the lower back was assessed during a modified Sorensen test, an isometric prone holding test for the back extensors against gravity and until exhaustion, or a staircase protocol (20%, 40%, 60%, and 80% of maximum voluntary contraction). Concomitantly, EMG was detected from the lumbar extensor muscles and subjects rated their perceived fatigue - and the patients also their pain - on a Borg CR- 10 scale. Recovery was assessed after the modified Sorensen test during 5-second contractions at 1-5 minutes of recovery then examined using an exponential time dependence model. To measure activity, participation, self-efficacy and health, the patients also filled in questionnaires. The reliability of the modified Sorensen test was also assessed. The modified Sorensen test with concomitant measures of EMG provided useful reliability (ICC>0.60) for evaluation purposes. Borg scale ratings correlated highly (r>0.7) with endurance time. The EMG median frequency slopes correlated low-to-moderately (r<0.54). At Borg rating 3 (moderate fatigu'), 5 (strong fatigue), and 7 (very strong fatigue), the medians of median frequency and endurance time had changed by 28-37%, 48-69%, 63-89% at each time respectively. This relation can be used in clinical testing where maximum endurance tests are unsuitable: a fixed time performance until Borg rating 5 may be used. That healthy women's back muscles were more fatigue-resistant than healthy men's was shown in a smaller median frequency decrease. However the opposite was indicated for patients with lumbar disc herniation. Elimination of pain due to surgery reduced muscle fatigue with respect to a smaller median frequency decrease but not enough to increase endurance time. Patients could not fatigue their muscles as much as healthy subjects could, as shown by significantly shorter endurance time and smaller median frequency decrease than healthy subjects. The applicability of the exponential time dependence model was excellent to all healthy subjects except three. Of the patients, only 14 had an exponential recovery process both before and after surgery indicating that the recovery process had been impaired due to the disc herniation. Measurements of fatigue and recovery were associated with activity limitations, participation restrictions, self-efficacy and health in patients with lumbar disc herniation. These interactions were seen by correlation coefficients of 0.53-0.95 between endurance time and some of the questionnaire answers, especially for women. Patients with non-significant fatigue and an impaired recovery process scored significantly worse on the questionnaires than those with both fatigue and non-impaired recovery.