Pain-related factors contributing to muscle inhibition in patients with chronic low back pain - An experimental investigation based on superimposed electrical stimulation

Objective: To evaluate muscle strength, as a component of physical deconditioning, and central activation ratio, representing the performance level during testing, in patients with chronic low back pain as compared to healthy controls, and to evaluate the contribution of cognitive-behavioral and pain-related factors to the central activation ration of patients with chronic low back pain. Methods: Twenty-five patients with chronic low back pain and 25 age and gender-matched controls participated. Muscle strength, that is, peak torque of the quadriceps muscle, was measured on a Cybex dynamometer. During peak torque, the quadriceps muscle was percutaneously stimulated using superimposed electrical stimulation, generating an additional twitch torque in case of submaximal performance. The central activation ratio was calculated as peak torque/(peak torque + additional twitch torque). To evaluate cognitive-behavioral and pain-related factors influencing the central activation ratio, measures of fear of injury, pain catastrophizing, psychologic distress, and pain intensity were used. Differences between groups were tested using either T tests or Mann-Whitney U tests. Associations were tested by partial correlation coefficients controlling for gender. Results: The male:female ratio was 15:10. Mean age and chronic low back pain duration were 42.7 (± 9.5) and 9.9 (± 8.3) years, respectively. Mean muscle torque (per kg lean body mass) in patients (1.95 Nm/kg ± 0.8) was less than in controls (3.16 Nm/kg ± 0.7) (P < 0.01). Median central activation ratio was lower in patients (P < 0.05). Patients experiencing increased psychologic distress and patients with a higher current pain level showed a lower central activation ratio (P < 0.05). Conclusions: When interpreting decreased muscle strength in terms of physical deconditioning in patients with chronic low back pain, submaximal performance has to be taken into account. The results suggest that patients with chronic low back pain who report increased psychologic distress and a higher level Of Current pain tend to show increased inhibition of muscle activity, leading to submaximal performance.