Elcctromyogram (EMG) was recorded by surface electrodes from lumbar paraspinal muscles during a 24-h period using Medilog 4 channel battery operated portable tape recorder. On replay records the total electrical activity amplitude and duration of the activity was evaluated. Nine patients with low back pain and unilateral or prevailing one-sided palpable paraspinal muscle spasm (PS) were studied. The control group consisted of 12 participants without pain or palpable spasm. Sustained electrical activity was documented during sleep in lumbar paraspinal muscles of patients with low back pain and palpable spasm. This study demonstrated that in normal subjects without low back pain there is no EMG activity in lumbar paraspinal muscles during sleep. The EMG activity in spasmotic muscles was statistically significantly higher than in the nonspasmotic control subjects without pain.