Alternating leg muscle activation during sleep and arousals: a new sleep-related motor phenomenon?

We describe a quickly alternating pattern of anterior tibialis activation, recorded during nocturnal polysomnography in 16 patients. Polysomnography, usually for sleep-disordered breathing, included surface electromyograms over the anterior tibialis of each leg. Cases were identified from approximately 1,500 studies reviewed in the course of standard clinical care. Patients were 12 men and 4 women (mean age, 41 +/- 15 years; range, 12-70 years). Brief activation of the anterior tibialis in one leg alternated with similar activation in the other leg. Activations occurred at a frequency of approximately 1 to 2 Hz, each lasted between 0.1 and 0.5 seconds, and sequences of alternating activations usually lasted between several and 20 seconds. The phenomenon occurred in all sleep stages but particularly during arousals. Ten of the 16 patients had periodic leg movements during sleep at a rate >/= 5.0 per hour, and 12 of the 16 patients were taking antidepressant medication. Alternating leg muscle activation (ALMA) during sleep, at this relatively high frequency, may be a newly described phenomenon. We speculate that ALMA could represent transient facilitation of a spinal central pattern generator for locomotion, perhaps due to serotonergic effects of antidepressant medication.