To further elucidate the pathogenesis of obstructive sleep apnea (OSA), we recorded sternocleidomastoid (SCM), genioglossal (GG), and abdominal (ABD) muscle activity, using surface electromyograms (EMGs), during 45 polygraphic studies in 39 children referred for possible OSA. For each muscle, an index of electromyographic (EMG) activity was developed, allowing an interpatient regression analysis of EMG amplitude versus highest PACO2 and lowest transcutaneous O2 tension (tcPO2) during sleep. Phasic inspiratory SCM activity was present during more than 50% of sleep time in 16 of 20 patients with OSA versus only 2 of 15 patients without OSA; SCM EMG activity increased with increasing PACO2 (r = 0.45, p less than 0.02) and decreasing tcPO2 (r = 0.51, p less than 0.01). Phasic inspiratory GG activity was present during more than 50% of sleep time in 15 of 18 patients with OSA versus none of 4 without OSA; GG EMG activity increased with increasing PACO2 (r = 0.51, p less than 0.05) and with decreasing tcPO2 (r = 0.60, p less than 0.02). Phasic expiratory ABD activity was present during more than 50% of sleep time in 10 to 20 patients with OSA versus 1 of 12 patients without OSA. These findings suggest the following conclusions: (1) inspiratory (SCM), airway-maintaining (GG), and expiratory (ABD) muscles contract during sleep-related partial airway obstruction in children; (2) augmented GG activity during periods of sleep-related partial airway obstruction suggests that pediatric OSA involves pathogenic mechanisms other than GG hypotonia; (3) expiratory ABD muscle activity suggests that some children with OSA have increased expiratory, as well as inspiratory, airway resistance during sleep.(ABSTRACT TRUNCATED AT 250 WORDS)