SLEEP STATE-DEPENDENT EFFECTS ON MICROVIB RATION AND MUSCLE TONE IN INFANTS

The effect of quiet (NREM) and active (REM) sleep on accelerometric re corded hand microvibration was studied in infants (N = 10) at the age of 2, 6, and 12 months. Additionally to microvibration, masseter surfa ce EMG as a measure of resting muscle tone, electrocardiogram as a mea sure of cardiac activity, and EEG for estimation of sleep state were r ecorded in a whole-night polysomnography. Root mean square amplitudes of microvibration were calculated in the frequency bands of 3-7 Hz (B1 ), 7 - 13 Hz (B2) and 13 - 30 Hz (B3). Additionally, mean frequency of microvibration, integrated EMG and heart rate were calculated. In qui et sleep the microvibration signal clearly shows the mechanical action of the heart beat followed by a complex oscillatory component. In act ive sleep, both, action of the heart beat and the oscillatory componen t was reduced (Fig. 1, 2). This was expressed by a decrease of B2 (- 1 5%), B1, B2 and mean frequency of microvibration remained unchanged (F ig.4). Integrated EMG was decreased (- 10%) and heart rate was not sig nificantly increased in active sleep (Fig. 3, 5). Further B1, B2, B3 a nd heart rate decreased with age, no effect was found with sleep durat ion. From these results it was concluded that reduction of elastic cou pling in the musculoskeletal system (= mucle tone) is responsible for the reduction of the 7 - 13 Hz component of microvibration during acti ve sleep. On the one hand reduction of muscle tone reduces transmissio n of cardiac impulses to distal parts of the body, and on the other ha nd musculoskeletal resonances which were excited by the heart beat bec ame more damped. Other factors contributing to microvibration, like re sonance properties of the wrist joint and of the skin-transducer are d iscussed. The parallel decrease of microvibration and heart rate with age, indicates that microvibration depends on heart rate too.