Vibrografic signs of autonomous muscle tone studied in long term space missions

Signs of resting muscle tone, assessed by cardiac driven microvibrations (MV), were studied in two cosmonauts during long term space missions on the Russian MIR station. In the 1 g environment, MV showed the typical 7-13 Hz resonance oscillations triggered by the heart beat. In 0 g, these pulsations were damped and the waveform became similar to an acceleration ballistocardiogram. If resting tone was slightly increased by extending the arm in 0 g, the resonance oscillations reappeared in most cases. By means of a simple vibromechanic analog it is demonstrated that the elastic component of muscle has to be reduced during the fully relaxed state in 0g.