A 7 to 13 Hz mechanographic signal with an amplitude of 1-5 μm is detectable over relaxed limbs. This phenomenon, termed microvibration (MV), continues during sleep and has been found a common feature of warmblooded organisms. MV is attributed to resonance oscillations driven by cardioballistic forces. It has been argued, however, that resting muscle tone influences MV. In a recent microgravity (0 g) study this could be confirmed: during relaxation in the free floating state of a cosmonaut the resonance oscillations (7-13 Hz) became strongly damped, whereas cardioballistic forcing ( < 5 Hz) remained in the MV signal. Here we report from a repetition of these measurements, but in addition to the previous study MV was also recorded during maneuvers to slightly increase muscle tone at 0 g. As a result such an increase in tone restored the MV oscillations, thus indicating MV as a function of muscle tone. For 1 g it is further concluded that some resting tone must exist in relaxed muscle tissue in order to respond with an oscillatory MV behavior.
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