The principal resonance frequency in the driving-point impedance of the human body decreases with increasing vibration magnitude – a non-linear response. An understanding of the non-linearities may advance understanding of the mechanisms controlling body movement and improve anthropodynamic modelling of responses to vibration at various magnitudes. This study investigated the effects of vibration magnitude and voluntary periodic muscle activity on the apparent mass resonance frequency using vertical random vibration in the frequency range 0.5 to 20 Hz. Each of 14 subjects was exposed to 14 combinations of two vibration magnitudes (0.25 and 2.0 ms-2 r.m.s.) in seven sitting conditions (two without voluntary periodic movement (A: upright; B: upper-body tensed), and five with voluntary periodic movement (C: back- abdomen bending; D: folding-stretching arms from back to front; E: stretching arms from rest to front; F: folding arms from elbow; G: deep breathing). Three conditions with voluntary periodic movement significantly reduced the difference in resonance frequency at the two vibration magnitudes compared with the difference in a static sitting condition. Without voluntary periodic movement (condition A: upright), the median resonance frequency was 5.47 Hz at the low vibration magnitude and 4.39 Hz at the high vibration magnitude. With voluntary periodic movement (C: back-abdomen bending), the resonance frequency was 4.69 Hz at the low magnitude and 4.59 Hz at the high vibration magnitude. It is concluded that back muscles, or other muscles in the upper body, influence biodynamic responses of the human body to vibration and that their influence is greater at low vibration magnitudes than at high vibration magnitudes.