Page 291 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

268 BIOMECHANICS OF THE HUMAN BODY

FIGURE 11.4 Effect of posture (N—”normal”; E—erect; B—with backrest), muscle tension (T—tensed
2
muscles), and stimulus magnitude (0.25, 0.5, 1.0, and 2.0 m/s ) on the apparent mass of a seated person
for four subjects (see text for explanation). (Fairley et al., 1989.)
in unison with the torso. The influence of posture, muscle tension, and stimulus magnitude on the
apparent mass of seated persons, in the vertical direction, is shown for four subjects in Fig. 11.4
(Fairley et al., 1989). The column of graphs to the left of the diagram shows the modulus of the
apparent mass measured with a comfortable, “normal,” upright posture and muscle tension
(labeled N), with this posture but an erect torso and the shoulders held back (E), with all muscles
in the upper body tensed (T), and, finally, with the subject leaning backward to rest against a rigid
backrest (B). The largest variation in apparent mass between these conditions was associated with
tensing the back muscles, which clearly increased the frequency of the characteristic peak in the
response (at around 5 Hz). In some subjects the frequency of this peak could be changed by a
factor of 2 by muscle tension. A variation in the apparent mass could also be induced by changing
−2
the stimulus magnitude, as is shown for four RMS accelerations (0.25, 0.5, 1.0, and 2.0 ms ) to the