Without passing on the remainder of the points being made I would note that most human bodies drive in resonance at ~5-7Hz and the first harmonic at ~10-12Hz.
Not coincidentally -- the frequency of furitama and tekubi furi is at about 10-12 Hz.
I would further note that oscillatory phenomena (alternating stress, opposite in sign) tend to be dominated by shear forces -- and shear forces are greatest at the surface of a structure, whether in bending or in torsion.
FWIW.
Enjoy.