Mechanical impedance is a measure of how much a structure resists motion when subjected to a harmonic force. It relates forces with velocities acting on a mechanical system. At resonant frequencies, the mechanical impedance will be lower, meaning less force is needed to cause a structure to move at a given velocity. However, in the previous studies on human subjects exposed to vibration, the peak amplitude of mechanical impedance has been observed at the resonance frequency.
Can anybody explain the reason for this contradiction?
At first glance, I agree, it's extremely confused. But why are you assuming the peak on the graph is a resonance? If the article's authors state so, it's a mistake.
The resonance frequency of the human body lies in the vicinity of 4-8 Hz. Therefore the peak in the graph indicates the resonance only. Similar responses have been reported in many other studies too.
I think you definition of impedance is slightly wrong. However it is also true that you can see a peak in impedance at a resonant frequency, and not only a dip. It depends which resonance it is and where you are driving it. On any body driven at resonance there will places where the drive sees a high impedance and places where the drive will see low impedance. For instance, on a skipping rope being swung or shaken at resonance, if it is driven near the ends it will be high impedance (lots of force but small movement), but if driven at the middle low impedance (low force but rapid large movement).
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