Indirect measurement of the fundamental natural frequency of a standing person

If a stationary person should be modelled as a single degree-of-freedom (SDOF) system in structural vibration, what are the natural frequency, damping ratio and the modal mass of the human SDOF system?

The natural frequency of a human body cannot be obtained directly using traditional methods and tools of structural dynamics, such as sensors (accelerometers) which cannot be conveniently mounted on a human body. However, a method has been developed to estimate the natural frequency of a standing person by experiment without touching the person whilst still using the methods of structural dynamics.

A simple formula can be derived based on the three frequency relationships derived in Section 19.2.2 using the measurements of the natural frequency of the empty structure and the resonance frequency/frequencies of the human-structure system as demonstrated in Section 19.3. For example, the demonstration given in Section 19.3.2 shows the natural frequency of the test rig alone of 6.10 Hz and the two resonance frequencies of the human-rig system of 5.20 Hz and 7.30 Hz respectively in Figure 19-9. Using Eq. 19-8 gives an estimated natural frequency of the standing person of 6.22 Hz in the lateral direction. It should be noted that this is an approximation because:

• the measurements in Fig. 19-9d are the resonance frequencies which include the effect of human body damping while Eq. 19-8 does not consider any effect of damping.
• the simple mass-spring-damper model is good enough for identifying the models of human body in structural vibration qualitatively. However, this body model is developed on fixed ground rather than on a vibrating structure.

The indirect measurement method needs to be improved; nevertheless it uses the methods of structural dynamics to study the biomechanics properties of a human body, which is fundamentally different from the methods of body biomechanics where shaking tables are used [19.9].