A model analysis of internal loads, energetics, and effects of wobbling mass during the whole-body vibration.

The internal loads, energetics, and the effects of wobbling mass during the whole-body vibration are studied in terms of analysis and comparison of two models: one is a system of four degrees-of-freedom with rigid and wobbling masses in both lower body and upper body (Model A), while the other one (Model B) is a system of three degrees-of-freedom with a rigid upper body and is otherwise identical to Model A. The main findings are the following: (1) The wobbling mass in the upper body is able to reduce the total internal load on the rigid mass of the upper body considerably. (2) "Partial" internal loads on a certain part of the body may be even larger than the total load on the same part of the body because of the phase differences among the partial loads. Therefore, a full consideration of safety during the whole-body vibration has to take not only the total, but also all the partial internal loads into account. (3) The fluctuation of power input and the fluctuation of mechanical energy could be much larger than the fluctuation of dissipation rate. (4) For frequencies higher than the resonance frequency range, the amplitude of the oscillation of the centre of mass of the body is so reduced that only the change of elastic potential energy dominates in the change of mechanical energy. Thus, a simple picture of energy flow is obtained as follows: for approximately one half of the oscillation period, the energy flows from the vibrator into the human body and is mainly stored in the muscle-tendon system, while for the remaining approximate half of the period, the energy flows from the muscle-tendon system back to the vibrator with a slightly smaller amount because a small part of the flown-in energy has been dissipated.