Absorbed power distribution in the whole-body system of a tractor operator.
INTRODUCTION AND OBJECTIVE
Many people are exposed to vibration (WBV) in their occupational lives. The biodynamic responses of the human body in sitting conditions have been widely measured under whole-body vibration (WBV). The measures are most often expressed in terms of force-motion relations at the driving-point, namely, mechanical impedance, apparent mass and absorbed power, and flow of vibration through the body, such as seat-to-head and body segments vibration transmissibility. The absorbed power is a measure of the energy absorbed by the subject due to the external forces applied to the system.


MATERIALS AND METHODS
The body behaves like a vibrating physical system with distributed energy-storage elements (masses, springs) and energy-dissipation elements (dampers). The total quantity of power can be divided into 2 components - one real and one imaginary. The real component reflects the energy-absorbing part of the system, due to the transformation of friction into heat within the tissues. The imaginary component reflects the energy-storing part of the system which does not consume any vibration energy.


RESULTS
The seated human is modeled as a series 4-DOF dynamic models. After introduction of the excitation, the response in particular segments of the model can be analyzed. As an example, the vibration power dissipated in an operator's body segments has been determined as a function of the agricultural combination operating speed 1.4 - 2.75 ms(-1).


CONCLUSIONS
The concept of absorbed power as a measurement for evaluation of WBV exposure opens a new area for research. The important character of absorbed power is that it has physical significance and therefore can be measured as well as computed analytically. The absorbed power relates to dissipation of energy attributed to relative motions of the visco-elastic tissues, muscles and skeletal system, which under prolonged exposures could lead to physical damage in the musculoskeletal system. A structural model of the human operator allows determination of the dynamic characteristics of the model, and study of the energy flow between the elements of the model.
Power absorbed during whole-body vertical vibration: Effects of sitting posture, backrest, and footrest
Abstract Although the discomfort or injury associated with whole-body vibration cannot be predicted directly from the power absorbed during exposure to vibration, the absorbed power may contribute to understanding of the biodynamics involved in such responses. From measurements of force and acceleration at the seat, the feet, and the backrest, the power absorbed at these three locations was calculated for subjects sitting in four postures (feet hanging, maximum thigh contact, average thigh contact, and minimum thigh contact) both with and without a rigid vertical backrest while exposed to four magnitudes (0.125, 0.25, 0.625, and 1.25 m s−2 rms) of random fore-and-aft vibration. The power absorbed by the body at the supporting seat surface when there was no backrest showed a peak around 1 Hz and another peak between 3 and 4 Hz. Supporting the back with the backrest decreased the power absorbed at the seat at low frequencies but increased the power absorbed at high frequencies. Foot support influenced both the magnitude and the frequency of the peaks in the absorbed power spectra as well as the total absorbed power. The measurements of absorbed power are consistent with backrests being beneficial during exposure to low frequency fore-and-aft vibration but detrimental with high frequency fore-and-aft vibration.
MEASUREMENT OF WHOLE-BODY VIBRATION EXPOSURE FROM GARBAGE TRUCKS
Abstract Japanese garbage truck drivers are exposed to mechanical whole-body vibration during their work. Some drivers have suffered from low back pain from this vibration. However, there is no evidence of a relationship between the whole-body vibration from the garbage trucks and low back pain or occupational disease, due to the lack of investigations. A field study was conducted in order to characterize the health risks associated with garbage truck work. Three different types of truck were tested at different loadings and on different road surfaces, with the vibrations measured at the driver/seat interface (x,y, andz-axes). The vibrations were compared with the health risk guidance according to Annex B of ISO 2631-1 [1]. The findings of this study indicated that Japanese garbage truck drivers should not operate trucks for 2.5 h in a day, under current working conditions.