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.
Absorbed power as a measure of whole body vehicular vibration exposure
In this thesis, absorbed power during exposure to vertical whole-body vibration is considered as a potential indicator of the physical stress affecting human health. The amount of vibration energy, either absorbed or exchanged between the source and body, may be a good measure of the physical stress on the body since it takes into consideration the interaction between the vibrating structure and the body as well as the relative motion between the vibrating body parts. Moreover, energy is a scalar quantity that makes it easy to add up contributions from all directions and all body segments to a single value. In order to study the energy content of the vibration transmitted to the whole body and to the different body segments, a four-degree-of-freedom linear biodynamic model used in earlier studies is chosen to represent the body. The local absorbed powers and the total power absorption under different excitations ( e.g. sinusoidal excitation, random excitation, and transient excitation) are derived. On the basis of the model and of the guidance provided in ISO 2631-1 (1997) to relate vibration exposure with the risks of health effects, health guidance caution zones are established based on absorbed power. The eigenvalues and natural frequencies, damping ratios and damping ratio matrix, the vertical transmissibilities from seat to different body segments, and driving-point mechanical impedance for the 4-DOF biodynamic model are also computed.
Fatigue effects on the coordinative pattern during cycling: kinetics and kinematics evaluation.
The aim of the present study was to analyze the net joint moment distribution, joint forces and kinematics during cycling to exhaustion. Right pedal forces and lower limb kinematics of ten cyclists were measured throughout a fatigue cycling test at 100% of PO(MAX). The absolute net joint moments, resultant force and kinematics were calculated for the hip, knee and ankle joint through inverse dynamics. The contribution of each joint to the total net joint moments was computed. Decreased pedaling cadence was observed followed by a decreased ankle moment contribution to the total joint moments in the end of the test. The total absolute joint moment, and the hip and knee moments has also increased with fatigue. Resultant force was increased, while kinematics has changed in the end of the test for hip, knee and ankle joints. Reduced ankle contribution to the total absolute joint moment combined with higher ankle force and changes in kinematics has indicated a different mechanical function for this joint. Kinetics and kinematics changes observed at hip and knee joint was expected due to their function as power sources. Kinematics changes would be explained as an attempt to overcome decreased contractile properties of muscles during fatigue.