Impedance response characteristics of the primate Mucaca mulatta exposed to seated whole-body gz vibration.

A mathematical model was used to quantify and describe the variability in the mechanical impedance response of the Rhesus monkey subjected to vibrations in the range 3-20 Hz at 0.5 g peak acceleration. Due to the similarities in response, a two-mass, one-degree-of-freedom (DOF) model was selected and the associated mechanical parameters determined using a nonlinear least-squares optimization program. For the six tests conducted on each of the four subjects, appreciable parameter variations were observed within a subject; however, the majority of the mean parameter values among different subjects and among the repeated tests on the population were within +/- 1 S.D. of each other. Significant differences were observed in the stiffness coefficient and the total mass among different subjects, and in the mass ratio (between inert and sprung masses) among the repeated tests. Variations in the profile shapes following resonance were described and limited by changes in the mass ratio and the damping factor. Higher mass ratios (greater than 1.0) were associated with lower damping factors (less than 0.50). The impedance response beyond resonance approached the response described by the impedance of the inert mass and the damper elements of the model combined in parallel, and supported the assumption that the lower torso was rigidly attached to the seat. Physically, the reactive force produced by the upper torso increasingly diminished following resonance, due to the load transmission/attenuation characteristics of the spinal structures at 0.5 g peak acceleration. The impedance measured at the seat becomes dominated by the transmitted damping force associated with the spine and the force generated by the rigid lower-torso mass.