Validation of Intra-Subject Variation in Biodynamic Responses of Seated Human Exposed to Whole-Body Vibration

Abstract

Many studies have been conducted to investigate the change in human response under various experimental conditions. Usually, these experiments were conducted using many subjects and the inter-subject variation was evaluated. However, the intra-subject variation in human response is also necessary for understanding the change in an individual's physical response to whole-body vibration (WBV). The aim of this study is to investigate the intra-subject variation in biodynamic responses (both apparent mass and seat-to-head transmissibility) of a seated human exposed to vertical whole-body vibration over time. In the experiments, nine male subjects were exposed to vertical random vibration (0.2-0.3 m/s 2 in r.m.s.) in the 0-30Hz frequency range. The measurement variation was also evaluated, wherein the measurements were repeated five times without any change to form the "baseline" for each subject, and the intra-subject variations were evaluated by comparing their responses with these "baseline." The intra-subject variation was examined from two different viewpoints: variation "within a day" and that "over several days." To determine the intra-subject variation "within a day", the five measurements were obtained at two-hour intervals on the same day. In the intra-subject variation "over several days", the five measurements were obtained again, but at the same time of the day on five consecutive days. The results show that the intra-subject variations (both "within a day" and "over several days") in biodynamic responses are larger than the "baseline." However, when the variation "within a day" in biodynamic responses is compared to that "over several days," no common trend is observed among subjects. Although the magnitude of intra-subject variation in biodynamic responses depends on each subject, both variations "within a day" and that "over several days" have a similar range of variation.

Change of Seat-to-Head Transmissibilities at Two-Hour Intervals with and without Whole-Body Vibration

“…The acceleration in the z-direction (vertical) was measured on the rigid seat surface using an accelerometer (Type 356A32, PCB Piezotronics, USA). The vibration of the heads of the test subjects was measured by the head-gear, which can measure tri-axial translations and tri-axial rotations of head motion with respect to the center of the head (11) . Three tri-axial accelerometers were attached to the head-gear; one on the forehead (B) and one above each ear (A and C), as shown in Figure 2…”

“…To examine the change of T_rms for "with vibration" condition, it is necessary to compare them with that for "without vibration" condition. In a preliminary study, we investigated the variation "within a day" in the seat-to-head transmissibility of a seated human (11) . To determine the intra-subject variation "within a day" in seat-to-head transmissibility, nine subjects participated in the experiment and five measurements were obtained at two-hour intervals for "without vibration" for each subject.…”

“…However, to understand the change in human response of a seated human exposed to WBV, a fundamental study is required to ascertain the human dynamic property changes under "with vibration" and "without vibration" from a physical reaction viewpoint. A preliminary study investigated the change of seat-to-head transmissibility for periods of two-hour, in which the subjects were not exposed to whole-body vibration (11) . It investigated the intra-subject variation in biodynamic responses of a seated human.…”

“…Transmissibilities for subject_A (b) Transmissibilities for subject_EFig. 3Head transmissibilities obtained for "before" and "after" at two-hour intervals with vertical vibration for two subjects (2-20Hz) T_rms in 10-20 HzFig.4T_rms evaluated in three frequency ranges; 2-20 Hz (a), 2-10 Hz (b) and[10][11][12][13][14][15][16][17][18][19][20] …”

Variation of Floor-to-Head Transmissibility in Seated Human during Prolonged Driving Simulation

“…Transmissibility in published reports has focused on changes in human response under various experimental conditions involving seated posture, excitation frequency and magnitude, and type of excitation (7)(8)(9)(10)(11)(12)(13)(14)(15) . These experiments have investigated human response when subjects were exposed to WBV for a short time.…”