The effects of low frequency z-axis whole-body vibration on performance of a complex manual control task
This thesis investigates continous manual control performance during exposure to z-axis whole-body vibration at frequencies between 0.5 and 10.0 Hz. The task involved first-order pursuit tracking with a simultaneous discrete target acquisition task. A major aim of the work was to determine the mechanisms underlying any vibration-induced impairment which occurred. The iterature is first reviewed (Chapter 2) and a model is presented summarising the mechanisms by which vibration has been suggested to disrupt performance (Chapter 3). Six experiments are then reported. Experiment 1 (Chapter 5) measured vibration-induced activity at the head, hand and the output of the system dynamics. The results are discussed with reference to the mechanisms which could disrupt performance. Experiment 2 (Chapter 6) investigated performance during exposure to vibration at frequencies from 0.5 to 5.0 Hz. The magnitude of performance disruption was approximately constant at vibration frequencies below 2 Hz, and increased with the frequency of vibration to 5.0 Hz. Experiments 3 (Chapter 7) and 4 (Chapter 8) showed that the disruption at frequencies above 2.0 Hz could be attributed to visual impairment arising from relative translational movement between subjects' eyes and the display: collimating the display removed the impairment. Linear spectral analysis techniques were used to separate root-mean-square (rms) tracking error into components linearly and not linearly correlated with movements of the target. Changes in total rms error were mainly accompanied by changes in the linear components: closed-loop system transfer functions showed increased phase lags between movements of the target and the response of the controlled element. In experiment 5 (Chapter 9), three simple tasks were used to isolate non-visual mechanisms of disruption. The results suggested that whole-body vibration at 0.5 and 4.0 Hz could interfere with neuro-muscular processes. The results of experiment 5, and the increased phase lag observed in experiment 4, indicate changes in the way the task was performed during vibration: these are described as secondary vibration effects. Experiment 6 investigated whether the effect of vibration on the system studied would be time-dependent. One-octave-band random vibration centred on 4 Hz was presented at a magnitude considerably above the ISO 2631 (1985) `fatigue-decreased-proficiency' limit for 180 minute vibration exposures. Performance declined with time, but vibration did not alter the time-dependence. The effect of duration was reduced when the task was performed over the entire duration on a second occasion. It is concluded that impairments in continuous tracking performance during whole-body vibration exposure were mainly caused by interference with visual and neuro-muscular processes. The results also show secondary effects which may represent adaptive change in performance during vibration. The behavioural model developed in Chapter 3 is used to summarise the mechanisms which were shown to be important, and to indicate other effects which could occur. Some suggstions for further research are offered.