The transmission of translational seat vibration to the head: The effect of measurement position at the head

Head motion has been measured in six axes on twelve subjects exposed to vertical seat vibration in the frequency range 0.5-25 Hz. The subjects sat on a rigid flat seat in two postures: back-off (no backrest) and back-on (subject's back in contact with the seat backrest). Translational acceleration has been calculated for various locations on the head and transmissibilities between vertical seat vibration and translational head motion determined for each location and all axes. The translational motion of the head was most affected by pitch motion of the head. This caused variations in fore-and-aft motion with position along the vertical axis of the head and variations in vertical motion with position along the fore-and-aft axis of the head. These variations are illustrated for each subject in both postures. The individual data allow the identification of various modes of vibration and show that seat-to-head transmissibility is greatly affected by pitch modes of the head and neck. The magnitude of motion occurring in some modes is dependent on body posture.</p

The transmission of translational floor vibration to the heads of standing subjects

The head motions of standing subjects have been measured while they were exposed to floor vibration occurring in each of the three translational axes: fore-and-aft, lateral and vertical. While exposed to fore-and-aft floor vibration, the 12 male subjects were instructed to stand in two postures: holding a handrail in front of them lightly; and holding the handrail rigidly. During exposure to lateral floor vibration subjects stood in three postures: feet together, feet 30 cm apart and feet 60 cm apart. The postures investigated during exposure to vertical floor vibration were: straight legs (i.e., locked), legs very slightly bent (i.e., unlocked) and legs bent. Variability within and between subjects (i.e., intra- and inter-subject variability) was investigated for all axes of excitation and all postures. Transmissibilities between the floor and the head were calculated for all conditions. During exposure to fore-and-aft floor vibration, the head motion occurred mostly in the mid-sagittal plane; a rigid grip on the handrail resulted in higher transmissibilities than a light grip.During exposure to lateral floor vibration, the head motion occurred mainly below 3 Hz and in the lateral axis; the 60 cm foot separation resulted in more head motion below 3 Hz than the other postures. During exposure to vertical floor vibration, head motion occurred principally in the mid-sagittal plane. For frequencies below about 5 Hz, a legs bent posture resulted in the highest transmissibilities, while a legs locked posture showed the lowest motion; this order was reversedat higher frequencies. Differences in transmissibility as large as 20:1 occurred between subjects for some conditions.</p

A review of the transmission of translational seat vibration to the head

Published studies of the transmission of translational seat vibration to the heads of seated subjects are reviewed in order to determine the variations in transmissibility. The review is restricted to vibration having the same direction at the seat and the head: (a) transmission between fore-and-aft seat vibration and fore-and-aft head motion, (b) transmission between lateral seat vibration and lateral head motion, and (c) transmission between vertical seat vibration and vertical head motion. Only studies reporting transmissibilities from six or more subjects are included in the review. Transmissibilities are shown for all studies included: 10 studies with fore-and-aft seat vibration, 14 studies with lateral seat vibration and 46 studies with vertical seat vibration. The studies involved a wide range of experimental conditions, including different sitting postures, various types of subject and various locations for measuring vibration on the head. The conditions of each experiment are tabulated. The transmissibility data have been used to calculate the median, interquartile range and range of transmissibility for each axis of vibration.</p

Transmission of roll and pitch seat vibration to the head

A series of experiments has investigated the transmission of roll and pitch seat vibration to the heads of seated subjects. Head motion was measured in all six axes using a light-weight bite-bar while seated subjects were exposed to random motion at frequencies of up to 5 Hz at 1.0 rad.s −2 r.m.s. Subjects sat on a rigid flat seat in two body postures: ‘back-on’ (back in contact with backrest) and ‘back-off’ (no backrest contact). The influence of the position of the centre of rotation was also investigated. Motion at the head occurred mostly in the lateral, roll and yaw axes during exposure to roll seat vibration and in the fore-and-aft, vertical and pitch axes during exposure to pitch seat vibration. A reduction in the magnitude of head motion occurred when the subjects sat in a 'back-off' posture compared with a 'back-on' posture. Varying the position of the centre of rotation along the lateral axis during roll seat vibration affected vertical and pitch head motion: least head motion occurred when the centre of rotation was in line with the subject's mid-sagittal plane. Varying the position of the centre of rotation along the vertical axis during roll seat vibration affected head motion in the mid-coronal plane: roll head motion decreased as the position of the centre of rotation was raised from below the seat surface to above the seat surface. Varying the centre of rotation (along the fore-and-aft and vertical axes) during pitch seat vibration altered head motion in the mid-sagittal plane. Head motion increased with increasing distance of the centre of rotation in front or behind the subject's ischial tuberosities and increased as the seat was raised from below the centre of rotation to above the centre of rotation.</p

The influence of seat backrest angle on human performance during whole-body vibration

This article was published in the journal, Ergonomics [© Taylor & Francis] and the definitive version is available at: http://dx.doi.org/10.1080/00140139.2011.634030This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2–8 Hz; 8–14 Hz and 14–20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms−2 root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue