Car driving with and without a movable back support: Effect on transmission of vibration through the trunk and on its consequences for muscle activation and spinal shrinkage Idsart Kingma* and Jaap H. van Diee¨n Research Institute MOVE, Faculty of Human Movement Sciences, VU University Amsterdam, Netherlands The aim of this study was to test the effect of a movable backrest on vibration transmission through the trunk during driving and on the physiological consequences thereof. Eleven healthy male subjects drove for about 1 h on normal roads with a movable and with a fixed backrest while surface electromyography (EMG) was measured at the level of the fifth lumbar vertebra (L5) and vertical accelerations were measured at the seat, backrest and at the spine at the levels of the second sacral vertebra (S2) and seventh cervical vertebra (C7). The movable backrest significantly reduced accelerations at C7 by up to 11.9% at the 5 Hz frequency band. The movable backrest also significantly reduced the coherence and transmission between S2 and C7 accelerations, but not the differential motion between these sensors. EMG at both sides of L5 was on average 28% lower when using the movable backrest. Spinal shrinkage was unaffected by backrest type. It is concluded that a movable backrest reduces the transmission of vibration through the trunk and that it reduces low back EMG. Car driving is associated with the risk of developing low back pain and this may be related to exposure to whole body vibration. This study found an effect of a simple ergonomics measure on the transmission of vibration through the trunk as well as on back muscle activation. Keywords: whole body vibration; driving; transmission; EMG; spinal shrinkage Introduction Whole body vibration induced by car driving was found to be related to the development of low back pain over three decades ago (Kelsey and Hardy 1975). Despite technical advances resulting in better comfort during driving, more recent surveys still find a relationship between driving and low back pain, even when truck drivers are excluded. For instance, in a study by Porter and Gyi (2002), frequent car drivers are reported to have been more days absent from work due to low back pain than infrequent drivers. Also, the percentage of subjects reporting to have had over 8 d of back pain in the last year was higher in subjects driving for their job 420 h/week than in subjects standing 420 h/week and in subjects sitting without driving for 420 h/week. There was no difference between frequent drivers and subjects frequently lifting in their job (Porter and Gyi 2002). In a recent review, Gallais and Griffin (2006) found, over three decades, nine studies specifically reporting on the relationship between car driving and low back pain. Most of these studies reported a positive relationship between car driving and low back pain. However, factors other than vibration, such as static posture during driving, were not controlled for (Gallais and Griffin 2006). In a review on the relationship between exposure to vibration and low back pain, Bovenzi and Hulshof (1999) did show a dose–response relationship between occupational exposure to vibration and low back pain. However, the mechanism causing the relationship between vibration and low back pain is not yet disentangled. Possible mechanisms are vibration- induced enhancement of muscle fatigue (Hansson et al. 1991) due to increased back muscle activation (Seroussi and Pope 1987, Blu¨thner et al. 2002), soft tissue damage due to creep induced by cyclic deformation (Solomonow et al. 1999), malnutrition of the intervertebral disc due to hampered fluid flow (Holm and Nachemson 1983) or vibration-induced enhancement of disc height reduction due to fluid loss (Pope et al. 1998). Several attempts have been made to introduce measures that might reduce the vibration induced to the spine during driving. For instance, Makhsous et al. (2005) showed that vibrations transmitted to the trunk can be reduced by using a seat with enhanced lumbar back support and reduced pressure on the tuberositas ischii. Lewis and Griffin (1996) investigated the effect of using a movable backrest. These authors stressed the importance of differential motions between the upper and lower parts of the trunk. Such motions cause cyclic strain of spinal tissues. *Corresponding author. Email: i_kingma@fbw.vu.nl Ergonomics Vol. 52, No. 7, July 2009, 830–839 ISSN 0014-0139 print/ISSN 1366-5847 online Ó 2009 Taylor & Francis DOI: 10.1080/00140130802559019 http://www.informaworld.com