When the most important muscles stabilising back are mainly passive during driving, the importance of passive stabilising structures and also the significance of other stabilisers of back such as abdominal muscles must be emphasised. The disturbed function of abdominal muscles can be associated with low back pain and may expose to injury [35,39]. Since the stabilising muscles are passive during driving, the lumbar spine is supported poorly and exposed to the effect of whole body vibration and injuries caused by sudden loads [9,28].
Ergonomic low back support maintains back in physiologic posture when the pressure is concentrated on discs regularly. Disc pressure and myoelectric activity has been observed to decrease when the backrest inclination was increased and the low back support was used [25]. Thus, ergonomic lumbar support may prevent LBP. This is sup- ported by the subjective beneficial effect.
Overload of spine exposes to microinjuries such as internal annular ruptures, which can lead even to total annular rupture and disc herniation. Internal annular ruptures cause pain, which can be diagnosed by discographic pain provo- cation. This pain can be provoked non-invasively by bony vibration test [32,34,40,41]. According to this study there was evidence for internal disc disruption being one potential cause of low back pain in bus drivers. The prevalence of internal disc disruption in bus drivers suffering from recurrent low back pain was approximately the same as find earlier by disco- graphic pain provocation [42]. Low back and neck–shoulder pain and fatigue increased more during driving in drivers who had positive vibration pain provocation test than those who had negative indicating that driving may expose for pain in those cases.
The “hypersensitivity” to bony vibration must also be taken into consideration. The partial disagreement between MRI and pain provocation findings could be explained by the facts that all internal disc disruption are not visible in MRI and that all disc disruptions are not painful and the total annular rupture is not necessarily painful by the vibration stimulation test.
Trapezius muscle loading during driving seems to be remarkably higher than paraspinal muscle loading in low back. It can be associated with neck–shoulder fatigue, which increased significantly in both groups and perhaps also with pain since as low as less than 1–2% MVC is recommended in sedentary work [43]. Muscle blood flow is related to neck–shoulder pain [44] and maybe a potential link between pain and muscle loading as well as muscle spindle excitability. However, the side difference in trapezius muscle was not associated with the location of pain in pain maps and the neck–shoulder pain did not increase significantly during driving. The increased left trapezius activity is probably due to that drivers mainly use their left hand in steering the bus and also by the EKG interference seen in some cases despite the filtering.
In conclusion the paraspinal muscle loading in urban bus drivers was very small and neither low back pain nor ergonomic low back support had an effect on it, however, trapezius muscle seems to be less active in drivers suffering from recurrent low back pain. Internal disc disruptions may expose to pain and fatigability during driving.