The hypothesis of an inverse association between regular walking or bicycling and LBP was partially supported, as the active journey to activities predicted less LBP in all regression models. However, the small difference of 1.6 km weekly walked or bicycled to activities seems too small for possible biological effects. Short active school journeys were associated with LBP in bivariate analyses, but the difference of 1.3 km weekly is also far too small for producing health effects. The small differences may imply that the results are statistical coincidences, and the results need to be confirmed in larger samples and prospective studies.
Another explanation of the findings, however, may be that the active journey to activities reflects other regular and irregular active journeys in daily life, performed possibly during the weekend, and does so better than the active school journey. This explanation is validated by findings in our previous study, in which physical performance was more strongly associated with active journey to activities than with active journey to school [19
]. One hypothesis, derived from Rose [17
] and Volinn [29
], is that the distances of the active journeys may be far too short to show considerable effects on the outcome, as walking or bicycling as much as 8 km or more to activities (upper quartile) only reduced the risk of LBP by 40% according to Table (0.948
=0.6). The alternative regression analysis, dichotomising the active transport at the same level, reduced the risk to 0.3. The box plot suggests a similar trend.
The results support Belgian findings, in cross-sectional and prospective studies, of less LBP in children walking to school compared to those brought to school by car or public transport [8
]. However, they contrast with a French study, in which long walking distance to school was associated with LBP; the latter association was only valid, however, for pupils carrying school bags exceeding 20% of their weight [28
]. Walking has also been described as a pain-relieving activity among adults suffering from LBP [3
]. Our results confirm the findings of positive associations between distance of active school journey, leg muscle strength and aerobic capacity in a Norwegian survey [21
]. The short distances covered by the active transport indicate that even short but frequent activity may be beneficial for low back health, and the results thus support the advice of 30 min daily walking given by American health authorities [24
]. Although walking and bicycling are quite different with regard to body position, both activities may represent a healthier activity for the low back than competitive sports and extremes of physical activity. A U-shaped association between LBP and physical activity was indicated in a Finnish survey [27
The hypothesis of an association between use of school bus and LBP was not supported, but sitting in the school bus was provocative for LBP in 17% of the rural adolescents. The lack of association may be due to the low level of exposure, the small study sample and the comfort of modern buses. Although vibration was not measured, sitting in the school bus probably involves an exposure to WBV of around 0.40m/s2
, according to the Swedish Institute of Work Environment (Wikström 2001, personal communication); the limit for a corresponding caution zone is approximately 10 h daily according to ISO [11
]. Even if that value may be lower for the softer tissues of children and adolescents, the exposure time of 50-min bus journey among the rural adolescents is still far below the caution zone. In addition, WBV may do less harm to children, because they probably do not sit in fixed positions in the bus. The results support conclusions reported in a review on risk factors for LBP, showing that studies now place less emphasis on conventional risk factors like WBV for disc degeneration and LBP [26
]. Another review, however, attaches great importance to vibration as a factor for LBP among drivers of trucks, tractors, etc [4
]. An alternative hypothesis, derived from Rose [17
], is that the WBV factor may be diluted by a high level of car use from childhood for both rural and urban groups of adolescents.
The small size of the study, the lack of a representative sample and the small differences indicate the need for careful interpretation. Due to the small size, there is also a danger that existing associations may not be uncovered. The similarities with other studies indicate that this study may bring some general information: the 1-year cumulative prevalence was similar to the 51% figure reported in a Danish survey [10
] and somewhat lower than French data of 83% [28
], and the associations found between the control variables and LBP correspond to other studies [2
]. By using ordinal logistic regression analyses, more information concerning LBP was utilised, and some more covariates could be included in the analyses. The small size of the schools and the careful collection of individual data means that the information given from the headmasters is likely to have been reliable. The cross-sectional design does not permit causal explanations, but school journeys in primary school were probably performed before the onset of LBP for many of the pupils, as LBP usually increases with age [2
Heavy backpacks may have been a confounder [15
], but no pupils mentioned that factor in answering the open questions concerning LBP-provoking activities. Smoking was not measured, and may have been another confounder [6
]. The headmasters reported that smoking was hardly ever observed among the pupils. Walking and bicycling to school were registered together, because the adolescents reported that it was impossible for them to discriminate. The pupils were not allowed to bicycle to school for the first 4 years in primary school in either area.
Use of cars and buses may impact on low back pain in three ways: exposure to sitting, to WBV, and reduction of active transport. Our results indicate that transport by school bus for 1 h daily may not be hazardous for low back health. The results also indicate that low levels of active transport may be associated with juvenile LBP, and raise the question of whether the reduction in active transport may be one cause of the increase in juvenile LBP. The reasons for not walking/bicycling to school may be due to lack of pedestrian routes, long distances, attitudes among parents and pupils, and/or the family organisation of daily life. The results from this and other studies indicate that pedestrian routes should be established in residential areas, and that parents should be informed that by using the car to take their children to school and to other activities, door to door, they deprive them of an easy way of taking physical exercise, and that this is probably associated with low back health. Further research is needed to investigate whether the findings of this study are reproducible in other and larger settings, and in other age groups. Future research might investigate associations between longer active and passive journeys, LBP and degenerative changes.