| Summary: | Abstract Low back pain (LBP) is a significant health problem in all developed countries. The high prevalence of LBP in youth is a cause of concern because a link has been reported between LBP in adolescence and chronic LBP (CLBP) in adulthood. In order to prevent CLBP in adulthood potential protective factors in youth should be identified. The association between trunk muscle strength and LBP has been widely studied but the results are conflicting. The current knowledge on the role of postural balance in relation to LPB is even more controversial. The aims of the thesis were to evaluate 1) the reproducibility of the inclinometric postural balance and maximal isometric trunk muscle strength measurements, 2) the association of low back symptoms with postural balance, trunk muscle strength, and cross-sectional area and fat content of extensor muscles, and 3) muscular fitness in relation to physical activity and television viewing. The study population belongs to the 1986 Northern Finland Birth Cohort (NFBC 1986), originally consisting of 9,479 children with an expected date of birth between July 1, 1985 and June 30, 1986. A total of 874 subjects completed the physical examination at a mean age of 19 years. Of those who participated in the physical examination 554 subjects took part in magnetic resonance imaging (MRI) of the lumbar spine, including extensor muscles, at a mean age of 21 years. Reproducibility of isometric trunk muscle testing was found to be comparable to other methods that are used to measure trunk muscle function. Low back symptoms were not associated with postural balance or trunk muscle strength. Neither was there an association between LBP and the cross-sectional area or fat content of the lumbar muscles. Trunk muscles were significantly stronger in those who participated in regular physical activity and weaker in those who watched TV more than two hours daily. In conclusion, physical activity has an association with muscular fitness whereas association with TV viewing is negative independently ...
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