From a population of 1503 school children, 38 15-year-old children reporting recurrent or continuous low-back pain (LBP) and 38 asymptomatic controls (34 boys and 42 girls) matched for age, sex and school class, were compared with respect to spinal mobility and trunk muscle strength. The subjects were interviewed on leisure time physical activity preceding the measurements. Of the subjects with recurrent or continuous LBP and of those completely without LBP experience 73.7% vs 42.1% reported that they pursued some physical activity two days a week or less (P = 0.006). Further, decreased spinal mobility, diminished endurance strength of the back muscles and diminished dynamic strength of the abdominal muscles were found in the group with a low frequency of activity. Mobility in children pursuing physical activity two days a week or less was decreased in lumbar extension measured by flexicurve, in forward bending, in side bending and in straight leg raising. Our results indicate that a very low frequency of physical activity in the young might be the risk factor for LBP. Pursuing sports as a leisure time activity is not harmful; on the contrary it has positive effects on spinal mobility and trunk muscle strength.