Physical activity in children is important, both for its direct benefits and for establishing potentially lasting future behaviors. Understanding the development of back strength in children is also important, because decreased back strength is associated with low back pain in adults. We hypothesized the following: (1) a substantial percentage of children do not participate in adequate physical activity; (2) the development of back strength corresponds to the development of strength of appendicular muscles; (3) there is a positive relationship between physical activity and back strength. The study included 53 boys and 43 girls, aged 10 to 19 yr, who had undergone isometric strength testing 4 yr previously. From responses to a questionnaire, each child's level of physical and sedentary activity was calculated. Isometric back flexion and extension were measured with the same method used 4 yr previously. Statistical analyses were performed, including quadratic regressions to estimate the rate of increase in strength, height, and weight. The following results were found: (1) during the month before testing, 21 children participated in physical activity for less than 30 min/day; (2) the level of physical activity was significantly associated with back flexion and back extension (P = 0.03 for both); (3) the peak rate of increase in back strength occurred approximately 1 yr after the peak rate of increase in height. We conclude the following: (1) measures should be taken to increase the involvement of children in athletic activities; 2) physical activity may be important in the development of back strength; (3) the pattern of back strength development seems to be the same as that for development of muscles of the appendicular skeleton.