NEW YORK (Reuters Health) – To build strong bones, boys and girls may need at least 25 minutes of vigorous exercise daily, a new study in Pediatrics shows.

Childhood and adolescence are key times for building bone mass and strength, Dr. Luis B. Sardinha of the Technical University of Lisbon in Portugal and colleagues note. While exercise is known to help people accumulate bone density in their early years, they add, the amount of activity children need for optimum bone health remains unclear. This is largely because past research has relied on children’s reports of their own activity, which are not considered reliable.

In the current study, Sardinha and his team used devices called accelerometers to objectively measure the intensity and duration of exercise in 143 girls and 150 boys who were an average of 9.7 years old. For a detailed picture of the children’s bone health, the researchers evaluated the compressive, bending, and impact strength of the femoral neck — is the upper part of the thigh bone.

Compressive and bending strength indicate a bone’s response to weight-bearing exercise, while impact strength shows a bone’s ability to withstand forces absorbed when a person is standing.

Overall, the researchers found, the boys had stronger femoral necks than did the girls. The main type of activity related to femoral neck strength was vigorous exercise, which the researchers define as brisk walking, jogging, running or jumping.

Among boys, those who got more than 26 minutes of this type of activity daily had greater compressive, bending and impact strength than boys who got 12 minutes or less each day. For girls, those who got at least 25 minutes of vigorous activity had greater bone mineral content in their femoral neck and more bending strength in the bone than those who were active for eight minutes a day or less.

“These results suggest a threshold below which the relationship between bone measures and physical activity might be weak,” Sardinha and his team say.

They conclude: “Daily vigorous physical activity for at least 25 minutes seems to improve femoral neck bone health in children.”

SOURCE: Pediatrics, September 2008.