In one school in the city of Malmo where children had 40 minutes per day of moderate physical activity, the risk ratio for fracture after 8 years was 0.48 (95% CI 0.25-0.91) compared with children in three other schools who had the national standard of 60 minutes of physical activity per week, reported Marcus Coster, PhD, of Lund University.
“About one-third of all children sustain a fracture before cessation of growth, and therefore preventive strategies to reduce fracture risk would have implications both for society and individuals,” he said at the American Society for Bone and Mineral Research meeting.
Short-term exercise intervention studies in children have shown that physical activity is associated with greater bone mass and improved neuromuscular function, but some studies have suggested that more vigorous activity is associated with a greater frequency of fractures.
“So we asked the question whether increased school exercise during the first 8 years of elementary school influence fracture risk as well as bone mass and muscle strength,” he said.
The children were enrolled in the Pediatric Osteoporosis Prevention study at ages 6 to 8 years. In the intervention school, 1,339 children participated, while in the three control schools, 2,195 children were included.
The physical activity program consisted of normal activities such as playing ball, running, and jumping, with the intervention group having a total of 200 minutes of physical activity per week. There was no specific strengthening program.
Fractures were identified through the region’s radiographic database, which includes all reported x-rays of fractures.
During the first few years there was an increase in fracture risk among children in the intervention group. “This was quite interesting but not that unexpected, since the increased exposure to physical activity would possibly expose the children to more trauma and therefore more injuries, and any protective or preventive result from the physical activity would not have had time to develop,” Coster said.
In the control group, the incidence of fractures was greatest in years 7 and 8, when the children were entering puberty, while this peak incidence was absent in the intervention group. In years 7, the incidence among control children was 37.1 per 1,000 person-years compared with 19.4 per 1,000 in the intervention group, while in year 8 the rates were 45.5 versus 21.7 per 1,000, respectively.
“This might suggest that our intervention program has prevented the peak incidence or at least has postponed it,” he said.
In 234 of the children, spine bone mineral density was measured with dual emission x-ray absorptiometry (DXA) and peak torque in knee extension and flexion using a dynamometer.
Compared with controls, the intervention children had a greater gain in bone mass, with an increase in bone mineral density (BMD) at the spine of 0.03 g/cm2 (95% CI 0-0.06, P=0.004). Changes in bone at other sites such as the femoral neck were not significant.
They also showed greater muscle strength, with peak torque of knee extension of 6.5 Nm (95% CI 0.4-12.6) and flexion of 7.7 Nm (95% CI 3.5-11.9, P=0.003).
“An 8-year intervention program that begins before puberty led to a highly clinically and statistically significant fracture risk reduction, most likely through physical activity-induced benefits in bone mineral density and muscle function,” he said.
In response to a question from the audience, Coster explained that the majority of the fractures were of the wrist or ulna, but they did not have BMD measurements for the distal radius.
He also noted that the children in the intervention group increased their physical activity outside of the school day, suggesting that the intervention led to changes in lifestyle.