The study, published in the British Journal of Sports Medicine, examined bone and muscle health in 295 school children from Frederikssund and Copenhagen over a whole school year.
The children participated in the 'FIT FIRST' training concept, which looks into the effects of various types of intense interval training at school.
Researchers compared the effects on children who took the normal school physical education (PE) classes with children who had intense exercise on the timetable for two hours a week in the form of ball games on small pitches or 'circuit training' consisting of gymnastic and strength exercises using their own body weight.
In the children in third grade who played ball games three against three or participated in circuit training for three x 40 minutes a week, muscular strength increased by 10 per cent and balance improved by 15 per cent.
More From This Section
The children's bone density increased by a whole 45 per cent compared to the control group.
"These types of sports are great ways for children to "put bone in the bank," said Krustrup, the leader of the project.
"Exercise in school for children aged 8-10 which improves bone density, muscular strength and balance is the first big step towards preventing osteoporosis later in life," he said.