Children's hearts are beating faster than ever because of a lack of exercise and too much time in front of the TV.
The news raises fears that today's kids will be at greater risk of heart disease when they grow up.
Since the 1980s, the average resting heart rate of children aged 9-11 has increased by two beats per minute (bpm) for boys and one bpm for girls, with the fastest increase coming after the mid-1990s.
Resting pulse rate is used as a marker for general fitness and heart health and the increases, although small, could indicate a greater likelihood of problems like heart disease and diabetes in future.
The researchers from University College London found that rising body weight and obesity among children could only account for part of the increase, suggesting that declining levels of physical activity could be making children less fit.
"Although modest, it is important to monitor these trends, especially in boys, as they appear to have increased at a faster rate in recent years," they wrote.
Dr Leah Li, who led the Medical Research Council-funded study, said: "The obvious [possible causes] are things like television and computer games – these things have increased dramatically over the past 30 years."
Christopher Allen, Senior Cardiac Nurse at the British Heart Foundation, added: "This study highlights a worrying increase in children's resting heart rates, which could raise their risk of cardiovascular disease later in life.
"A high resting heart rate can mean a low level of fitness, whether a child is a normal weight or overweight."
The findings were based on measurements taken in five studies and surveys conducted between 1980 and 2008, which included a total of 23,000 British children aged nine to 11.