Sedentary behaviour increases in children between the ages of nine and 12 - even if they are otherwise physically active, according to research at the Universities of Strathclyde and Newcastle.
The study of around 500 children at the age of nine, and more than 400 of the same children three years later, found that they had all become more sedentary by the age of 12.
Girls were found to have a larger increase than boys in their time spent sedentary - defined as sitting or lying while using low amounts of energy. They also had larger decreases in 'fragmentation' - the frequency of breaks in sedentary behaviour.
Dr Xanne Janssen, a Lecturer in Strathclyde's School of Psychological Sciences & Health, led the research. She said: "There has been a lot of research focused on physical activity but there's been a lot less on sedentary behaviour, which we looked at in this study.
"There were smaller rises in sedentary behaviour among children who went to sports clubs, while the rises were larger among older children and during winter."
"While some of the factors, such as season and gender, aren't modifiable in themselves, interventions could help deal with sedentary behaviour. This could include providing children with more opportunities to attend sports clubs, particularly in winter, or encouraging them to break up their sedentary behaviour more often - this doesn't have to be a big increase in activity but can simply be walking or standing up and moving around.
"Further research, including exploration of potential factors in early and teenage years, will be needed to gain a more in-depth understanding of approaches which could lead to changes in sedentary behaviour among children and young people."
Children participating in the research wore belts recording their activity for a week and kept diaries to provide further detail.
The research has been published in the journal Preventive Medicine Reports. It forms part of the Gateshead Millennium Study, a continuing project between Strathclyde and Newcastle examining the health and physical activity of young people born between June 1999 and May 2000.
The study was supported by grants from the Scottish Government Chief Scientist Office, the UK National Prevention Research Initiative and Gateshead Health NHS Foundation Trust.