Inactive mothers may be preventing their children getting enough exercise by setting a bad example, a study suggests.
Scientists who measured the physical activity levels of mothers and their four-year-old children found a direct association between the two.
"The more activity a mother did, the more active was her child," said lead researcher Kathryn Hesketh, from University College London.
"Although it is not possible to tell from this study whether active children were making their mothers run around after them, it is likely that activity in one of the pair influences activity in the other.
"For every minute of moderate-to-vigorous activity a mother engaged in, her child was more likely to engage in 10% more of the same level of activity.
"If a mother was one hour less sedentary per day, her child may have spent 10 minutes less sedentary per day.
"Such small minute-by-minute differences may therefore represent a non-trivial amount of activity over the course of a week, month and year."
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The study included 554 mothers, many of whom worked and had children who attended day-care facilities.
Both mothers and children were fitted with combined movement and heart rate monitors that recorded their activity levels for up to a week.
"This approach allowed us to capture accurately both mothers' and children's physical activity levels for the whole of the measurement period, matching hour for hour maternal-child activity levels," said co-author Dr Esher van Sluijs, from the Centre for Diet and Activity Research at Cambridge University.
"This comparison provided us with detailed information about how the association between mothers and children's activity changed throughout the day, and how factors such as childcare attendance and maternal education influenced this relationship."
Just 53% of the mothers taking part in the study engaged in 30 minutes of moderate-to-vigorous activity at least once a week, the results published in the journal Pediatrics showed.
Government health guidelines recommend 150 minutes of "moderate intensity physical activity" such as brisk walking over the course of a week.
New parents are known to be less active than their childless peers. Once women become mothers their activity levels frequently fail to return to pre-parenthood levels.
"There are many competing priorities for new parents and making time to be active may not always be top of the list," said Ms Hesketh.
"However, small increases in maternal activity levels may lead to benefits for mothers and children. And if activity in mothers and children can be encouraged or incorporated into daily activities, so that more time is spent moving, activity levels are likely to increase in both.
"In return, this is likely to have long-term health benefits for both."