Boys are less fit than they were 20 years ago - regardless of their weight, reveals new research.
Even the slim ones are not in as physically good shape as their dads were at the same age, according to the study.
The findings add to growing evidence that today's kids are the first generation since World War Two to be less fit than their parents.
The shift from active play outdoors to indoor screen based activities is behind the alarming trend.
Spanish researchers tested the aerobic aptitude of normal and obese 11 year old boys from Malaga in 1996 and again in 2016.
They said their findings, presented at the European Congress on Obesity in Austria, highlights the need for a focus on child fitness rather than just weight.
Dr Jose Garcia, of the Faculty of Science Education, Malaga University, and colleagues called for more initiatives to increase fitness levels in children.
He said: "Our results suggest measuring BMI (body mass index) alone may not be enough to monitor children's future health and reinforce the need for promoting physical activity, especially aerobic fitness, to improve the capacity of the heart and lungs and better post exercise recovery."
The number of children in the UK doing an hour of exercise a day falls by nearly 40% between the ages of five and 12.
By the final year of primary school, just 17 per cent of pupils are doing the recommended 60 minutes of physical activity every day.
Public Health England has described the drop in activity levels as "concerning".
Earlier this year research featured on a BBC TV programme found the least fit British children 20 years ago would be considered among the fittest children today.
The Inside Out report about childhood fitness initiative 'Super Movers' said the least fit child in a class of 30 in 1998 would be among the five fittest in 2018.
Dr Gavin Sandercock, a sports scientist at the University of Essex, said: "If we could time travel to hold a one-mile race so today's parents and their children were both 10 years old, mums and dads would win it by about 90 seconds.
"About a third of children have clinically low aerobic fitness."
In the latest study 132 healthy weight and 72 obese boys in 1996 were compared with 213 normal weight and 139 obese counterparts in 2016 to measure changes in aerobic fitness over time.
The boys wore a heart rate monitor during a 'shuttle' test requiring them to run 20 metres between two points until they could no longer do it before a bleep sounded.
Pulse beats per minute (BMP) were recorded at the end and every minute during recovery.
Results showed that healthy weight boys in 2016 were markedly less fit than their predecessors in 1996 - running an average of 5.1 shuttles in 1996 and 4.8 in 2016.
In contrast, there were no significant differences in obese boys over the period - 4.2 versus 4.1.
But, importantly, both normal and obese boys showed much lower cardiac efficacy and worse heart rate recovery at the end of the test and throughout recovery in 2016 compared to 1996.
For example, in 2016 normal boys' average heart rate at the end of the test fell from 181 bpm to 147 bpm after 1 minute to 136 bmp after 2 minutes;.
In 1996 average heart rate was 198 bmp at the end of the beep test, and fell to 155 bmp after 1 minute and to 133 bmp after 2 minutes.
Dr Garcia added: "We know most children do not take part in enough physical activity, compared to current WHO (World Health Organisation) recommendations of at least 60 minutes of moderate-to-vigorous exercise every day such as swimming, football, or dancing."
According to the NHS just 23 percent of boys and 20 percent of girls aged five to 15 meet the national recommended level of activity.
The less efficient the heart, the more it has to beat per minute to pump blood around the body.
Heart rate recovery, the speed at which the heart rate returns to normal after exercise, is an indicator of physical fitness and the risk of cardiovascular diseases such as heart disease and stroke.
Individuals in better cardiovascular condition tend to have lower heart rates during peak exercise, and return to their resting heart rate more quickly after physical activity.
A recent study showed that children with higher body mass index (BMI) have slower heart rate recovery, suggesting a possible link between healthy body weight and faster heart rate recovery.