The World Health Organisation recommends young adults should do moderate to ­vigorous exercise for about an hour a day – but this goal isn’t being met by four out of five adolescents.

And researchers have also found less intense exercise ­endorsed by the WHO isn’t linked to the same level of fitness as ­vigorous workouts.

To achieve heart/lung fitness (CRF), teenagers should exercise vigorously for at least 20 minutes a day according to Oxford University researcher ­Alexander Jones and colleagues reporting in the US journal Pediatrics.

Interestingly, the benefits of CRF plateau after about 20 minutes of vigorous exercise, with the optimum being about 14 minutes. Furthermore, teens in the upper quarter of vigorous activity score higher than those in the lowest quarter. Moderate and light exercise don’t promote CRF.

In an accompanying editorial, Michele LaBotz and Sarah Hoffman of Tufts University School of Medicine in Boston, wrote that the WHO’s current guidance “may not be sufficient to improve CRF in adolescents”.

Jones and colleagues said their recommendation of 20 minutes per day (instead of an hour) may be more attainable for teens, as well as adults.

“We provide grounds for clearer public health messaging on how to improve CRF in this population,” they argue. “One possible reason [why many teens do not meet the WHO goal] is that this duration is quite long, requiring a daily time commitment that some may find difficult to ­maintain. A shorter target of 20minutes might be easier to schedule daily and a focus on vigorous physical activity would simplify messages about the intensity of activity that is likely to improve CRF,” they noted.

For their study, Jones and his team assessed physical activity and CRF in 339 children aged 13 to 14 (170 boys and 169 girls) in the Oxfordshire Sedentariness, Obesity, and ­Cardiometabolic Risk in Adolescents: A Trial of Exercise in Schools study.

They used wrist-worn accelerometers and 20m shuttle runs to calculate the effects of exercise on CRF.

On average, girls exercised less often and had lower CRF than boys.

“However, we found no evidence of the relationship between physical activity and CRF differed significantly according to sex, supporting the use of the combined model and unified recommendations for all,” they reported.

LaBotz and Hoffman also noted that there are more benefits to exercise than CRF.

“It is important to consider that other benefits associated with physical activity may accrue at different rates, and that accumulating 60 minutes of daily moderate to vigorous physical activity may have benefits beyond that of improving CRF,” they explained.