I think we can safely state that adults, across the centuries, have worried that the children of the next generation will be soft and unfit for the rigours of life.
The latest explanation of this Spartan anxiety comes in a study of resting pulse rates in 9-11 year old children in the UK between 1980 and 2008.
This study shows that there has a been a small but steady drift upwards in the mean resting pulse of children of both sexes over this period, much amplified by the scale of the graphs in this paper.
The researchers conclude that only a small portion of this can be attributed to obesity, and the rest is probably the result of reduced physical activity.
Peters, H., Whincup, P.H., Cook, D.G., Law, C. & Li, L. (2014) Trends in resting Pulse Rates in 9-11 Year Old Children in the UK 1980-2008. Archives of Disease in Childhood. 99(1), pp.10-14.