Hi Jonathan, Your son is half right! Recent research has shown that static stretches done prior to a race can reduce muscle strength for up to an hour after stretching. which may put you at greater risk of injury. However these sorts of stretches are perfectly valid for post race stretching as part of a warm down exercise. In time for this year’s Flora London Marathon we have recorded some videos on the best pre and post race stretches on the Lucozade Sport website, if you go to www.lucozade.com/flm you can see these all demonstrated. Thanks for your question and I hope these help.