the junior model straps would not stand up to the impact from adult springs. Think how small harvey's springs were and how light he is.
It took ages to deveop the adult calf cuffs ( sprog emil and ET had them and Jason made his own) which can cope. They cannot be purchased in the UK at the moment atleast. At the moment the only way to get them is to know some1 on the continent who could buy them then ship them to you.