![Image courtesy of Jess Loughborough](https://battleshields.wordpress.com/wp-content/uploads/2015/06/4538366098_934bb67878_o.jpg?w=1024&h=680)
Roman shields or any other shields that have a central boss, are held using a central handgrip, not straps. It is never strapped to the arm as it defeats the purpose of the protective metal boss itself.
The image is of the Ermine Street Guard, the biggest Roman reenactment society in the UK.