Personally I don’t use arms but totally agree with others ,one of the most important things about injection sites is that you rotate them regularly , failure to do this will result in bruising and lumps , which in turn leads to poor absorption of insulin which leads to control problems. I know it’s easier sometimes to just lift the T shirt, jab and on you go but try and get into habit of leg, bum, stomach , arm or variations of all one day after other , there is plenty of area in all of those areas to make bruise , lumps and poor absorption risk totally minimal.