I used to work in an NHS clinic where regular injections were given to patients suffering from long term conditions, mostly interrupted outer quadrant of their buttocks, but some into the upperarm as well. We were always taught to instruct our Patient to allow their arm to become limp and floppy, and being in a standing position does aid this, but there are exceptions, when someone is needle phobic and may become dizzy and faint during the process, when their is no other option but to inject them whilst sitting or even flat on the examination couch. Tensing up can cause a degree of bruising within the muscle which can then be uncomfortable for a few days afterwards. Simple painkillers such as Paracetamol can help to ease any discomfort you may have during this time.