Although I can easily tremolo sixteenth notes on a single string at 140bpm I notice that when I change from eighth to sixteenth notes there is a slight tightening up of the bicep of my picking arm. Tbh I only really notice this by holding my bicep with my left hand whilst tremoloing an open string. Does this happen to everyone or should I be working to get rid of it? Hope I’ve made myself clear 
Don’t worry about it unless it causes you discomfort. Most likely the muscles are just tensing a little to hold the arm-hand apparatus in place as you pick up speed.
Andy Wood mentions something similar in his interview, where he says he’s applying a force into the body of the guitar ‘to prevent it from flying away’ as he picks up speed. I experience this too, when I use the same DSX movement he does. It’s just a little bit, but it’s noticeable.