leg stiffens up

hi there, got a wee issue these last few weeks, its a strange one so hear it goes dont have any symptoms at all when not running and none when running slow pace the problem starts when i increase to my threshold pace, my right leg seems to stiffen up usually in the calfs, after this i have trouble increasing  pace and sometimes i have to stop and  stretch.this helps but affect the rest of my run.....i wear orthrotics....any advice before i head to the physio?

Is the muscle being worked beyond it's fitness level?  Or perhaps you aren't giving it enough time to recover following running?  Try letting it fully recover and do plenty of stretching....after you've warmed it up of course.  Otherwise it might be down to lack of salt...but that's unlikely unless you've worked hard on cutting it out of your diet.