Now that I have graduated & running around 12k per week I have started to get low back stiffness towards the end of my runs. This is usually after about 25 minutes & even if I try to relax my shoulders etc I can't seem to loosen this area. Not aware of my gait changing as I get tired & also seems to be getting worse despite getting fitter. Interestingly don't have back ache rest of the time just the last 10mins of my runs