Hi there - I agree with Vam44 in getting your back investigated closer. However - your history with the knee-op - it could also be some post-op problems - compensation etc. Your calfmm issue could be ischaemic, which then leads to compensation and the back pain. Maybe if you do have some spondolysis/spondolysthesis issues swimming may not be good - not breast stroke at least. Keep up the work in the gym - but beware the your back may have an issue.