Hopefully this will make sense - recently every time I play, after awhile (1+ hours) my knees will start to feel "heavy" - there's no pain and I can still play normally, but I am very conscious of it between points especially and it is slightly uncomfortable. I'm not sure how else to describe it. The feeling goes away within an hour or so after I stop playing. Anyone experience something like this? Is this the start of something more serious? I'm 29 years old and play approximately 4 times a week, 2 hours each session, only on hardcourts. Thanks for sharing any insight/advice.