I had ACL operation on right knee2 years ago and have not played football since although I am 3 weeks away from playing again. It has been my only motivation to get back into condition. I had cartilage surgery on my left knee, think they trimmed it about 9 months ago. They said I have no ligament in it but it feels stable. I have strengthened them using the cross trainer which can be boring but listening to good music passes the time. I also run on the grass with good trainers and have had no effects.