I have arthritis in both kness and found the leg exercises using the machines were actually worsening the problem.

The best exercises I've found are for leg lifts laying on a bed and using weights on the ankles. On the back...lifted up on a slant and held in place for a count of five; side raises the same. You can also do the front and side raises standing. Also good for the thighs are resting on hands and knees and lifting a leg of the ground(bed) out side ways fully extended or bent and holding for a count. Also good are back leg extentions.