You don't hear of the term "closed chain" much any more. It was first proposed in Steindler's 1955 text, "Kinesiology of the human body". His definition is, "a condition or environment in which the distal segment meets considerable resistance that restrains free motion". It got too confusing as to what defines "considerable resistance" or "restrains". But, here are two of my favorites. The first is a closed chain "mountain climber". The athlete has a pair of football girdles wrapped around his feet for glide on a painted concrete floor. On a traditional gymnasium floor, a pair of socks will do just fine. The exercise is appropriate for just about any LE rehab. The nature of the injury, and the stage of healing will determine whether the legs move in sync or out of sync, the plane, and the direction. I hate to use the term, but it's a great "core" & cardiovascular exercise.
In the 2nd picture, the athlete is in a narrow stride stance, holding two powerballs over his head and shaking them right to left, in sync, as quickly as possible for 30 seconds. I also have them go sagittal plane hip to overhead, then rotational @ shoulder. Beginning proprioception training for a LE injury from the top down is sometimes a good way to set your athletes up for success.