Are 'open chain' exercises bad for the knee?
In the early part of rehabilitation it is important not to overload this exercise with weights, as this is likely to do more harm than good.
"...rehabilitation professionals need not be afraid ... of the proper, appropriate use of [open kinetic chain] knee extension....it is a useful and necessary exercise to improve quadriceps strength .... [but] make sure you are wise in your selection of the appropriate amount of resistance, arc of motion, time frame from surgery, and the type of patient."