Quad strength is very important in preventing knee issues. The strong muscles around the knee help support the joint so cartilage isn't worn away. Chair Exercise #11 strengthens the quads so try Stepping on the Gas.

STEP ON THE GAS:  Loop the band around the arch of the right foot and grasp the end closest to you with both hands.  Bring the knee up to the chest and while holding  the band tight extend the leg to the floor.  Perform 2-3 sets of 10-15 reps.  Repeat with the left leg.