You can't see the back of your thighs but you know they need some work. Try these simple hamstring curls at your desk.
Stand with your hands resting on the back of your chair for balance. Pull your deep abs up and in--this will help with balance too.
Move 1) Lift your left foot a little bit off the floor.
Move 2) Bend your left knee and slowly lift your foot towards your booty, until your shin is parallel to the floor. Lower the leg slowly and repeat 10 - 15 times, then switch legs.