For those that suffer from bad knees, sitting at a desk all day is extremely detrimental to their health.
The key to keeping your knees strong is mobility. As long as you leave your chair periodically, standing and walking around during your breaks, you should be okay.
You can also do some stretches right in your chair, ones that will strengthen all your leg muscles to support your knees. So even if you have no time for the gym or a walk, you at least have these desk exercises.
Muscle targeted: Calves
Move towards the edge of your chair, sitting with your feet flat on the floor and legs at a 90 degree angle. Keep your hands on your knees. Slowly raise your heels off the ground, simultaneously pushing your knees. Lift as high as you can, then hold for 5 seconds.
Straight Leg Lifts
Muscle targeted: Quadriceps (Thighs)
Sit on the edge of your chair with your feet flat on the ground. Straighten out your right leg in front of you to tighten your thighs. Hold for 5 seconds, then bend your knee and bring your right foot back to the ground. Repeat on your left leg, doing this 5 times on each leg. You can raise your legs at the same time, but just remember to relax in between each repetition. Do this several times a day.
Seated Toe Lifts
Muscle targeted: Hamstrings (Back of thighs)
Again, sit on the edge of your chair with your feet flat on the floor. Dig your heels into the ground and lift your toes. Hold this for 5 seconds and repeat this 5 times, breathing throughout and resting in between each rep. Do this a few times during your shift. You can alternate legs if you prefer to stretch one at a time.
Muscle Targeted: Inner quads (Inner thighs)
Sitting on the edge of your chair, put your fist between your thighs, then squeeze. Hold this for 10 seconds, then release.