If the muscles in your legs are weak then you are more likely to endure a knee injury or knee pain. To prevent this, you may start simple stretching exercises and gradually extend their duration and difficulty. Remember not to put a lot of stress as you need to strengthen your joints and not injure them. Here re some exercises that will help you gain strength:

Exercises for your quadriceps:

Strengthening contractions

Sit on a bench or chair and extend your legs. Put your heels on the round and stretch your legs as much as you can while contracting your thigh muscles. Hold for 10 seconds and repeat ten times.

Leg lifts

Lie on your back and bend your right knee so that your foot rests on the ground. Now raise your left leg straight so that it comes to the level of your right knee. Hold for a 3 seconds and then gradually lower it. Repeat the exercise ten times before switching the leg.

Knee dips

Stand upright but ensure that your knees are not locked. Lift your right leg off the ground and try balancing on your left leg. Once you've gained balance, try lowering yourself a few inches and then raising yourself up. Ensure that your left knee does not lock and do the dips ten times before switching to right leg.

Squats

Stand upright with a little arc in your back and lower yourself. Imitate the posture of a chair and try bending your knees to 90 degree angle and bringing your upper legs parallel to the floor. Raise yourself up gradually and repeat ten times. Initially you may start with bends lesser than 90 degrees.

Exercises for your hamstrings:

Strengthening contractions

Sit on a chair and extend your legs. Put your heels on the floor and without moving them, pull your legs back so that your hamstrings feel stretched. Hold for a few seconds and then relax. Repeat ten times.

Strengthening curls

Lie on your stomach and put your left foot on your right heel. Now pull your right leg upwards while with your left leg resist its motion.

Stretch

Place your right leg on the floor and left foot on a chair. Keep your legs straight and bend forwards until you feel your hamstrings in your left leg stretching.

Sit in a chair and extend your one leg. While keeping the heel on the ground bend forwards but do not try touching the toes. Repeat the exercise for a number of times before switching to the other leg.