The knees are used in all our movements, so they are exposed to great pressure and stress due to our body weight.
Namely, every pound of fat you carry equates to 10 pounds of pressure on the knee joints when running, and 3 pounds of pressure while walking.
Therefore, the licensed physical therapist Dr. Jo recommends a compilation of several simple exercises which strengthen the knees and relieve pain due to the reduced stress they are exposed to.
Exercise #1 – Lunges
Hang onto that stack of books, and place the feet on either side of the stack, faced forward, with the back upright. Then, slowly drop the body straight down so that the back knee touches the stack of books. Elevate the body slowly and repeat two more times and switch sides.
Exercise #2 – Squats
Stand facing in the direction opposite the flat surface, and put the feet a shoulder-width apart. Push the bottom back and down until it touches the surface of a chair or a coffee table, and then, pop back upright. Repeat two more times.
Exercise #3 – Step Ups
You need a few thick hardcover books and stack them. Next, place one leg on the step, while the knee remains behind the toes. Next, start placing the body weight on the leg, and slowly pull the other leg up to the stack. Lower the leg and repeat twice, and switch sides.
The following video from Dr. Jo provides extra tips about these exercises: