Knees are very complicated structures and we put a lot of pressure on them during our lives.
Especially those who participate in impact sports like running; skiing and soccer are likely to experience knee pain. This can be caused from wear and tear or from injury.
Those who spend a lot of time standing during the day should ensure they have proper supportive footwear to prevent knee pain, and take small stretching or exercise breaks to increase circulation.
Ligaments surround the knees, and if you injure your knee it's a good idea to apply ice to reduce inflammation and to avoid putting weight on or bending the knee too much.
If your knee pain is very bad or frequent then you should get it checked. See below here what kind of therapy that can help. There are also natural products like Arnica salves that can reduce inflammation when applied on the knee.