What causes knee pain? Injuries can make your knees hurt, but chronic knee pain is usually caused by changes that take place over time. Three conditions that cause chronic knee pain are osteoarthritis, patellofemoral syndrome, and degenerative meniscal tears.   Osteoarthritis. As you get older, the cartilage in the knees thins and breaks down. Your knees often become inflamed and painful. Strong muscles can support the bones and relieve pressure on the knees.   Patellofemoral pain. When the kneecap doesn’t move properly, the cartilage beneath it becomes irritated, causing pain. If you have a muscle imbalance that pulls the kneecap out of position, you may develop this kind of pain. Strengthening weak muscles can help.   Degenerative meniscal tears. A C-shaped piece of cartilage called the meniscus cushions the knee joint. Aging can cause the meniscus to weaken and tear.   How does exercise relieve knee pain? Ways that movement improves the health of bones and cartilage:  

• Increases blood flow
• Circulates the fluids that lubricate the knees
• Brings nutrients to the tissues
• Removes waste materials
• Builds muscle

  How can exercise hurt the knees? To work out safely with knee pain, know the exercises not to do. High-impact exercises can hurt the joints. Walking, biking, and swimming are examples of lower-impact exercises.  Many doctors do not recommend exercises that put stress on the knee, such as squats.   Skip the leg extensions Leg extensions could hurt your knees. To do this exercise, you sit in a chair or bench and straighten the leg until it is parallel to the floor. This position puts a load on the knee. If you would like to perform a modified version, you can raise the leg halfway rather than to full extension.   In the gym, you can use a leg extension machine to make the same motion — except that the machine adds weight to the movement. Because of the force exerted on the knee joint, this is not a good exercise for those with knee pain. Also, it doesn’t add much to functional strength since it mainly works the quadriceps, the muscles on the front of the thigh. It doesn’t work the muscles on the sides and back of the leg.   Be careful with squats Squats also put a load on the knees. To get the benefit of squats without pain, go just halfway down. You can also do wall squats. Place your back on a wall with your feet about 18 inches away from the wall facing forward. Slide the back down as far as you can without pain before sliding back up. Keep the knees above the ankles.     Lunges can cause problems Like squats, lunges are an excellent exercise for those who don’t have knee pain. You begin a lunge by stepping forward with one leg and lowering the other leg. In the final position, both legs are close to a 90-degree angle. This degree of flexion stresses the knee. You can perform a lunge with less bend. Still, it may be better to choose a different exercise.   What about running? Most people believe running is bad for those with bad knees. Some evidence suggests the opposite. One study found that running may decrease your risk of needing a knee replacement.   You may be able to run without damaging your knees if you:  

• Avoid running on concrete
• Wear the right shoes
• Stop if your knee pain increases
• Stop if you develop a new type of knee pain

  Get medical advice Talk to your doctor or physical therapist before starting an exercise program, especially a running program. See your doctor if you develop pain that:  

• Is sharp and stabbing
• Makes you walk with a limp
• Lasts more than 2 hours after your workout
• Worsens at night
• Causes swelling, heat, or redness in the joints
• Doesn’t improve with rest or ice

  Enjoy the benefits of exercise If you plan your workout carefully, you can enjoy the benefits of exercise without hurting your knees. You may have:  

• Less pain
• Greater energy
• Better sleep
• Improved function in your daily life.

Exercise may also help you lose weight, which reduces the stress on joints and may improve your knee pain.   References: https://www.medicinenet.com/exercises_and_activities_not_do_if_have_bad_knees/article.htm