If you find yourself suffering from knee pain after exercise, it could be due to something obvious like below or it could mean something more.
1. Insufficient warm up or cool down during exercise
Before you begin your cardio like running or aerobic exercise, you need to warm up by doing very light aerobic exercises to warm up the muscles. Then you can follow by stretching before moving on more intensive aerobic exercises or strength training.
The same goes after the end of your workout sessions- you need to do static stretches (meaning stretch and hold for a minute if you can) to cool down. Stretching helps to prevent muscle cramps.
This is common enough and a lot of other sites mentioned the above reason. But a less mentioned reason is:
2. Incorrect exercise posture during exercise
Have you ever notice that some people make very loud thumping sounds when running on the treadmill? Yours truly is guilty of the same. When that is the case, you are using the wrong part of your feet and in the process, you may put too much of strain on your knees as well. More alarm bells should ring if you usually walk with the wrong posture- ie bow legs, knocked knees, duck’s feet (feet turned inwards) or feet that turns outward too much. By nature, certain posture misalignment, due to muscle imbalance leaves a lot of strain on your knees.
Rightfully when we exercise, we should use our leg muscles- the quadriceps, hamstrings, calf muscles, etc to absorb the shock of exercise. You should not put pressure on your knees- ie using your knees to push down the stepper on the stepper machine or moving the pedal on the bicycle. If you are doing that, you would end up hurting your knees more in the long run.
When you have constant knee pain, that gets worse after exercise, the solution is not to avoid exercise (it could make your condition worse and hasten the onset of arthritis or osteoporosis). A lot of people with knee pain (and are usually overweight) tend to stay away from exercising. Health professionals through dozens of studies and research encourage people with knee pain and arthritis to start with light aerobic activity and do strength training- train up surrounding muscles like quadriceps, hamstring and calf muscles to support your weight.
But a lot of studies did not place emphasis on doing these exercises correctly. You must consciously use your leg muscles when exercising instead of putting the pressure on the knees. You can start using cardio machines that are more friendly on the knees such as a stationery bicycle or the cross trainer machine.
And on the rest of the day, be conscious in your daily movement not to put too much of pressure on your knees. Like what Dr Bookspan said, there is no point of exercising correctly for 10 minutes when you spend the rest of the day in the wrong posture.