The term "bad knees" is often used interchangeably to describe chronic knee pain. Sometimes knee pain is due to muscular imbalances, sometimes it is a lingering injury, or sometimes it's used to describe someone who has had knee replacement surgery.
If there are muscular imbalances at play, they can be treated which will alleviate the pain. However, this section is simply to provide a way to train (exercise) without pain, or without making the condition worse.