If someone says they have "bad knees," it means that their knees are not in good condition and can cause them pain or difficulty while moving or walking.
Sentences with «bad knees»
But, if it will get people with bad knees to have some exercise, it's still not a bad option. (pathtofatloss.com)
Getting bad knees from doing squats comes from using bad form or half - squats. (fitness-baron.com)