Is the treadmill bad for our knees?

No!!! it can actually improve impact forces by altering your stride pattern.

There are lots of research articles looking at kinetics, joint mechanics and joint loading while on a treadmill compared to normal running pattern. When on the treadmill, the researchers found significant increases in stride cadence (steps per minute), shortening of stride length, and a shorter stride duration for all participants.

A shorter stride length and increased cadence, has been shown to reduce the impact force to the ankles and knees, and better disperse the impact across the joints; this reduces the stress on the anterior portion of the knees.