I run in some capacity almost every day and just like anyone else that is in their 40's I experience knee pain from time to time. There is this old saying by people that running on a treadmill reduces knee stress and when you look at a machine and start thinking about the technology that is involved in these sometimes really expensive contraptions it is easy to start to believe that this is true. I got bad news for anyone out there though... It isn't.
This rumor almost certainly got started by someone who was really proud of the fact that they bought a treadmill or even more likely, the manufacturers of treadmills. When you think about how the treadmill bounces a bit when you run on it, you might think that it is reducing the impact but come on guys, this isn't a trampoline... it doesn't really have that much "give" to it because if it did, it wouldn't work.
The reason why runners' knees end up hurting, whether you are young or old (especially if you are older) is because of the repeated impact of your body weight that comes crushing down on your knees with each step. Since the treadmill itself has, by design, a hard flat surface that you are walking / jogging / running on, the stress that hits your knees is the same as if you were outdoors.
In many ways a treadmill can help to prevent injury because the surface is always the same no matter how you run. This cannot be said to be true of trail, or even street running. I've never heard of anyone twisting an ankle whilst running on a treadmill because there are no unseen hazards. I'm afraid that is where the perceived advantage ends though.
In fact, since the nature of a treadmill is that the machine, not you, determines the pace, it is actually a lot more likely that you will run incorrectly on a treadmill than if you were outside or on a track. I don't know about you, but when I am running outdoors I have never experienced a phenomenon where I was struggling to keep up with the ground that is moving below me faster than I can keep up with.
Generally speaking, kinesiologists agree that to run "correctly" you need to be making the contact between your feet and the ground while spreading out the impact on as much of the surface area of your foot as possible. Then next time you are walking somewhere pay attention to how your foot absorbs the "impact" naturally spread across your entire foot. This is how someone would run outdoors as well again, because the floor isn't moving underneath them."
When running on a treadmill, especially if you are pushing it faster than you should be in an effort to scare yourself into being faster, there is a great chance that you are actually running incorrectly and not distributing the impact in a natural way and then this impact ends up affecting, you guessed it... your knees!
A secret that a well-respected personal trainer told me in Chicago once is that when running on a treadmill, you need to focus on your posture much more than your speed. You need to maintain a straight backed posture as much as possible and attempt to "engage your core" by being as "12 o'clock" as possible. He told me that if you focus on getting your shoulders as far away from your ears as possible, the human body has a tendency to enforce this good posture on its own.
Mix up your workouts
Basically every sport scientist agrees that the number one thing that a person can do in order to prevent joint pain or injury is to mix up what you do for cardio as much as possible.
Doing too much of anything can result in over-exerting a particular muscle-group or joints and while it seem intuitive, one of the best ways to avoid this is to have a variety of exercises that you do. If your gym has treadmills they almost certainly have bikes, rowing machines, and ellipticals as well. Why not go ahead and change it up from day-to-day?
The bottom line is that if someone ever tells you that a treadmill is "better for your knees" they have probably been subjected to bad advice or is a victim of marketing. Remember that the number one directive of a company selling a product is to create a reason why you NEED to buy that product, not necessarily to be telling the truth.
Also, running on a treadmill is boring AF. If you have the opportunity to simply go on a walk with a jog mixed in you are going to be far more entertained and also never experience the need to keep up with the floor that is moving below you!