Treadmills have a motor which drives the belt, you dont. It works the lift, but not the drive. Long term causing problems. Then you have Running road or track and field, impact, good running technique can Help. But the fitter you are usually the impact increases, its something I noticed years ago. So getting the thinking cap on in 84 had an idea, but wasnt till 98 put the patent in,. just got an extension and world patent on something. This should or will solve the problem. Running as hard as you want without impact, while at the same time making the Heart work 35% more efficient, solves everything. Plus going by tests repairs injurys in half the time
Always outdoors. Your body cheats you on treadmills since you can just lean forwards and shuffle your feet and your speed is constant. Outdoors either your legs move faster or you ain't going nowhere.
No less than 2% incline and even then the treadmill SUCKS. It just isn't equivalent. It is still great if you can't get out on the road, but don't think it is the same. The only thing it is good for is doing great intervals as you can see the time and speed right there. Treadmill might trump skipping rope, but nothing trumps real roadwork.
Running outdoors is more enjoyable, but treadmills get you just as fit. I ifind that my outdoor times improve just from my treadmill training [and some cycling]. I do most of my training on a treadmill now after wrecking my left knee and I rarely get pain from it. When I run outdoors, and try to run everyday, I end up limping badly after a couple of weeks. I'd say the only real disadvantage to treadmill running is that you tend to overheat and get soaked in sweat. I get round this by running 20 minutes then change my shirt and let my body cool down by doing some strengthwork.
Only muscles I want to work on the treadmill is legs plus heart and lungs .So an incline should do the trick for fast intense runs .Bagwork and body weight training for the rest .Also hand amd neck but mostly bodyweight for me except a neck harness , hand gripper and a plate or a sandbag for legs .