In some ways, running on a treadmill can be worse than running outdoors. Mark's Daily Apple explains how, along with a simple tweak that will help you get more from your treadmill runs: set it at an incline.
Besides running on a flat, unchanging surface rather than an outdoor terrain that causes your body to adapt, the treadmill doesn't engage your posterior chain (the muscle group that includes the lower back, glutes, hamstrings and calf muscles) as much as running outside:
Runners typically generate the majority of their power with their posterior chains: the glutes and the hamstrings. It's the hip extension, powered by the glutes and hamstrings, that propels the runner across the terrain. Since treadmill running cuts way back on peak hip extension, instead favouring hip flexion, the already sorely underdeveloped (from sitting and office work and generally sedentary living) posterior chain receives even less attention. Why engage the glute to bring your leg back and your body forward if the tread belt does it for you?
The posterior chain is important for pretty much any kind of movement, so you don't want to lose out on working those muscles.
Mark recommends setting an incline, since that will engage your posterior chain more, citing a study that found running with a 1 per cent grade "recreated the energetic cost of flat ground running".
Check out the full post for a few more suggestions on improving your treadmill runs... and why you might want to run outdoors more when you can.
5 Reasons to Run Outside Instead of on a Treadmill [Mark's Daily Apple]