Most runners always seem to have a preference if offered the opportunity to run outdoors versus a treadmill. Environmentally, both offer their advantages and disadvantages. Running outdoors you get the sights and sounds of the area around you, the opportunity to run with some friends if you so wish, and if you’re really lucky you may even get to soak up a few rays. Whereas indoors on a treadmill you get the benefits of climate control, the convenience of not having to leave your home, and the chance to indulge in your latest Netflix binge. Looking past those inherent differences, we still hear the question as to whether running outdoors is biomechanically different to running indoors on a treadmill.
One study by O Riley et al. in 2008 looked at the biomechanical differences at a steady speed while running outdoors versus on a treadmill. They found that there were some differences (cadence, stride length, propulsive force), but overall the two were still quite similar.
Another set of recent studies by Caekenberghe et al. in 2013 looked at the biomechanical differences between accelerating (speeding up) while running outdoors versus on a treadmill. On the whole they found definite differences as outdoors you adopt a greater forward lean, you strike more under your body rather than out in front, and have a greater push backwards. These adaptations were absent on the treadmill as the belt accelerated.
So overall the answer is yes they’re different, but I would say there’s no harm in substituting one for the other and in fact the variation may be a welcome change for the body every now and then. I would say however, if you’re a runner training for outdoor events, make sure the majority of your running is done outdoors to be as specific as possible with your training.