When running outside you are faced with wind resistance and different types of terrain, both of which are not present when exercising on a treadmill. The same applies to cycling. If you go for runs through your town or a nearby park, you might also have to deal with things such as random rocks, other people, and uneven paths, all unexpected obstacles that require more ankle flexing and coordination. Training outdoors can help improve your reaction time and hand-eye coordination in non-fitness situations as well. However if you want to get an outdoor-quality jog in at the gym, studies show that typical wind resistance can be replicated on a treadmill by setting the incline to between 1% and 1.5%. Keep in mind that many other beneficial aspects of running outside, such as going downhill, are not feasible in the gym.