The question was still not very clear on what are you talking about, and it's not definite on what you are looking at probably you just wanted to know roughly what others think about advantages and disadvantages of Treadmill running compared with natural running.

** How's the condition of runners and place of running outdoor/indoor, should be more descriptive on what are you talking about**


In my opinion, if runners in both conditions are in good health running in door on even surfaces should be safer than on treadmil 'cause no mechanical errors will be involved when the runners can control their own speed also could slow down at own pace when get tired. On the other hand, running in door on treadmil could be safer to runners comparing to running naturally out door in terms of minimizing the risk to get injured from external factors such as car accidents, uneven foot path (could cause ankle and foot injuries), heat stroke (if the temperature is too high and clothing is not appropriate), etc.

However, if the runners are not in good health eg. diabetic runners - running in door on treadmil should be much safer for them 'cause a more appropriate level of hardness can be programmed and controlled easier than running out door. Disadvantages of running naturally out door for this group of runners should not be much differed from the healthy group.

Hope this could help!