Exercise that involves repetitive motion of the arms, legs and hips is known as aerobic activity. A treadmill is an aerobic machine used in gyms and homes. Using this machine and running outside have their similarities and differences. What you choose depends on your personal interests and preferences.
Impact is a primary concern with people who suffer from lower back pain or arthritis. Working out on a treadmill and running outside both cause high impact, but because there's a little spring to it, the impact is slightly less on a treadmill. The difference is negligible if you walk instead of run.
The treadmill comes equipped with speed and incline adjustments. To make your workouts more challenging, you simply press the appropriate button and adapt to the change. When you run outside, you are completely in control of your speed and elevation. If you want to increase the challenge, you have to run harder and run up hills.
Wind or lack of it is a major difference between the treadmill and running outside. When you are indoors, there is no wind whatsoever. Outdoors, you might have to battle high winds or contend with nothing but a light breeze. In any case, even the slightest wind will make your workout more challenging. To compensate for lack of wind, you can increase the incline on a treadmill.
Difference in Terrain
A treadmill has a smooth, flat belt that continually moves around as you walk or run. Unless you run on a track outdoors, there is a very slim chance you will be on smooth terrain. In most cases, you will have to deal with uneven ground, potholes, rocks and any number of other obstacles. This holds true, especially when you run through trails in the woods. If you have a goal to run in a race, such as a 5K, your best bet is to run outside. Your body will get acclimated to the terrain and it will not be a shock to your system on race day.
Regardless if you run outside or use a treadmill, the same muscles get activated. These include the glutes, quadriceps, hamstrings, calves, hip flexors and abdominals. The abs contract to keep your body stable. If you run up hill or turn the incline up on the treadmill, you will place more emphasis on your calves, hamstrings and glutes.
When you run outside, you will have to deal with uphills, but you also have the luxury of an occasional downhill. This is generally not the case with a treadmill. Except for a few machines that allow slight downhill grades, once you turn it on, the belt is flat and it can only be raised.
Treadmills have handrails that run along the top edge. When you first get going on the machine, you can grasp these for balance. Although they are beneficial in this case, they can also be abused. If you are running or walking while leaning on the handrails, you will cheat yourself out of work. When you run outside, you have nothing to grab onto for support.