Treadmill vs Outdoor running

Kineseology (study of the movement of the body) point of view

There is a measurable difference in the muscles being trained on a treadmill than the ones that make you . The major difference tends to be in the calves, which propel the body forward at the end of the stride. The quads push the body forward to some extent, but the majority of the motion comes from lower down. The hamstrings pull the lower leg into a recovery position so that the hip flexors can move the whole assembly back to the starting position. On a treadmill, the brain quickly realizes that the firing of the calves isn't moving the body forward in relation to the rest of the room and that lessing of the energy used won't cause any dimunition of speed. Since the body gradually finds the most efficient way to accomplish any physical task, it slowly shuts down the calves until they're only working at about 35% of what they would outdoors. For people that feel that they work harder on treadmills, it's probably either mental or they have a different gait.

Other possible Explanations

Physically, there may be some slight differences between how your body expends energy running on a track versus a treadmill.

• The treadmill belt offers some help by pulling your feet back underneath your body, so you are potentially exerting less energy to move your feet and legs than if you were not on a treadmill.
• When you run indoors on a treadmill, you do not have to overcome wind resistance. The lack of wind means you'll spend less energy running four miles on a treadmill than you will running four miles outdoors. However, it may depend on how fast you are running. For the average person, running 5 - 9 miles per hour (mph) will result in little difference. Some studies say outdoor running expends up to 5 percent more calories; if you run faster than 9 mph, running outdoors could utilize up to 10 percent more calories because you are working harder against wind resistance. Other studies say there's no difference. One study demonstrated the way to balance energy use between indoor and outdoor running is to set the treadmill at an incline (or grade) of 1 percent.
• Running indoors maintains or offers stable elements. Runners not only avoid wind, but also other potential natural elements, such as cold air, rain, or sand (if you run on a beach), which demand extra energy.
• Running on a treadmill versus pavement (the composition of the track surface you run on is unclear) provides a softer surface, making it a little easier for your joints. People with knee pain or soreness might opt for a treadmill versus the road outside for this reason.

adapted from http://www.goaskalice.columbia.edu/3049.html