If all is sequenced well, and if proper form is used, running is partially muscle (work) and elasticity (energy return). Both of which your natural human form is more than capable of; in fact, your body is built for these functions. Contrary to popular shoe-salespeoples' (and some doctors') beliefs, cushioning in your running shoe alters your running stride in a negative way: by limiting the natural elasticity in your running stride. Let's look at the mechanics.
When we land on the ground during our foot strike our muscles tighten and springy tendons lengthen. Our weight is absorbed by springs in the feet and legs. If we hold that muscular tension until it's time to lift that foot back up, we get to take advantage of the spring tension of those tendons. The stored energy in those springs act like a helping hand - a little springy assist to get us into our next stride.
Seems like a system designed pretty well. So how do we mess it up? Think about it this way: If you sprung that spring tension prematurely then you'd have to use ALL muscle to get you into your next stride. That's how we mess it up: by springing the tension early with well-cushioned footwear. Cushioning absorbs the stored spring energy and you don't get the assist you would normally.
Here's a visual that reinforces the point: Imagine you're bouncing a basket ball. The stored elastic energy gets the ball 80% of the way up to your hand. The other 20% comes from you using downward muscular force. Think about how hard you have to push that basketball so it will return to your hand, dribbling down the court. Now think about how hard you'd have to bounce the ball if it were on a softer surface, like a gymnastics mat. A thick mat, like a gymnastics mat, functions as a cushion that will absorb a great deal of that stored spring energy. The stored energy is not returned to the ball. To get the ball back up to your hand you have to push hard enough to make up for the loss in the ball AND the mat. The springiness of the gymnastics mat (or any other cushioning-type surface) is horrible for energy return.
Similarly, no spring constant (in the form of cushioning) in a shoe will help the runner get that energy back. The runner's muscles have to make up that work.
Save your energy for running, not cushioning and see your speed and efficiency improve and your injury rate decrease (provided you use natural form).
Note we have to "hold that muscular tension" - seems like a lot of work, right? To reduce how long muscles are engaged while you run: speed up your cadence! Remember: Shock absorbed by springiness in your body is returned again in your next stride.