Why are we not actually running, when we dream that we are running?

The movements we make to walk, get hold of things, even to scratch the end of our nose ... are controlled by nerve impulses coming from the Grey cortex of the brain. These instructions are transmitted by a nerve impulse, a kind of electrical current, which passes at a varying rate through the white nerve fibers in the brain, the brain stem, the spinal cord and nerves going to the muscles.
When we dream that we are running, rock-climbing or swimming, a part of the brain stem concerned with movement sends out a message damping down all the main muscles of the body. This means that any movement instructions the brain is sending out during a dream do not get beyond the brain stem.
We move in our sleep, we turn over, our limbs twitch and our eyes are very active while we are dreaming. But the movements we are dreaming of are completely neutralized. Sometimes, the instruction suggested in a dream is so strong that the centers in the brain stem are momentarily overwhelmed by the strength of the nerve impulses. In this instance they allow part of the instruction to get through, but this has the effect of awakening us immediately. In young children, the nerve centers are not fully operational. They are still not quite ‘mature’. They allow some slight movements to escape, resulting in sleep-talking or laughter. This also happens in adults, but less often.