Found another gear (Read 449 times)

After an injury, I'm two weeks back into running. I started off much slower than I used to run, 9:45-10:15/mile as opposed to 8:15-8:45. I wear a heart rate monitor, and found that at that slower pace my HR was right where it should be. If I sped up a little, the HR predictably went up accordingly. Yesterday, I decided to just cut loose a little bit and enjoy a faster run. I averaged around an 8:30 mile. Amazingly, my HR was the same, and sometimes lower, than when I had done the slower runs. It was like I had shifted into the next gear. Has anyone else had this happen and do you have an explanation? I'm guessing that it has to do with efficiency, and that my form gets more natural at speeds to which I'm more accustomed.