A.M. 20 with Mike, part of it with the kids. Did a pickup for 1.25 in 6:59. Mike was struggling - something is wrong. We ran into John Kotter and Taylor Farnsworth and joined them for part of the run. I told John he had at least a 2;13 marathon in him, if not 2:10. He thought he only had 2:20. I explained some things, among them the concept of virtual leggedness, which I have made up, or at least I have made up the term for it. Virtual leggedness is the total leg power output when running at a constant speed divided by the power output of the strongest leg. Thus, a person with one leg has the virtual leggedness (VL) of 1, a perfectly symmetric runner has the VL of 2, a perfectly symmetric four legged animal will have it at 4, and your average runner will have it somewhere between 1 and 2, but much closer to 2. Maybe 1.9 or so. The closer to 2 the better for a human. VL is an important contributor to quality X. To measure VL ideally you need a treadmill with built-in force plates. In lieu of that, it can be estimated by running on a hard surface where it is possible to leave visible footprints without affecting your form, and then comparing the stride lengths of the right and left pushoff.
A runner with a higher VL will not only outsprint a runner with lower VL given the same muscular strength relative to body weight, but also will win in the marathon given the same mile speed and the same aerobic/metabolic capacities. To understand this idea, the following exaggerated illustration can be helpful. Time yourself hopping 50 meters on one leg. Then time yourself hopping for half a mile on one leg. Compare the times to what you can do in 100 meters sprinting, and in an all out mile. You will likely get your 100 meter running time in the 50 meter one leg hop, but you will be quite far off your running mile PR in the half mile hop if you are able to finish at all.