SACH-SAWCH

From heel landing and let down to mid stance the quadriceps controls inertially induced knee FLEXION to shorten the leg (Hip to Ankle line length). Again, inertial sensing. So, knee flexion into midstance [hip above mid-foot] and then follows knee extension past midstance. That, too, happens inertially. Quadriceps does not do it. Not active knee extension? Mostly the center of mass is flying forward by inertia and the knee extends to fill the gap that would otherwise occur. Some length comes from the ankle rise over what moves forward as the forefoot metatarsal head rollover. H ip A nkle R elative Position, a vector. Inman does not note

this detail, but looking at a Hip to Ankle line (a functional virtual invisible leg) the angle to the ground at heel contact and that line as the foot rolls off the forefoot create an arc angle whose apex is well below the floor. If that big arc centered below the floor was a wheel, then a small arc of turn has little rise & fall.

A smaller arc (shorter stride) of a bigger wheel reduces vertical anti-gravity needs. An alternative to sine wave? Another energy saver was hiding from Inman in his own great studies. Look how close this stiff legged BIG wheel arc comes to the sine wave that we just described. However, it needs a centered ground apex as a center of motion or else it becomes a lifting cam which destroys energy and is jarring. The forefoot is too forward for this job … unless....