The Sole Can Only Push: Foot Mechanics and Ground-Reaction-Force Control (The Kuriyama Protocol, Chapter 2)

Part 7 of a series; the second of fifteen chapter-by-chapter articles. Part 1: 10.5281/zenodo.23587879 · Part 2: 10.5281/zenodo.23587670 · Part 3: 10.5281/zenodo.23526074 · Part 4: 10.5281/zenodo.23609857 · Part 5: 10.5281/zenodo.23610607 · Part 6: 10.5281/zenodo.23682055 In standing and walking, the sole is almost the only boundary through which the body exchanges force with the environment, and it is also where ground information enters: actuator contact and sensor share one location. Chapter 2 of The Kuriyama Protocol specifies that boundary in five sections, each with claim, verbatim field observations, derivation, literature check, verdict and prediction: - Footwear as a cast. Shoes immobilize most of the foot's joints, and soft layers smooth the pressure pattern. An elastic layer of thickness t spreads a point load over roughly 1–2 t, so a sole removes relief finer than its thickness while keeping slopes and steps. Age-related plantar insensitivity is real (Perry, 2006); environment adds to it rather than replacing it. - A push-only boundary: vertical ground reaction force ≥ 0 and |horizontal| ≤ μ × vertical (the friction cone). - Tripod and truss: plantar-fascia tension T = F·L / (4h); with illustrative values T ≈ 570 N, rising about 30% if the arch drops by 1 cm. - Bow, lever and spring: the windlass mechanism stiffens the foot at push-off without additional command, a variable stiffness at near-zero switching cost. - Loading position follows intended force direction: horizontal/vertical GRF = tan θ; the variable to instruct is lean, not loading position. Combined with the friction cone: θ_max = arctan μ, about 31° on a dry floor but about 6° on ice. Four gaps found while writing, not stated in the book: (1) "decline is caused by environment, not ageing" overstated the case; (2) soles do not make plantar information "nearly constant" but remove it from the fine end first; (3) the truss formula holds only for a fixed loading configuration (the windlass phase shows the opposite tension–height relation), and the book's arch-support prediction was confounded by direct load-bearing; (4) plantar receptors are broadly distributed, so the teaching claim of regions with "more receptors" should read "lower thresholds" (Kennedy & Inglis, 2002). Five falsifiable predictions are registered (numbered 29–33, continuing the series): - P29. Thicker soles progressively remove the fine-scale component of centre-of-pressure fluctuation on uneven ground more than the coarse component; returning barefoot after thick soles transiently increases sway. - P30. After blinded one-sided plantar treatment, rate of force development in a single-leg hop increases more than peak force. - P31. With added backpack load in quiet standing, plantar-fascia stiffness (shear-wave elastography) rises with load, more steeply in individuals with larger L/h. No arch supports. - P32. Restricted first-MTP dorsiflexion reduces arch rise at push-off and increases calf EMG at the same walking speed. - P33. "Tilt pelvis and shin forward" yields larger forward GRF and lower ankle EMG than "weight on your toes"; the difference shrinks on a low-friction surface. Refutations are welcome. The weakest part is the sole-as-spatial-filter estimate, which the author has not measured. The author has no formal research training. Corrections will be issued as new versions, with earlier versions kept retrievable. The article body is in Japanese. An English summary, the registered predictions and the reference list are included in the document. Contact: [email protected]