Abstract: A running shoe and a flexion plate insert. An upper and an outsole together define an inner volume. A tensioning system selectively constricts the shoe. Trimmable traction formations project from the shoe outsole, and cuttable drainage plugs permit draining of water. A flexion plate exhibiting asymmetric flexion characteristics is selectively retained atop the outsole by engagement of an aperture in the flexion plate with a mounting plug retained within the shoe or by a removable first midsole. The mounting plug and the aperture have eccentric portions that are out of alignment when the flexion plate and the first midsole are aligned and that align when the first midsole and the flexion plate are disposed at a given, non-zero angle. The first midsole can be an upper midsole, and a second, lower midsole can be disposed in a facing relationship with the upper midsole with the flexion plate therebetween.