2.1. Modeling of OLB in the frontal plane with a double inverted pendulum

Details of the double inverted pendulum model of balancing on one leg and some of the assumptions of the model can be seen in Fig 1. Maximum isometric strengths for ankle inversion / eversion, and hip abduction / adduction in neutral position were found from the literature for men and women in three different groups to represent the effect of age and neuropathy on OLB: healthy younger adults (20–30 years), healthy older adults (65+ years), and older patients with PN [7, 27, 28]. In order to apply the isometric strengths to our model, they were first normalized by the reported mean mass and height of the subjects in each group, and scaled back to the mass and height of the model (please see S1 Text). Maximum ankle inversion / eversion strengths were considered constant, and passive tissue contribution as negligible within the ankle angles common during OLB.

(A) One link represents the stance leg (SL) and the other the rest of the body (RB). The trunk is assumed to be straight without any lateral bending, and the contralateral leg is kept at a neutral abduction angle. (B) Parameters of the model are calculated from published mass-link information of a mid-size male aviator (m₁ = 14 Kg, m₂ = 67 Kg, l = 57 cm, h = 88 cm, r = 19 cm, α = 56°) [29]. (C) Two independent states (θ₁, θ₂) are required to describe balancing quasistatically on one leg. (D) Free Body Diagram for the equilibrium of the quasistatic double inverted pendulum.