Schematic explanation of ²Δα. The azimuthal changes in probe angle, α_P, and lever angle, α_L, over two steps are referred to as ²Δα_P and ²Δα_L, respectively. When walking straight, the probe attached to the myoV lever arm returns to its original orientation every two steps, so that both ²Δα_P and ²Δα_L equal zero and ²Δα_P = ²Δα_L. It can be shown that ²Δα_P = ²Δα_L is a general relationship, regardless of straight walking, so long as β_P switches regularly between two states (Section 3 of the Appendix).