Data processing. (A) For each stride k, a local forward direction was defined by the best fit (total least squares) line through footfalls k-3 through k+3. (B) With respect to this line, measurements were defined for forward foot placement sk, lateral foot placement wk, and angular foot placement Ψk. (C) Stride period Tk was defined by sequential threshold crossings of horizontal plane velocity magnitude, with a threshold value of 0.2 m/s.