Fig. 3. Apparatus used to rigidly restrict human ankle motion and control rolling characteristics of the foot. (A) Subjects wore a boot and arc apparatus bilaterally, each consisting of a rigid walking boot modified to accept wooden arc shapes of varying radius. (B) `Arc foot' shapes of varying arc radius (defined as fraction of leg length) were rigidly attached with pyramidal prosthesis adapters. Arcs ranged in radius 0.02–0.40 m in absolute dimensions, and each subtended a sufficient range of angles to ensure continuous rolling ground contact throughout the stance phase. Arcs had matched mass of 1.1±0.1 kg, and boots had mass 0.85 or 1.05 kg, depending on size.