Fig. 1.

Overview of the experimental conditions. A: illustration of the unstable object. B: illustration of the posture of the subjects and the procedure with the 3 experimental conditions: stable surface, unstable surface pretraining, and unstable surface posttraining, intermitted by 30 min of training. In between trials on stable and unstable surfaces, subjects performed 2 practice trials of 16 s on the unstable surface. The total period between measurements on stable and unstable surfaces was ∼5 min. Rest periods between trials within each block of 6 trials lasted 20–30 s. The blocks below the time line represent the 18 trials, with NONE referring to no sensory manipulations, VES referring to vestibular manipulation, and VIS to visual manipulation. Note that the order of the latter 2 conditions varied over subjects. Motor strategies used in balancing were analyzed based on the trials without manipulations. Surface effects were analyzed based on averages over the trials on the stable surface and averages over the trials in the unstable posttraining condition. Training effects were analyzed based on separate trials of the unstable pretraining and unstable posttraining conditions.