Fig. 4: System setup and experimental training protocol.

The participants coupled the line of visualized postural sway (VP) to the target line (T) under the guidance of online visual feedback. Balance performance of the trained stabilometer task was measured in the pre-test (Day1) and post-test (Day 3). During the practice session (Day 2), the participants in the control group were trained with traditional visual feedback that displayed visualized errors (VE) equivalent to real errors (RE) in the computer monitor at eye level (VE = RE). By real-time mathematical transformation, the error amplification (EA) group were trained with visual feedback that virtually double the real errors (RE) during the training session (VE = 2*RE). (EA: error amplification, RP: real postural sway).