###### Abstract  
Anticipatory postural adjustment (APA) existed before a self-induced perturbation is an important motor control skill for balance and gait initiation, but cannot be easily monitored. During proactive balance test, a self-initiated weight shifting is produced. This might be an optimal paradigm for APA measurement. The purpose of this study was to investigate if APAs existed in the proactive balance test which consists of quantifiable weight shifting. The feature and reliability of the APAs were also evaluated. We firstly built a proactive balance test program on the commercially available Wii balance board. The program could generate adjustable target direction and distance for guiding subjects performing quantifiable weight shifting. The  
The proactive balance tests measure self-initiated weight shifting abilities in various directions [14; 15] and are commonly used as an option for clinical balance measurement. This has been implemented by using a stabilometric platform or force plate to quantitatively measure center of pressure (COP) during weight shifting [9; 16; 17; 18; 19; 20; 21; 22; 23; 24] and has been reported to be able to distinguish fallers from non-fallers [25; 26]. Recently, there have been some studies using Wii balance to develop proactive balance measure/training system at home. A review study showed that custom applications using the Wii Balance Board as a proxy for scientific force platforms have great promise as a low cost and portable way to assess balance [27].  
In addition to proactive balance ability, the anticipatory postural adjustments (APAs) had been suggested to be important motor control strategy for dynamic balance abilities in daily life activities. Studies showed that during a self-initiated arm forward reaching task, the COP moved backward before the actual arm movement. This COP movement was defined as APA that was used to overcome the foreseeing COP perturbation generated by the arm movement [28; 29; 30]. APA is not only for maintaining postural stability during such self-induced perturbations, but also for moving the COM to the new target [31].  
Notwithstanding the fact that APA measurements have been suggested to be important for patients with gait initiation difficulties, e.g., those who suffer from [PERSON]'s disease [32; 33; 34], there are currently no home and/or institute-based systems available to quantify APA measurements. Conventional measurements of APA usually use the fast arm-raise task to create a self-induced perturbation [28; 29; 30; 35]. However, this testing paradigm is not feasible in all clinical conditions. For example, a patient might not be able to generate fast arm movements due to bradykinesia. Therefore, designing a paradigm to measure APA with low physical demand is important.  
Researchers showed that an APA-related COP shifting could be detected before the subjects raise one leg from two-legs standing posture [31]. The APA was to overcome the expected postural perturbation generated by weight shifting. In proactive balance tests, subjects are asked to produce quantifiable weight shifting movements. It is plausible to hypothesize that the APA of COP shifting could be detectable in proactive balance tests. This paradigm to measure APA might be optimal since it does not require subjects to generate fast movement. Therefore, the purpose of this study is to investigate if APAs could be detected in proactive balancing task. The reliability and modulation of APAs according to the amount and directions of weight shifting were also evaluated. This study also develops a testing system to prove the concepts.