## 4 Discussion
### Apa  
Our study successfully detected APA in 100% of proactive balancing trials in all target directions. To our knowledge, this is the first study to successfully quantify APA components during proactive balance tests. This success rate is higher than those reported in  
Figure 6: The APA time (a), APA distance (b), and APA correction (c) in short (60%, black line) and long (80%, red line) target distances in all movement directions. * Statistical significance between short and long target distance (\(p\) < 0.05) +Statistical significance between a specific target direction and 0 degree target direction (\(p\) < 0.05). previous studies which used fast arm-raise tasks to detect APA [31; 42]. [PERSON] and [PERSON] (2001) reported that APA could not be detected in some arm-raise tasks, which caused little perturbation of equilibrium [31; 42]. The high APA detecting rate during proactive balance test was possibly due to the reason that the proactive balance test produced higher amplitude of perturbation of equilibrium than the arm raising movements did. Although the generation of APA requires substantial amount of equilibrium perturbation, our results showed that the APA time was invariant to target distances. This suggested that the APA time might be pre-programmed and the modulation of APA time was independent of external biomechanical challenge. Previous studies suggested that, during gait initiation, the first APA was pre-programmed and requires the bilateral inhibition of soleus and activation of tibialis anterior [43; 44; 45]. Although the pattern of APA during proactive balance test was not identical to the APA during gait initiation, our study supported the invariant pre-programmed feature of APA modulation during proactive balance test. There are two types of APA patterns detected in our study, one phase or plural phases. The APA had only one phase in 0-(left), 90-(forward), or 180-(right) degree target directions which were all orthogonal directions. The APA had more than one phases in 45 (front-left) and 135 (front-right) degree target directions which were diagonal directions. In the biomechanical point of view, the diagonal movement directions were more complicated than orthogonal movement directions. The orthogonal movement direction required the control of simple anterior-posterior or medio-lateral shifts of COP. However, the diagonal movement directions required simultaneously controlling of anterior-posterior and medio-lateral shifting of COP in a proportional manner. [PERSON] et al. [44] proposed a postural model control in the shift of COP. The ankle strategy which was dominated by ankle muscles operated the anterior-posterior (90 degrees) direction; whereas the hip strategy dominated by hip abductor/adductor muscles [46] operated the medio-lateral (0 and 180 degrees) direction. The 45 and 135 degrees direction of COP might need coordination of both ankle and hip strategies and, thus, generate more than one phases in the APA. Weight shifting to diagonal directions might me more related to gait initiation. Previous studies showed that, during gait initiation, there are also two phases of APA. The COP transit in lateral and posterior directions toward the swing foot heel to form the first phase of APA, and then shift laterally toward the stance foot to form the second phase of APA [43; 44; 45]. Researchers suggested that the second phase of APA could increase the efficiency for gait initiation which reduced the load on the swing leg and are necessary for forward progression [45; 47; 48]. Whether the APAs detected in the diagonal weight shifting directions correlates with gait initiation in patients is suggested to be studied in the future. Clinically, forward reaching task is commonly used in evaluating the balance ability in health and diseases [49; 50]. However, [PERSON] et al. reported that, for PD patients, the forward reaching did not identify every person at risk of fall and suggested other evaluations should be added to increase the sensitivity [49].