FoF is defined as a lasting concern about falling that causes a person to limit or even stop the daily activities that he/she is capable of [4, 5]. Inspired by this definition, we used LoS, a postural task in the area of individual abilities, and the postural control strategies to objectively identify FoF. We calibrated the LoS task based on seventy-five percent of each participant's ability. Using the LoS task, we introduced a new postural stability-indicator to predict the level of FoF in PD patients. In this regard, with a new method for analyzing the CoP data, three indices, FTR1, FTR2, and FTR3 were introduced. Ultimately, the ratio between FTR1 and FTR2 (FTR1/2) showed a 92.1% overall accuracy to predict the level of FoF in participants.
The reliability of FTR1, FTR2, and FTR3 was tested with an ANOVA-based ICC model. Three different test conditions were used (levels of height). Our results showed that FTR1 and FTR2 had a high relative and absolute reliability in all conditions. Also, acceptable to high relative reliability and acceptable absolute reliability were obtained for FTR3. These results showed intra-session reliability for our measures. We, therefore, recommend researchers to select these measures in future research and assessment.
Kumar et al. showed a significant correlation between the FoF and functional balance (-0.97, p < 0.01) and mobility (0.95, p < 0.05) measures in the elderly population . Also, some studies in the PD population [16, 18] showed a correlation between them. PT and TUG had negative correlations with postural stability and BBS had a positive correlation with postural stability . Therefore, it can be concluded the low FoF group has better postural stability than the high FoF one. It is in agreement with previous studies, which confirmed a significant correlation between FoF and postural stability [15, 22, 23].
According to our results, FTR1 and FTR2 were strongly correlated with clinical postural stability measures. Also, FTR1 and FTR2 respectively had a negative and positive correlation with FES-I. Our results confirmed that participants with a prolonged presence of CoP in the RFA1 had better postural stability and lower FoF than those with a prolonged presence of CoP in the RFA2. It seems that patients with high FoF probably had lower accuracy in controlling and guiding their CoP toward targets and returning to the home position, due to insufficient postural stability; therefore, they spend more time in RFA2. In contrast, participants with low FoF probably had higher performance in controlling the CoP motion (sufficient postural stability). Therefore, they could quickly hit the targets and returned to the home position, and spent more time in RFA1. These results are supported by a previous study , which demonstrated that the directional control is poorer for PD patients in comparison to the healthy population. Regarding the HY stages being positively correlated with FoF (rs = 0.47, p < 0.001) , a possible reason for this similarity is that their participants , similar to high FoF participants in our study (mean ± SD of HY = 2.63 ± 0.5), were in HY stages of 2 to 3.
The effects of the threatening conditions on postural strategies
Previous studies investigated the behavioral correlations of FoF with increased heights ; therefore, we ran the LoS task at two other elevated levels to investigate preferred postural strategies in patients with low and high FoF.
It seems that participants in both groups modified their postural strategies in the high threat conditions (Fig. 1). According to the results of the repeated measure ANOVA, the low-FoF participants significantly decreased their presence time in RFA3 and spent more time in other areas (Fig. 1B). High FoF participants, in addition to decreased presence time in RFA3 and the total score of LoS task, significantly increased their presence time in the RFA1 (Fig. 1A, C). It indicates that in the challenging conditions, subjects of the high-FoF group attempt to reduce the peripheral mobility of the CoP and increase the presence time at home position. These results comply with the reduced spatial mobility in the FoF population, reported by previous studies [4, 5]. Jefferis et al.  showed that elderly men with FoF had lower excursions from the home position and more mobility difficulties than those without FoF. This observation shows that our perspective is in line with the concept of FoF.
Adkin et al.  mentioned that the central nervous system (CNS) progressively tightens the control of posture when the postural threat increases. Their study, accompanied by other research works [38, 39], well established that the CoP sway amplitude significantly decreases with increasing level of threat. In the present study, we ran the LoS task on different heights in an ascending order, which corresponds to the level of the threat. Therefore, the mentioned CNS’s changes in our participants are also expected. Some studies revealed the underlying neuromuscular strategy [37, 40]. They suggest that after increasing levels of threat, the CNS applies a ‘stiffening strategy’ [29, 37, 40] that leads to reducing the degree of freedom. In this strategy, reflexive muscle co-contractions occur around the ankle joint to maintain the body in the desired position in response to the threat [18, 41, 42]. In our study, participants were asked to use an ankle strategy rather than a hip strategy. It is possible that due to the effects of stiffening strategies around the ankle joint in threatening conditions, participants in the low and high FoF groups switched to a hip strategy for performing the LoS task. Therefore, we suggest future studies to identify the postural segmental strategies and the neuro-muscular pattern underlying this behavior by kinematic and electromyography devices.
One possible reason for inefficient postural adjustment to perform the task is poor movement planning . In a study on older adults with low and high FoF, participants in both groups showed an initial attentional bias toward fall-threatening words, compared to threatening words unrelated to falling . Zaback et al.  also reported the attention shifted to movement processes, threat stimuli, and self-regulation in a postural-threat condition. They concluded that these attention shifts are associated with changes in postural control . On the other hand, many researchers have interpreted stiffening strategies as an intuitive preparatory strategy for accommodating potential destabilizing situations [29, 37, 40, 45]. Based on the described evidences, we expected that in threatening conditions, a stiffening strategy was automatically developed in both groups of PD with low and high FoF. Regarding FTR1/2 and the total score of LoS (Fig. 1), it seems that CNS, based on the intensity of FoF, showed a different adaptation with a stiffening strategy. Participants with high FoF probably were unable to overcome the stiffening strategy, and it led to restricting themselves to complete the LoS task (hitting the targets) (Fig. 1C). Attentional control theory predicts that anxious people, due to failure in shifting attention from task-irrelevant toward task-relevant information, are unable to properly plan the movement . Based on independent t-test for MoCA and HADS-anxiety subscale (p = 0.08, p = 0.18, respectively), It seems that the cause for group differences in movement planning was unrelated to general cognitive or anxiety levels. According to the HADS-depression subscale (p = 0.04), scores of the high FoF participants were significantly higher than the low FoF ones. A study  reported that depression disorder is associated with neurocognitive changes related to coordinate motor output. Therefore, depressive symptoms may be the cause of improper motor planning in PD patients with high FoF. In line with this perception, Franzén et al. , for the first time, showed depressive symptoms as the strongest independent variable (β = 0.40, p < 0.001) to predict concerns about falling in PD. Depression is closely associated with PD [48, 49]. The development of depression in PD is more likely to be caused by the nigrostriatal pathway degeneration than the outcome of the awareness of the disease's prognosis . Motor symptoms of PD emerge when 50% of dopaminergic neurons degenerate, while depressive symptoms are prevalent even before the onset of motor symptoms [50, 51]. Some of the factors that consistently correlate with depression in PD include earlier-onset, advanced stage, psychiatric comorbidity (e.g. anxiety), and the presence of cognitive decline . It seems that by managing depression in the PD population, we can prevent additional syndrome, such as fear of falling. Cognitive-behavioral therapy showed satisfactory effects to control the FoF  and depression [51, 53-55] in PD, which needs further study. Furthermore, we suggest that researchers consider the level of depression while investigating the mechanism of FoF in participants with PD.
In the present study, for the first time, we investigated the behavior of PD patients in the LoS task in presence of height threat. Increasing the height should be above some threshold to impact the FTR’s. There were no significant changes in the FTR’s from the ground to the 20cm height (Fig. 1). Another study has considered 19cm as a low threat condition , which is in line with our findings. Although the highest threat level (40cm height) in this study was lower than those reported in the previous studies (140cm, 160cm, and 320cm) [36, 56], it left a significant effect on postural stability in the PD patients. Possible reasons are the multi-directional and dynamic nature of the LoS task and the simultaneous three-directional threat (setting the balance board on the front and side edges of the wooden platforms). Whereas, tasks in the previous studies often were done in the quiet stance, away from the edge or with a uni-directional threat [36, 56]. In agreement with our study, Yiou et al.  showed that simultaneous multi-direction threats affect postural stability in dynamic tasks even in lower heights. Another rationale for using these heights was to identify the lowest heights that could significantly change the postural stability in PD patients. This finding simplifies the future studies for performing a dynamic task in PD patients at a high threat elevation. For future studies, we suggest that researchers investigate the impact of height threat on postural stability for other pathological diseases or the elderly population.
Predicting the level of FoF
We demonstrated that by the proposed protocol on the ground level and FTR1/2 index, it is possible to predict the FoF level in PD subjects with an overall accuracy of 92.1% (Table 4). As mentioned above, the sufficiency of postural stability has a strong negative correlation with FoF. However, participants 3 and 6 (P3 and P6) of the low FoF group, and participant 26 (P26) of the high FoF group, based on FTR1/2, exhibited different behavioral strategies from their groups and fell in the opposite group. P3 and P6 had HY stages from 1 to 1.5 and had high postural stability, based on the objective postural stability measures (BBS score = 53 and 55, TUG = 7.07 and 5.97, and PT = 0). P26 was in HY stage of 3 and had postural instability and fall risk  based on clinical evidence (BBS = 43, TUG = 7.53 and PT = 1). As mentioned in methods, before LoS evaluation, participants learned the LoS task (6 repetitions). A previous study , confirmed the importance of repetition in the adaptation of emotional states. Therefore, P26 performed the LoS task in his secure LoS area, with sufficient postural stability similar to the low FoF participants. To confirm this result, using high threat conditions (40cm height), without pre-training at this height, we stimulated FoF in participants. Based on the different intensity of FoF in two groups, we expected different strategies, which were confirmed in the previous section. As we expected, based on FTR1/2-40cm, P26 was placed in the high FoF group (his report in FES-I). P6 was still identified as high FoF. He was available for a follow-up, and his FES-I score obtained eight months later, which indicated a sharp increase from 18 (baseline test) to 27. This suggests that his response to FES-I might have been biased at the baseline test or he had been developing FoF identified by FTR1/2. In other words, it seems that maladaptive high-FoF might have developed in the early stages of disease, before the clinical diagnosis of the postural instability. Others have also reported that PD influences the movement preparation phase, even before the clinical detection of postural instability ; The cause has been reported to be an injury to the basal ganglia, which leads to the loss of automatic selection and execution of motor plans [60, 61]. Therefore, The FTR1/2 index seems to have the potential to be a mechanical biomarker to sense FoF-related postural instability.
This work had some limitations which should be considered in the interpretations of the results: off drug state and female PD subjects were not included in this study.