# Validation of Smartphone Sway Analysis for Fall Prevention
## 4 Discussion  
Measurement of sway characteristics of an elderly is of great importance to evaluate his/her probability to have a fall. As mentioned above the currently accepted methods rely on the patient coming to the doctor's office and performing a sway test on a force plate. While this approach provides the necessary information, it is not a convenient way to assess the sway variables. The developed here approach for measurement of the sway variables by the smartphone application provided possibility to get the subject's sway information without need to come to the doctor's office. The results presented here show high correlation between a mobile device application and that of the long-accepted results from a force plate. A similar study in sports medicine had a correlation of 0.63 with regards to positional sway [9]. Recent study comparing force plate data with the commercially available smartphone application showed partial correlation between the two approaches [16]. Another study was done to compare results from the force plate-based analysis with a commercially available system [11] and only fair reliability was achieved in the range of 0.48 to 0.59, that is much lower than the one achieved in the present study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **APLP** & **APLA** & **AVP** & **AVA** \\ \hline APLP & 1.00 & 0.83 & 0.96 & 0.89 \\ APLA & & 1.00 & 0.73 & 0.92 \\ AVP & & & 1.00 & 0.85 \\ AVA & & & & 1.00 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Correlation results for the anterior-posterior direction (33 subjects).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **TPLP** & **TPLA** & **XTPLP** & **XTPLA** & **YTPLP** & **YTPLA** \\ \hline TPLP & 1.00 & 0.83 & 0.43 & 0.65 & 0.96 & 084 \\ TPLA & 1.00 & 0.39 & 0.85 & 0.80 & 0.99 \\ XTPLP & & 1.00 & 0.59 & 0.17 & 0.31 \\ XTPLA & & & 1.00 & 0.55 & 0.76 \\ YTPLP & & & & 1.00 & 0.83 \\ YTPLA & & & & 1.00 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Correlation results for the total path lengths (33 subjects).  
While there were differences in correlation depending on the direction, it is of less concern due to the increased risk of falls associated with swaying in the anterior-posterior direction [1]. In this direction, the correlation was found to be approximately 0.83, giving a much better correlation in the direction that falls are most associated with. The better correlation between the smartphone and force plate results than seen in previous works may be attributed to the smartphone placement at the tibia, thus providing more accurate information about the location of the center of pressure.  
Raw data shown in Figure 2 represents general shape of the plots as measured by the force plate and smartphone application. The 'crab-claw' peaks of the path length follow the similar general shape for the force plate and the application, this is supported by the overall strong correlations that have not been matched in other studies.  
Calculated path length of the center of pressure (COP) from the force plate and smartphone application were numerically significantly different, however they correlated well one with another, as seen from the Table 4, especially in AP direction. This is a direction that clinically corelated with falls in elderly.  
Since current study is based on the use of asymptomatic subjects aged 20 to 22 years the extrapolation to patient populations with balance impairments may not be appropriate. Further investigations will include the assessment of validity of these two approaches in these populations.