## 2 Materials and Methods
### The Reliability of the Force Sensor  
The reliability of the FS was tested by measuring nine subjects with different weights. The participants were measured by using both a force platform in our gait lab and the FS Model 6A68D, respectively. The participants stood still on the FS or force platform and the weights were recorded for 5 s. The average weights from all participants were statistically compared as Table 3.  
In this study, we aimed to compare FS (Force Sensor) and FP (Force Platform) so that we could use FS in the experiment, as FP is widely considered a golden standard in biomechanics labs. The SECA scale used is an ordinary device, which has been used in our hospital for many years, and thus may have the unpredicted errors. The SECA readings were used as test data for two instruments (i.e., FS and FP) for the comparison between FS and FP rather than between FS/FP and SECA. In the test, the subjects could have stood on FS and FP to get two sets of readings directly.  
In Table 3, the equation of (A \(-\) B)\(\times\)100/A was used to calculate difference percentage. The mean of abs difference% between weight and FS or FP readings are roughly 0.88% or 0.85%. Two instruments gave similar differences consistently. These differences between the instruments and scale may come from the old scale. It is noted that the difference between FS and FP as the mean of abs difference% is 0.13%, which is reasonably good. In other words, FS is fine to be used in our experiment. Further, the FS and FP readings was used to run a \"reliability analysis\" in SPSS to get a [PERSON]'s Alpha 1.000 and Interclass Correlation Coefficient (ICC) as 1.0, meaning that the reliability is fine. The Pearson Correlation Coefficient was 1 as Figure 2.  
Following Table 3, the readings were further dealt with in Table 4. Using Table 4, we used a classical statistical method (Bland and Altman) ([PERSON] and [PERSON], 1968) to ascertain repeatability or agreement between the FS and FP readings, and the results show that the difference range of 95% CI (confidence interval) are between 0.81-1.96 \(\times\) 1.32 to 0.81 + 1.96 \(\times\) 1.32, i.e., \(-\)1.8  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**No.** & **Body** & **Weight** & **FS (N)** & **FP** & **W \& FS** & **W \& FS** & **W \& FP** & **FS \& FP** & **FS \& FP** \\
**Mass (kg)** & **(N)** & **FS (N)** & **(N)** & **diff\%** & **abs diff\%** & **diff\%** & **diff\%** & **abs\%** & **diff\%** & **abs diff\%** \\ \hline
1 & 50.9 & 499.33 & 477.4 & 476.9 & 4.39 & 4.39 & 4.49 & 4.49 & 0.11 & 0.11 \\
2 & 65.7 & 644.52 & 643.2 & 642.8 & 0.20 & 0.20 & 0.27 & 0.27 & 0.07 & 0.07 \\
3 & 67.4 & 661.19 & 664.7 & 661.2 & \(-\)0.53 & 0.53 & 0.00 & 0.00 & 0.53 & 0.53 \\
4 & 68.1 & 668.06 & 670.8 & 671 & \(-\)0.41 & 0.41 & \(-\)0.44 & 0.44 & \(-\)0.03 & 0.03 \\
5 & 71.7 & 703.38 & 695.7 & 695.9 & 1.09 & 1.09 & 1.06 & 1.06 & \(-\)0.03 & 0.03 \\
6 & 83 & 814.23 & 813.6 & 814 & 0.08 & 0.08 & 0.03 & 0.03 & \(-\)0.05 & 0.05 \\
7 & 90 & 882.9 & 876.8 & 876.4 & 0.69 & 0.69 & 0.74 & 0.74 & 0.04 & 0.04 \\
8 & 94.6 & 928.02 & 932.4 & 931.6 & \(-\)0.47 & 0.47 & \(-\)0.39 & 0.39 & 0.09 & 0.09 \\
9 & 103.6 & 1016.3 & 1016.5 & 1014 & \(-\)0.02 & 0.02 & 0.23 & 0.23 & 0.25 & 0.25 \\ \hline mean & 77.22 & 757.55 & 754.57 & 753.76 & 0.56 & 0.88 & 0.67 & 0.85 & 0.11 & 0.13 \\ SD & & & & 1.54 & 1.36 & 1.51 & 1.41 & 0.18 & 0.16 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Reliability of the force sensor.  
Figure 2: Linear correlation between FS and FP readings. The Pearson correlation coefficient is 0.999, \(p\) < 0.0001.  
to 3.39, where 0.81 and 1.32 are mean and SD of differences (Table 4). If the difference is related to the readings' means, we can estimate that the differences have 95% CI within 0.35%, i.e., (2 \(\times\) 1.32 \(\times\) 100/((754.56 + 753.75)/2)), where 754.56 and 753.75 are the means of FS and FP readings (Table 4). FS and FP show no significant difference in readings indicating that FS is a valid, reliable tool to be used in the data collection, like FP.