## 2 Experiment Methods
### Data Collection  
As shown in Figure 1, this research divides the student respondent samples into four parts (BMI 1, BMI 2, BMI 3, and BMI 4) using the BMI Percentile Calculator of the Centers for Disease Control and Prevention [31]. BMI 1 represents underweight adolescents, with a BMI\(\%\) < 5 th percentile, and BMI 2 is characterized as a healthy weight: 5 th percentile \(\leq\) BMI\(\%\) < 85 th percentile. BMI 3 represents an overweight group: 85 th percentile \(\leq\) BMI\(\%\) < 95 th percentile. The BMI 4 group is considered obese, with a BMI\(\%\) \(\geq\) 95 th percentile. We collected walking data from freshman students in high school in order to collect controlled data. High school students have grown up enough to walk well, and they are less exposed to drugs and alcohol than adults. A total of 244 freshmen in high school participated in the study, and their walking data were collected by the mHealth application. Among the 244 students, 32 students reported having traumatic brain injury (TBI) experience, 46 students experienced pain while walking, and 7 students had difficulties in walking. The 74 students with pre-existing conditions that may have influenced their walking patterns, such as TBI, were eliminated from the study. Therefore, the walking data of 170 students among 244 freshmen were used for the research. Table 1 reflects the body mass index (BMI) for age (in months) chart for the freshmen, as suggested by the Centers for Disease Control and Prevention [32]. The students in the samples were 14 years old at the time of the data collection: October 2019. Thus, as presented in Table 1, we followed the BMIs for the age chart for 174.5 months old. Demographic information, along with the BMI categories of the student participants (n = 170), are displayed in Table 2. In terms of gender, 86 (50.59%) female and 84 (49.41%) male students participated in the research. The same number of students were under-weighted (BMI 1), representing 2.33% of female students and 2.38% of male students. More than 60% of students fell under the healthy weight group (BMI 3), 55 female students representing 63.95%, and 51 males representing 60.71%. The percentages of female and male students either overweight or obese are quite similar: 33.72% and 36.91%, respectively. However, for BMI 4, the number of male students with obesity was 17, which doubled the 8 female students in the same category. Among the student samples, White was the dominant race, consisting of more than 75%, followed by Hispanic (10.59%), and other races (7.06%). Similar to the gender, there were 83 students with a healthy weight in the White category, consisting of 62.41%. There were about 3% of White students who were in the underweight group, while there were 20 who were obese, consisting of 15.04%. All the Black students, a sample of three, fell in the overweight group (BMI 3). Among the 18 Hispanic students, 13 students were in the healthy weight group, BMI 2, while 4 students were in the overweight group. Half of the Asian students were in BMI 2, and there was one student each for both BMI 3 and BMI 4. The students of other races were those with more than two races. A total of 5 students (41.67%) of other races were of healthy weights, and the rest of them were in BMI 3 (33.33%) and BMI 4 (25%). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Gender** & **3\%** & **5\%** & **10\%** & **25\%** & **50\%** & **75\%** & **85\%** & **90\%** & **95\%** & **97\%** \\ \hline Female & 15.69 & 16.06 & 16.68 & 17.91 & 19.65 & 22.02 & 23.71 & 25.10 & 27.70 & 28.27 \\ \hline Male & 15.93 & 16.27 & 16.84 & 17.95 & 19.51 & 21.60 & 23.06 & 24.25 & 26.45 & 29.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: BMI percentages for 174.5 months old. Figure 1: CDC BMI Percentile Calculator for Child and Teen [31]. During the experiment, all students wore the smartphone in the pocket of a waistband located at the center of the body, as shown in Figure 2a.