# Validation of Smartphone Sway Analysis for Fall Prevention
## 2 Materials and Methods
### Data Collection  
Each subject stood with their arms at their side, relaxed, shoes removed with socks in the middle of the force plate, equidistant to its edges. Each subject was allowed to select their own foot placement on the force plate, since it has been shown that self-selection of foot placement serves to stabilize posture [15]. The subjects had a phone attached with a Velcro strap to their left tibia (Figure 1). All of the available literature cited in this paper relied on the smartphone being placed in the areas of the hip, lower back or breast of the subject. Similar positions were tested during preparation for this research, and it was found that these positions could not faithfully represent the position of the body's center of pressure, since during sway one tends to bend the body at the hip and knee joints to maintain better equilibrium; therefore, the tibia position was selected, as shown in the Figure 1.  
The phone was oriented upright and made to face forward, towards the front of the plate, so that the front of the plate was in the medial-lateral direction and oriented in a similar direction as the subject standing on the plate, as well as being away from the subject's tibia so that the phone application could be accessed at about the middle of the subject's left tibia both horizontally and vertically.  
The subject then closed their eyes and the app on the phone and the TekScan (307 W First St, Boston, MA 02127, USA) force plate (MatScan(tm) pressure mat\") began to record data when simultaneously activated by the researcher. The force plate data was recorded by TekScan's software, FootResearch 5.2.3, supplied by the TekScan, Inc. The sex, height, and weight of the subjects were recorded, as was the phone's position in relation to the ground. The phone application was developed in an Android Studio 4.1.3, SDK 15 and uploaded onto a Samsung Galaxy j7 Pro phone for use in data collection.  
The data acquisition length was set to 70 s. The data recorded from the smartphone application was saved as an ASCII file for later processing. The position of the resultant vertical force (Center of Pressure) on the force plate between the subject and the plate was extracted from the data recorded by the force plate and was saved as an ASCII file for data processing. The sampling rate was 50 Hz. Data from the application was recorded as angular orientation of the mounted phone. By using the distance of the phone was from the ground, its linear movement was calculated in both anterior-posterior (AP) and medial-lateral (LM) directions. This linear movement of the phone and the mid tibia, since the phone was attached to it, was assumed to be a representation of the center of pressure changes. The absolute position of the center of pressure between the foot and ground were not of interest in this work since we were looking for a correlation between the actual  
Figure 1: Subject standing on the force plate with the smartphone attached to the left tibia during data acquisition.  
center of pressure as measured by the force plate and the angular data as recorded by the smartphone.