## Case Study: Product Development
### System Validation
#### 6.4.2 Form of Activity Recognition  
One of the requirements, stated by the future users and stakeholders, was to detect a fall of the monitored person in a distinguishable way from the other activities such as standing, sitting, lying down and walking. In order to perform the activity recognition, the SMV and accelerations in the \(x\)-, \(y\)- and \(z\)-axis are measured, and based on the test data, the appropriate identification thresholds for each activity have been justified and set. To adjust the identification thresholds of a fall, the SMV and accelerations in the \(x\)-, \(y\)- and \(z\)-axis of fall tests of two volunteers were analyzed. Figure 11 presents an example of accelerations in the \(x\)-, \(y\)- and \(z\)-axis along with the SMV measurement of the forward fall test. As one can see, due to the characteristic peaks concerning the beginning of the fall and the contact with the floor, it is possible to justify such thresholds to recognize the fall. Moreover, the decreased levels in the \(y\)- and \(z\)-axis indicate that the person is lying, which also confirms a fall if at the initial instant, standing positions were recognized. The final fall test consisted of 350 different falls including forward, backward, lateral falls to left or right, fainting with rotation to the left or right side and tumbling preceded by flexing the knees, 50 times each case. Up to 342 falls were identified correctly, which gives a satisfying fall detection validity of 98%. As was predicted, the proposed system cannot directly distinguish between standing and sitting postures, which can be seen in Figure 12a,b, presenting the SMV along with accelerometer readings from the \(x\)-, \(y\)- and \(z\)-axes for standing and sitting activities, respectively. The signals of all four measured variables are not specific for different activities, and the noticeable changes around 0 g of the \(x\)- and \(y\)-axis in Figure 12a occur due to the natural movement of the body. Therefore, it proves that the observed differences are not sufficient to distinguish between these two activities. Figure 11: Forward fall test: accelerations in \(x\)-, \(y\)- and \(z\)-axis and SMV characteristic. The solution of the problem could be to identify the dynamic activities of sitting down and standing up along with lying down and getting up, instead of static activities of sitting or standing. The volunteers were asked to perform the activities of getting up and sitting down, shown in Figure 13a,b, respectively. As is visible, when the activity of getting up starts, the acceleration in the \(y\)-axis and hence SMV changes quickly, and when the volunteer straightens, both acceleration in the \(y\)-axis and SMV gently stabilize. During the sitting down activity, the curves are opposite, while bending, the change is mild, and at the end of sitting down, both acceleration in the \(y\)-axis and SMV changes quickly. These differences allow for finding the identification thresholds to distinguish between the activities of standing up and sitting down. Figure 14 shows the SMV and accelerometer readings for the activities of lying down on a bed or sofa and getting up from them. During the activity of lying down shown in Figure 14a, the acceleration in the \(z\)-axis rapidly decreases, and the acceleration of the \(y\)-axis sharply increases, which is caused by an orientation change of the accelerometer and shifting of an axis, which is most influenced by the gravity force. During the activity of getting up presented in Figure 14b, the acceleration curves of the \(z\)- and \(y\)-axes behave conversely; the \(z\)-axis rapidly increases; and the acceleration of the y-axis sharply decreases. These phenomena allow one to distinguish between those two activities. Figure 12: Accelerations in the \(x\)-, \(y\)- and \(z\)-axis and SMV readings during activities: (**a**) standing; (**b**) sitting. Figure 13: Accelerations in the \(x\)-, \(y\)- and \(z\)-axis and \(SMV\) readings during activities: (**a**) getting up and (**b**) sitting down.