## 4 Experimental Study
### Experimental Data Monitoring  
The experimental set-up involved a set of hardware and software tools to collect the necessary data to properly apply the proposed method.  
About the hardware, the case study involved the use of the following tools, as shown in Figure 5:  
1. HTC Vive Trackers 3.0: they are small, interchangeable motion tracking accessories that can be attached to any part of the body, by proper straps, in order to achieve the motion capture of a joint or a body segment. Each tracker calculates its position based on the infrared signals emitted from a set of base stations that have to be properly positioned in the space. In this specific case, the user wears the trackers, according to a predefined configuration (i.e., two trackers on the arms, one on the belt and two on the legs).
2. Empatica E4: it is a wrist-wearable device able to record a set of physiological data of the user, using different types of sensors. In particular, the photoplethysmogram (PPG) sensor measures the Blood Volume Pulse (BVP), from which the heart rate (HR) and inter beat interval (IBI) can be derived. Moreover, the electrodermal activity (EDA) sensor measures the changes in skin conductance resulting from the sympathetic nervous system activity. The device also has a 3-axis accelerometer and an infrared thermopile.
3. Tobii Pro Glasses 2: this is a mobile, wearable eye tracking system that looks like a pair of glasses, equipped with four infrared cameras (two cameras for each eye) that records eye movements. The system consists of the head unit (glasses) and the recording unit, in which are stored batteries, and an SD memory card that is usually hung on the belt. A full HD camera in the head unit provides the user point of view. Glasses are also equipped with a microphone, accelerometers, and gyroscopes to track the head movements.
4. External camera: this provides the video recording of the user and the workspace environment from a fixed position.  
Figure 5: Human monitoring devices used during user testing.  
Furthermore, the case study exploits different tools:  
1. XRErgo: a standalone application developed in Unity 3D, which allows the evaluation of the users' postures and working conditions in both real environments and virtual simulations with motion capture. In the case study, it exploits the HTC Vive trackers as the motion capture system to create a digital twin of the operator, animated in real-time, and assesses postural risk by _RULA_ method.
2. Steam VR: software that streams the real time position of the Vive Trackers in XRErgo attached to arms, legs and back in XRErgo in order to perform the ergonomic analysis.
3. E4 realtime: an application for the real time streaming of data from Empatica to a smartphone or a tablet. This is used to control Empatica E4 calibration and data recording. The application automatically uploads to the cloud the data collected during the tests, to be further analyzed and processed.
4. Kubios HRV Standard: this tool provides a detailed HRV analysis to calculate _RR_ data from HR and HRV data collected during the tests.  
The temperature in the room was measured and was constant at 24 \({}^{\circ}\)C, thanks to the air conditioning system. The room does not have windows and during the task execution the doors were closed so the light sources are totally artificial, provided by neon lamps positioned on the ceiling. During task execution in the laboratory there was silence, and there were no other sources of noise.