## 4 Experimental Study
### The Experimental Protocol for User Testing  
The proposed method for the ergonomic assessment as described in Section 3 has been adjusted to match the needs of the specific case study. A detailed assessment protocol has been defined to successfully apply the proposed method to the selected case and to make data collection feasible. Figure 3: Case study mock-up replicated in lab and testing workstation. Figure 2: Product layout and tractor engine part involved in the case study. Tests were executed involving one user at a time. Firstly, a brief pre-test questionnaire had been defined and administered to the user to collect demographic information and to understand the familiarity with devices and operations performed during the test. A specific code had been assigned to each participant to keep the data anonymous. In particular, the survey allowed the analysis of:  
1. Demographic data such as gender, age, weight, height, educational qualification, occupational role;
2. Previous experience with manual working tasks like the ones performed during the trial (e.g., use of wrenches, screwing, working with electrical wires, etc. );
3. Familiarity with the use of wearable devices (e.g., activity trackers, smartwatches, sensorised t-shirts, etc.). After the demographic survey, an instructional video was shown to the user in order to carefully explain the sequence of tasks that must be fulfilled during the trial. After that, the assessment protocol was presented, as shown in Figure 4, followed by a Q&A session to clarify any eventual doubts on the various testing phases. In order to better discriminate between different levels of stress and cognitive load that may be experienced during the task execution, the protocol was organized into three different phases, dividing the entire task sequence into three parts. Each part provided a different level of cognitive workload, in order to test the human parameters variation and to check the robustness of the task design. Users were not aware of the improved or reduced level of workload created during each phase: only the researchers knew. When the procedure was clear and understood, the test could begin. Figure 4: Workload assessment protocol. Subsequently, each participant was assisted in wearing the smart devices for the motion capture and monitoring of human physiological parameters. The motion capture was used to track the user's full-body movements during the execution of all subtasks to analyze the physical risk and detect any improper and dangerous postures. Biometric monitoring was performed from the beginning until the end of the test, both during the tasks execution and while answering the questionnaires, to support the objective analysis of physical and cognitive conditions. The overall duration of the test for each user was about thirty minutes, and the temporal length of the experimental procedure was similar to the one needed for the engine oil filter replacement at the shop floor. The entire procedure was video-recorded using two cameras: an external camera in a fixed position, which provided an observer's viewpoint, and the eye-tracking camera mounted on eye-tracking glasses, which provided the users' viewpoint. Three-minutes of signal recording from the wearable sensors (i.e., ECG data, EDA, eye-tracking data) in resting condition (upright, being still) were established at the beginning and at the end of the task execution to, respectively, analyze the baseline and the supposed variations in physiological signals, before and after performing the tasks. Also, in these two slots, the NAS assessment was requested about the level of perceived stress. NAS 0 is the assessment at the beginning, collected during the baseline before any task execution, while NAS 4 is the assessment at the end during resting after the entire task execution. Such NAS measures are useful when evaluating the trend of the perceived stress before, during, and after the execution of the different tasks, and thus to compare the perceived stress between restful and stressful conditions.