## 4 Results: Analysis of the Integration of VR, AR, and MR in the Design Process
### Results of Usability Indicator Analysis
#### 4.1.2 User Performance  
User performance was evaluated from an ergonomic perspective, focusing on comfort and human error.  
The comfort assessment considered fatigue generation or decline in physical and cognitive performance caused by prolonged exposure to stress factors--in this case, the controllers (Meta Quest 3 headset, hand controllers) and the 'Gravity Sketch' interface. Key aspects included (1) physical fatigue resulting from maintaining strained postures (head, neck, and shoulders) and repetitive movements (arms and wrists), which caused muscle overload; (2) visual fatigue, associated with sustained eye concentration in the digital environment of 'Gravity Sketch', which led to dizziness, headaches, and loss of focus during extended modelling sessions. Fatigue was exacerbated by the initial lack of familiarity of the designer with immersive environments, and these symptoms limited effective work duration, requiring interruptions or breaks in all Design Units. However, this difficulty gradually decreased as experience with the system increased.  
Human error was evaluated using a composite indicator that integrates various factors related to user performance during VR design sessions (Table 6). The definition and rationale of each variable included in the human error (HE) index are detailed in the Methods Section. Successful completions (tasks completed correctly) and different types of errors and difficulties were considered, including failures, restarts due to serious errors, interruptions caused by interface problems, loss of work control, and the time spent correcting errors relative to total task time. The results show a high rate of iteration, correction, and task modification during the design process. The ability to undo actions and edit directly in VR/AR models reduces the severity of errors, although it does not reduce their frequency. In most Design Units (such as the ring, candle holder, vehicle, mouse, or outdoor chair), multiple interruptions and partial restarts were recorded; these situations were more frequent during the learning phase. This reflects a great dependence on mastering the interface to ensure smooth interaction. Although technology allows quick error recovery, interruptions due to lack of knowledge of functions, export difficulties, or loss of model control were recurrent. The high proportion of tasks not successfully completed, compared to those completed, reveals navigation difficulties within the interface and loss of control over the model, attributed to disconnection from the physical reality and low cognitive adaptation to fully virtual environments. In most Design Units, solutions were not achieved linearly, but through iterative trial-and-error processes. Although the time spent correcting technical errors was relatively low compared to the total work time, the accumulation of failures, restarts, and unsatisfactory decisions directly affected the  
Figure 4: Verification of dimensional accuracy with Gravity Sketch: (**a**) ring anthropometric adjusting with hand model; (**b**) ring 3D printing; (**c**) ring dimensional accuracy; (**d**) environment scale validation; (**e**) candle holder real geometry adjustment; (**f**) candle holder 3D printing; (**g**) dimensional validation; (**h**) candle holder dimensional accuracy.  
efficiency of the process. However, this characteristic also promoted creativity by turning errors into opportunities for new ideas (as happened in the design of the reading chair and the high stool).