## 2 Materials and Methods
### Instruments for Outcomes Measurement and Stratification
#### 2.3.2 Pressure Pain Threshold  
PPT assessment provides an objective measure of MPS and quantifies generalized mechanical hyperalgesia in individuals with FM [34; 35]. PPT is also a valid proxy to  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**General** & **Mean** & **SD** \\ \hline Age & 40.55 & 11.23 \\ Anthropometry & & \\ Height (cm) & 160.27 & 7.40 \\ Weight (kg) & 76.06 & 11.93 \\ \hline
**2016 ACR diagnostic criteria** & & **SD** \\ (**Range of Scores**) & **Mean** & **SD** \\ \hline WPI (0–19) & 11.82 & 3.52 \\ SS (0–12) & 8.09 & 1.70 \\ Fatigue (0–3) & 2.55 & 0.52 \\ Walking unrefreshed (0–3) & 2.36 & 0.51 \\ Cognitive symptoms (0–3) & 1.82 & 0.60 \\ Other symptoms (0–3) & 1.36 & 0.81 \\ \hline
**No. of Comorbidities** & **n** & **\%** \\ \hline
0 & 3 & 27.3 \\
1 & 5 & 45.5 \\
2 & 1 & 9.1 \\
3 & 1 & 9.1 \\
4 & 1 & 9.1 \\ \hline
**Physical Activity Level** & **n** & **\%** \\ \hline Sedentary & 5 & 45.5 \\ Lightly active & 4 & 36.4 \\ Moderately active & 2 & 18.2 \\ Vigorously active & 0 & 0 \\ \hline
**Educational Level** & **n** & **\%** \\ \hline Primary & 0 & 0 \\ Secondary & 2 & 18.2 \\ Technical & 5 & 45.5 \\ Professional & 2 & 18.2 \\ Postgraduate & 2 & 18.2 \\ \hline \hline \end{tabular} Abbreviations: SD: Standard deviation, ACR: American College of Rheumatology, WPI: Widespread Pain Index, SS: Symptom Severity Scale, n: Sample, %: percent.  
\end{table}
Table 1: Sociodemographic characteristics of the sample.  
evaluate the presence of exercise-induced hypoalgesia, often impaired in FM patients [22]. To this end, PPTs were measured at three anatomical sites based on previous protocols: (i) upper trapezius--assessed at the center of the muscle belly, specifically at the midpoint between the C7 vertebra and the acromion [36]; (ii) the lumbar spine--assessed at the erector spinae muscle, 2 cm lateral to the interspinous space between the L4 and L5 vertebrae [37]; and (iii) knee--assessed at the joint line, 2 to 3 cm medial to the inferior pole of the patella [38]. PPTs were measured in kg/cm\({}^{2}\) using a Wagner FPX 25 digital algometer (Wagner Instruments, Greenwich, CT, USA), which has a 1 cm diameter rubber probe. An experienced evaluator performed all measurements on the dominant side. PPT measurements were performed following previously validated protocols [37,38]: before each measurement, the evaluator gave the same instructions to the participants: \"I am going to apply pressure progressively on your neck/back/knee using this instrument. I want you to let me know when the sensation changes from comfortable pressure to mildly unpleasant pain\". PPTs were measured by placing the algometer perpendicular to the skin surface of the area to be assessed. Pressure was increased at a continuous rate of 0.5 kg/cm\({}^{2}\)/s. The evaluator stopped the measurement when the subject notified. Each site was measured three times, with a 3-min rest between trials. The mean of the three measurements at each site was used for data analysis. PPTs were assessed before the intervention, immediately after the first iVRE session (to quantify the exercise-induced hypoalgesia mechanism) [22], and at the end of the protocol.