## 3 Results  
### Participants  
A total of 235 (\(n=235\)) participants with headache were screened for possible eligibility criteria. Of these, 208 patients (72% women) satisfied all the eligibility criteria, agreed to participate, and signed the written informed consent. The reasons for exclusion were co-morbid migraine (\(n=15\)), fibromyalgia (\(n=5\)), medication overuse headache (\(n=4\)), or previous whiplash (\(n=3\)). A total of 108 (\(n=108\), 52%) were classified as frequent episodic tension-type headache (FETTH), and 100 (48%) were classified as chronic tension-type headache (CTTH) accordingly to the ICHD-III diagnostic criteria. A total of 60 (42%) were currently taking prophylactic medication intake, i.e., amitriptyline, on a regular basis, but none fulfilled the diagnosis of medication overuse headache. The SC reveals three clusters with different distributions within the variables as visualized in Figure 3. To further analyze the differences of each cluster, the mean and standard deviation of each feature were computed for each cluster and compared (Table 2). By analyzing Table 2, the three clusters were identified and characterized: the first one (cluster number 2) included patients with high headache frequency (all CTTH) and the other two (clusters number 0 and 1) including individuals with less headache frequency (75% FETTH). In general, individuals of cluster 2 showed higher headache  
Figure 2: Plot of the first 15 eigenvalues, and application of the eigengap heuristic to select the optimal number of clusters. intensity, longer headache duration, more headache-related disability (HDI-P, HDI-E), widespread pressure pain hypersensitivity (lower PPTs and DPTs but higher pain-evoked DPT), worse sleep quality, higher number of active TrPs and lower health-related quality of life (SF-36), but between-cluster differences were heterogeneous. Figure 3: Plots of the distribution of the features for each of the three clusters. Gender feature has been represented as a bar plot at the end. The outline of each plot represents whether the corrected one-way ANOVA test was significant (green) or not (red) for that feature. Clusters 0 and 2 were significantly different for headache intensity, frequency, and duration, HADS-D, HDI-E, HDI-P, sleep quality, and SF-36 scores (all, \(p<0.01\)), but not for psychophysical outcomes such as PPTs and DPTs (all, \(p>0.254\)) and the number of active TrPs: patients within the cluster 2 exhibited worse headache features, higher depression and related-burden, and worse health-related quality of life, but similar widespread pressure pain sensitivity and active TrPs than those within cluster 0. Patients within cluster 2 showed significantly higher scores for headache frequency, HADS-D, HDI-E, HDI-P, sleep quality, SF-36 scores, widespread pressure pain sensitivity, e.g., PPTs and DPTs, and the number of active TrPs (all, \(p<0.01\)), but similar headache intensity (\(p=0.152\)) and headache duration (\(p=0.785\)) than those within cluster 1. Finally, participants from clusters 0 and 1 exhibited similar clinical (headache frequency: \(p=0.831\); intensity: \(p=0.770\); duration: \(p=0.123\)), psychological (HADS-A: \(p=0.956\); HADS-D: \(p=0.486\); HDI-P: \(p=0.075\); HDI-E: \(p=0.265\); sleep quality: \(p=0.808\)), and most health-related scores (SF-36 scores, P from 0.266 to 0.789). They differed significantly for sensory psychophysical outcomes, such as widespread PPTs and DPTs (all, \(p<0.001\)) and the number of active TrPs (\(p=0.045\)), bodily pain (\(p=0.003\)): individuals within cluster 0 were similar in all headache-related outcomes, but exhibited higher widespread pressure pain sensitivity (i.e., decreased PPTs and DPTs), higher number of active TrPs and higher bodily pain than those assigned to cluster 1.