*2.1. Participants*

Forty subjects with a diagnosis of CTTH (age range 30–65 years, median 50.35, SD 10.12) and another forty healthy controls (HC) with no headache (age range 23–59 years, median 40.65, SD 10.51) were included in the study. Subjects with CTTH were recruited from the Neurology Department of the Virgen de la Victoria University Hospital in Malaga (Spain). The study was approved by the Ethics Committee of the University of Malaga. All subjects participated voluntarily and signed an informed consent form before inclusion. This study complies with the ethical criteria defined in the Declaration of Helsinki of 2014 and Organic Act March 2018, of 5 December, on the Protection of Personal Data and Guarantee of Digital Rights.

The diagnosis was made by a neurologist specialized in headache, following the criteria of the International Classification of Headache Disorders [1]. The electrophysiology study was performed by a specialist in clinical neurophysiology. Psychometric data were collected by a clinical neuropsychologist. All raters were blinded to the results of the other investigators. The data were analyzed by an independent evaluator.

The inclusion criteria for subjects with CTTH and healthy controls were: aged between 20–65 years and no psychotropic drugs or analgesics taken in the 72 h prior to the study. Individuals diagnosed with a CNS or PNS disorder or who might present technical skin and/or subcutaneous tissue issues that could make it difficult to stimulate or register sensory responses were excluded [28]. Patients with more than one type of headache (such as chronic tension-type headache and migraine) were not included.

## *2.2. Materials*

2.2.1. Electrophysiology Study

The electrophysiology study was performed with a Sierra Wave electromyography system, version 7 (Cadwell, WA, USA), with integrated electrical neurostimulator and disposable surface electrodes and dermal temperature probe from the same brand.
