## 1 Introduction  
Fibromyalgiagia (FM) is a syndrome characterized by persistent widespread pain [1], often accompanied by other symptoms such as fatigue, muscle tenderness, cognitive disturbances, mood and sleep disorders, as well as physical symptoms, like dizziness, constipation, and nausea [2,3]. The prevalence of FM varies widely, ranging from 1.4% to 13.4% in the general population [4,5]. It more frequently affects women, who represent over 90% of diagnosed cases and have a higher risk of developing severe symptoms [6]. Although potential links to inflammatory, immunological, genetic, and psychosocial factors have been suggested, the exact causes and etiopathogenesis of FM remain unclear [7]. The clinical presentation of FM can lead to disability and reduced quality of life, significantly impacting daily activities and overall well-being [8,9]. Moreover, compared to other rheumatic conditions, such as rheumatoid arthritis, spondyloarthritis, and Sjogren's syndrome, FM has a more pronounced effect on quality of life; patients experience greater difficulties adapting to the condition, and often rely on less effective strategies to cope with pain [10]. Pain is one of the most common symptoms prompting FM patients to seek medical consultation due to its complexity, multidimensional nature, persistence, management challenges, and widespread impact [8]. These clinical characteristics of pain are associated with dysfunction in the neural circuits involved in the perception, transmission, and processing of nociceptive stimuli [7]. This dysfunction is reflected in a reduced ability to modulate pain through descending mechanisms, high concentrations of excitatory neurotransmitters in the nociceptive pathway, and changes in central and peripheral nervous system plasticity, contributing to sensitization within the nociceptive system, consistent with the concept of nicotinic pain [4,7,11]. While perceived pain intensity (PI) is commonly assessed using the numeric rating scale (NRS) [12], pressure pain thresholds (PPTs) provide a valuable tool for measuring mechanical pain sensitivity (MPS) in chronic pain conditions, such as FM [13]. Studies have shown that patients with FM exhibit significantly lower PPTs--indicative of mechanical hyperalgesia--compared to healthy controls [14,15]. This finding may be associated with increased pain sensitization [13,16], psychological alterations [17], and may also have a predictive relationship with PI [18]. Current clinical guidelines recommend a combination of pharmacological and non-pharmacological interventions, tailored to individual characteristics, to reduce symptom severity, improve quality of life, and help patients adapt to and coexist with a challenging and persistent condition [19,20]. Physical exercise is widely recommended as a first-line non-pharmacological therapeutic option for patients with FM, whether used alone or in conjunction with other interventions [20]. However, the literature indicates that exercise may initially worsen symptoms during the early stages of rehabilitation, potentially impacting adherence to physiotherapy protocols [20,21]. This issue is particularly pertinent given the complex interplay of factors, including a diminished hypoalgesic response to exercise [22], pain-avoidance behavior [23], and additional cognitive, motivational, and emotional challenges that FM patients must overcome to remain engaged in their rehabilitation [24]. Virtual reality (VR) has emerged as a promising therapeutic tool, offering various modalities, including non-immersive, semi-immersive, and fully immersive systems (iVR) [25,26,27]. Among these, iVR systems provide a more comprehensive multisensory experience by integrating visual, tactile, vestibular, and auditory stimuli, which enhances the sense of embodiment compared to less immersive systems [28]. Despite its potential, notable gaps remain in understanding how iVR influences pain management, particularly concerning the underlying pain mechanisms involved and its effectiveness relative to baseline prognostic risk [27].