###### Abstract
However, very few studies have examined the relationship between exercise modalities and EIH in persons with CNSLBP. Chronic pain conditions like CNSLBP are also increasingly associated with low-grade systemic inflammation [22]. For instance, elevated levels of pro-inflammatory cytokines interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-\(\alpha\)) have been associated with chronic pain states [23]. In pre-clinical studies, these cytokines have been demonstrated to sensitize nociceptors at the peripheral and central levels [24; 25]. As such, they represent potential therapeutic targets for pain modulation [26]. Interestingly, exercise induces significant acute inflammatory responses and can even lead to disproportional reactions in persons with chronic musculoskeletal pain [27; 28; 29]. However, it is unclear if this is associated with pain sensitivity outcomes (such as a disrupted EIH) in persons with CNSLBP. In addition, both inflammatory responses and pain sensitivity outcomes appear to be influenced by psychosocial factors in chronic pain disorders [30]. In particular, higher anxiety, pain catastrophizing, and depression have been associated with an altered inflammatory state in fibromyalgia and osteoarthritis [31; 32]. Likewise, sleep disturbances have been associated with higher circulating IL-6 levels in CLBP [33]. High-intensity interval training (HIIT) is an innovative ET modality that has been advocated as optimizing the treatment effects of ET. HIIT involves short bursts of vigorous exercise interspersed with periods of passive or active rest [34]. When performed systematically, it improves cardiovascular fitness along with multiple other positive outcomes on general health indicators [35]. Furthermore, recent systematic reviews have noted positive effects on diseases-related outcomes such as long-term pain intensity and disability, while providing interesting advantages in the treatment of chronic musculoskeletal pain populations including being time-efficient, increasing motivation, and limiting general exhaustion by the volume of the training [36; 37]. Additionally, research also indicates that HIIT may improve acute pain sensitivity more as several indications of a dose-response effect of exercise intensity and EIH have been found [17]. As such, it is tempting to speculate that HIIT is a promising therapeutic intervention for CNSLBP [38]. However, given its vigorous nature, HIIT might also provoke excessive inflammatory responses in chronic pain conditions. Some studies have exploredthe effects of HIIT on clinical pain and systemic inflammation [39], but there is a notable lack of research specifically addressing the acute effects of exercise intensity on these outcomes in individuals with CNSLBP. Therefore, this exploratory study aimed to investigate to what extent (1) a single HIIT protocol affects acute pain sensitivity and inflammatory parameters in persons with CNSLBP, (2) the acute effect of a single HIIT protocol on pain sensitivity and inflammatory markers differs between persons with CNSLBP and healthy controls (HC), (3) the acute effect of a single HIIT protocol on pain sensitivity and inflammatory markers in persons with CNSLBP differs from a single moderate-intensity continuous training (MICT) protocol, and (4) the acute effect of a single HIIT protocol on pain sensitivity is correlated to the inflammatory markers in persons with CNSLBP.