###### Abstract  
Chronic nonspecific low back pain (CNSLBP) might be associated with increased pain sensitivity and inflammation. High-intensity interval training (HIIT) has been suggested to reduce pain outcomes and inflammatory markers, but its effects compared to moderate-intensity continuous training (MICT) remain unclear. This study aimed to evaluate the acute effects of HIIT on pain sensitivity and inflammatory markers in persons with CNSLBP compared to healthy controls (HCs) and to determine how these effects differ from MICT. Twenty persons with CNSLBP and twenty HCs were assessed before (PRE) and after (POST) a single HIIT and MICT protocol for pain sensitivity (cuff pressure pain threshold (cPPT), temporal summation of pain (TS), conditioned pain modulation (CPM)), and inflammatory markers (IL-6, TNF-\(\alpha\)). Data were analyzed using one-way ANOVAs, paired \(t\)-tests, and correlation analyses. At PRE, persons with CNSLBP exhibited lower cPPT (\(28.2\pm 7.1\), \(\Delta=-5.5\), \(p=0.040\)), higher TS (\(1.11\pm 0.89\), \(\Delta=0.79\), \(p=0.042\)), and lower CPM (\(36.2\pm 11.6\), \(\Delta=-10.0\), \(p=0.023\)) compared to HCs. HIIT resulted in PRE-POST improvements in cPPT (\(38.9\pm 12.6\), \(\Delta=5.2\), \(p=0.019\)) in HCs. No PRE-POST differences were observed in pain processing in those with CLBP. No PRE or PRE-POST differences were observed in the inflammatory markers in either group. The current exploratory study suggests that a single HIIT session might have a beneficial effect on pain sensitivity in HCs but does not alter acute pain sensitivity or inflammatory markers in persons with CNSLBP. Further research is needed to clarify the involved mechanisms and explore the (relation with the) long-term effects of HIIT. chronic low back pain; exercise; high-intensity training; pain sensitivity; inflammation ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). 2025152918202515  
prolonged work absenteeism, and increased medical costs [3; 4; 5]. Consequently, CNSLBP is a pervasive societal issue consuming a substantial amount of healthcare resources [3]. Regular physical activity and structured exercise improve physical and psychological health [6; 7]. Moreover, various forms of exercise therapy (ET) have proven to be effective in relieving pain associated with musculoskeletal disorders [8; 9]. Hence, clinical practice guidelines, including those for persons with CNSLBP, systematically recommend ET as a therapeutic modality [10]. While ET might benefit CNSLBP, its pain-relieving effect is only approximately 15% and responses to exercise vary considerably [11; 12]. Additionally, other studies showed no change or even brief exacerbations of pain following exercise [13]. These 'flare-ups' of acute pain are believed to be related to increased pain sensitivity in people with musculoskeletal pain [11]. Exercise-induced hypoalgesia (EIH), referring to a short-term endogenous pain-inhibiting response directly after exercise, is well documented in healthy persons [14]. EIH is measured by changes in pain thresholds before and after exercise and is often assessed using pressure pain thresholds (PPTs). Pre-treatment EIH (i.e., as a pain sensitivity measure) has been linked with the pain-relieving therapy of exercise and therefore might act as an early indicator of a person's potential responsiveness to ET [15]. However, while EIH is influenced by exercise type, dose, and intensity [16; 17], the optimal intensity required to induce EIH in CNSLBP remains poorly understood [18]. Of interest, studies have shown that EIH can be affected in a variety of musculoskeletal disorders [13; 19; 20]. Likewise, persons with CNSLBP display alterations in pain sensitivity, including modified PPTs [21], which may, at least partly, explain individual differences in responses to acute exercise in this specific population [20].