Intensity matters: high-intensity interval resistance exercise induces greater immune activation than low-intensity resistance exercise 45 min post-workout

Abstract Purpose Acute physical exercise triggers a transient immune response. For endurance exercise, these effects are intensity-dependent. In contrast, the role of exercise intensity in resistance exercise (RE) and its contribution to immune modulation remains less understood. This randomised crossover trial aims to compare the acute effects of High-Intensity Interval Resistance Exercise (HIIRE) versus Low-Intensity Resistance Exercise (LIRE) on routine clinical cellular inflammatory markers—neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and systemic immune-inflammation index (SII)—in 24 healthy, RE-trained young men. Methods Measurements were taken at baseline (T 0 ), immediately post-exercise (T 1 ), and 45 min post-exercise (T 2 ). It was hypothesised that HIIRE would elicit a significantly greater increase in NLR, PLR, and SII from baseline (T 0 ) to post-exercise (T 1 and T 2 ) compared to LIRE. Results Baseline-adjusted ANCOVAs revealed significant interactions for cellular inflammatory markers ( η 2 p = .072; NLR: p < .001, η 2 p = .217; PLR: p < .001, η 2 p = .147; SII: p < .001, η 2 p = .204). Post hoc analyses locate this difference for NLR, PLR, and SII 45 min post-exercise, with greater increases of HIIRE. Conclusion The results suggest a complex immune response involving pro-inflammatory signalling and immune cell redistribution induced by acute RE, with intensity being a main driver. This underscores the pivotal role of intensity in modulating acute immune responses to RE, with potential implications for exercise prescription targeting inflammatory processes.