| Summary: | This study aimed to systematically review the effects of the different velocity loss (VL) thresholds during resistance training (RT) on strength and athletic adaptations. The VL was analyzed as both a categorical and continuous variable. For the categorical analysis, individual VL thresholds were divided into Low-Mod<sub>VL</sub> (≤ 25% VL) or Mod-High<sub>VL</sub> (> 25% VL). The efficacy of these VL thresholds was examined using between-group (Low-Mod<sub>VL</sub> vs. Mod-High<sub>VL</sub>) and within-group (pre–post effects in each group) analyses. For the continuous analysis, the relationship (R<sup>2</sup>) between each individual VL threshold and its respective effect size (ES) in each outcome was examined. Ten studies (308 resistance-trained young men) were finally included. The Low-Mod<sub>VL</sub> group trained using a significantly (<i>p</i> ≤ 0.001) lower VL (16.1 ± 6.2 vs. 39.8 ± 9.0%) and volume (212.0 ± 102.3 vs. 384.0 ± 95.0 repetitions) compared with Mod-High<sub>VL</sub>. Between-group analyses yielded higher efficacy of Low-Mod<sub>VL</sub> over Mod-High<sub>VL</sub> to increase performance against low (ES = 0.31, <i>p</i> = 0.01) and moderate/high loads (ES = 0.21, <i>p</i> = 0.07). Within-group analyses revealed superior effects after training using Low-Mod<sub>VL</sub> thresholds in all strength (Low-Mod<sub>VL</sub>, ES = 0.79–2.39 vs. Mod-High<sub>VL</sub>, ES = 0.59–1.91) and athletic (Low-Mod<sub>VL</sub>, ES = 0.35–0.59 vs. Mod-High<sub>VL</sub>, ES = 0.05–0.36) parameters. Relationship analyses showed that the adaptations produced decreased as the VL threshold increased, especially for the low loads (R<sup>2</sup> = 0.73, <i>p</i> = 0.01), local endurance (R<sup>2</sup> = 0.93, <i>p</i> = 0.04), and sprint ability (R<sup>2</sup> = 0.61, <i>p</i> = 0.06). These findings prove that low–moderate levels of intra-set fatigue (≤25% VL) are more effective and efficient stimuli than moderate–high levels (> 25% VL) to promote strength and athletic adaptations.
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