###### Abstract  
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-ModVL (\(\leq\)25% VL) or Mod-HighVL (\(>\)25% VL). The efficacy of these VL thresholds was examined using between-group (Low-ModVL vs. Mod-HighVL) and within-group (pre-post effects in each group) analyses. For the continuous analysis, the relationship (R2) 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-ModVL group trained using a significantly (\(p\leq 0.001\)) lower VL (16.1 \(\pm\) 6.2 vs. 39.8 \(\pm\) 9.0%) and volume (212.0 \(\pm\) 102.3 vs. 384.0 \(\pm\) 95.0 repetitions) compared with Mod-HighVL. Between-group analyses yielded higher efficacy of Low-ModVL over Mod-HighVL to increase performance against low (ES = 0.31, \(p\) = 0.01) and moderate/high loads (ES = 0.21, \(p\) = 0.07). Within-group analyses revealed superior effects after training using Low-ModVL thresholds in all strength (Low-ModVL, ES = 0.79-2.39 vs. Mod-HighVL, ES = 0.59-1.91) and athletic (Low-ModVL, ES = 0.35-0.59 vs. Mod-HighVL, ES = 0.05-0.36) parameters. Relationship analyses showed that the adaptations produced decreased as the VL threshold increased, especially for the low loads (R2 = 0.73, \(p\) = 0.01), local endurance (R2 = 0.93, \(p\) = 0.04), and sprint ability (R2 = 0.61, \(p\) = 0.06). These findings prove that low-moderate levels of intra-set fatigue (\(\leq\)25% VL) are more effective and efficient stimuli than moderate-high levels (\(>\)25% VL) to promote strength and athletic adaptations.  
intra-set fatigue; strength training; sprint; jump; performance +
Footnote †: journal: Medical Image Computing  
dichotomous question (i.e., to reach muscle failure or not) limits to some extent the practical applications of their findings. For instance, considering that a lifter could perform ~19 repetitions against the 65% of his/her one-repetition maximum (1 RM) in the full squat exercise [14], the single instruction based on whether reaching muscle failure or not lacks sufficient concision to accurately program the training stimulus. On the other hand, while the knowledge concerning the influence of intra-set fatigue on strength capacity is limited, no study to date has systematically reviewed the literature to analyze the effects of this training variable on athletic performance. Therefore, it would be of great practical value to clear up the effects produced by the different levels of intra-set fatigue on strength and athletic adaptations, by gathering experimental studies programming this training variable through an accurate and objective methodology.  
In the last decade, the velocity loss (VL) approach has been found as the most accurate method to program a target intra-set fatigue [13]. This methodology relies on the monitorization of the progressive decline of the barbell velocity, which is caused by the reduction in the shortening velocity generated by a fatigue status [21; 22]. The validity of the VL as an accurate indicator to objectively assess the neuromuscular fatigue incurred is based on the strong relationship found between intra-set VL and parameters of mechanical and metabolic stress [15]. In general, these relationships proved that the greater the VL reached, the higher the mechanical and metabolic fatigue incurred within the training set.  
Therefore, considering the more accurate information provided by the VL approach over traditional methodologies used to set intra-set fatigue, this study aimed to systematically review the scientific evidence examining the effects of the different VL thresholds during RT on strength and athletic adaptations.