## 4 Discussion
### Muscle Strength  
The present research revealed that Low-\(\text{Mod}_{\text{VL}}\) thresholds produced higher increments in muscle strength than \(\text{Mod}\text{-}\text{High}_{\text{VL}}\), especially at submaximal loads (Figures 2 and 3). On the other hand, although within-group analysis showed that 1 RM and local muscle endurance could also be favored by \(\text{Low-}\text{Mod}_{\text{VL}}\) thresholds (Table 4), no large between-group differences were found concerning these variables (Figures 4 and 5). These findings offer a wide view of the adaptative effects across the force-velocity relationship produced by different levels of intra-set fatigue during RT programs. To date, most interventions examining this aspect included the 1 RM as the single variable to evaluate changes in dynamic strength [9; 20]. Nevertheless, the 1 RM only informs about an individual point within the force-velocity relationship, which corresponds to the force applied to the maximal absolute load the athlete could lift once [41]. Hence, this parameter lacks information about the ability of the athlete to exert force to the other loads included within the force-velocity spectrum, especially to those he has to deal with on the field/track. Thus, the information provided by the 1 RM can be complemented by analyzing pre-post  
Figure 8: Between-group analysis comparing the effects of Low-\(\text{Mod}_{\text{VL}}\) (\(\leq\)25% intra-set VL) and Mod-\(\text{High}_{\text{VL}}\) (>25% intra-set VL) to promote changes in jump capacity. Green squares: ES for each individual comparison; Black diamond: pooled ESs. [PERSON] et al. [31]; [PERSON] et al. [35; 37; 38]; [PERSON] et al. [34; 40]. Figure 7: Between-group analysis comparing the effects of Low-\(\text{Mod}_{\text{VL}}\) (\(\leq\)25% intra-set VL) and Mod-\(\text{High}_{\text{VL}}\) (>25% intra-set VL) to promote changes in sprint capacity. Green squares: ES for each individual comparison; Black diamond: pooled ESs. [PERSON] et al. [31]; [PERSON] et al. [35; 37; 38]; [PERSON] et al. [34; 40]. changes in barbell velocity [42], which in turn would reflect the changes in the levels of force applied [43]. In this regard, we found that, although the effectiveness to increase the 1 RM did not largely differ between VL groups (Figure 4), performance at sub-maximal loads (i.e., low loads: \(\leq\)60% 1 RM; moderate/high loads: \(>\)60% 1 RM) were maximized by Low-Mod\({}_{\text{VL}}\) thresholds (Figures 2 and 3, Table 4). In particular, all between-group comparisons examining changes at low loads favored the Low-Mod\({}_{\text{VL}}\) group. Representing these results on a force-velocity curve (force = X-axis; velocity = Y-axis), a greater upward shift of this curve (especially at its medial and distal sections) was generated after training using low-moderate levels of intra-set fatigue. In practice, this fact suggests that those sports actions whose performance directly depends on the force (and so velocity) applied to the weight of the athlete (e.g., jumps, sprints, changes of direction) or of an object (e.g., ball or javelin) would benefit from using Low-Mod\({}_{\text{VL}}\) thresholds during RT. On the other hand, contrary to traditional beliefs supporting that higher training volume would produce greater enhancements in local muscle endurance [44], this review did not find large differences between Low-Mod\({}_{\text{VL}}\) and Mod-High\({}_{\text{VL}}\) thresholds to improve this capacity (Figure 5). Indeed, the within-group analysis showed superior effectiveness of Low-Mod\({}_{\text{VL}}\) thresholds (ES = 1.50 vs. 1.14, Table 4). Among other factors, this finding could be explained by the slightly higher increments in the 1 RM achieved by the Low-Mod\({}_{\text{VL}}\) group (Table 4). As has been found by previous studies [34; 40], changes in local muscle endurance are mainly related to the changes in 1 RM (r = 0.63 to 0.71, \(p<0.05\)).