## 4 Discussion  
The purpose of this systematic review and meta-analysis was to understand whether different non sport-specific strength training, namely strength exercises that do not recall the sport-specific movements, could be related to best performances in specific endurance sports (running, cycling, triathlon and cross-country skiing). In general, the aerobic parameters (VO2 max or MAS) did not change from pre- to post-intervention, while the endurance performance increased. The gain in endurance performance, therefore, could only be linked to mechanical factors and these--as expected--were different in the different sports. In agreement with these findings, Authors [32; 33] showed that aerobic capacity was not inhibited by concurrent training (endurance and strength training). Rather, a better neuromuscular coordination delaying the onset of fatigue after strength intervention and a skeletal muscles changes in fiber composition might be the mechanism responsible for the increase of aerobic performance [34; 35; 36]. Therefore, we looked across literature for the type(s) of non-sport-specific strength training (MST, PST, EST and RST) reported as more efficient in increasing endurance ability, focusing on WE (running economy, cycling economy, double poling economy) (Figure 2) and endurance performance tests (TT, TTE and Po) (Figure 3). Even if different study designs and methodologies could possibly influence the results, the quality of the studies considered was acceptable, considering the PEDro score. In these studies, all NST intervention types impacted on performance (cf. with Figures 2 and 3, positive combined effect size), improving several physiological parameters relevant for endurance athletes. However, considering the aforementioned 4 endurance sports, we found data on the comparison between different types of NST only for running, whereas for the other disciplines only MST was considered. Indeed, maximal strength training is the most studied type of exercise among non-sport-specific strength training across the plentiful literature, while scarce data is available on plyometric, explosive or resistance strength training. Therefore, the data reported in this meta-analysis could help in prompting new studies on endurance sports. As reported in Figure 2, the work economy was positively affected by MST in general, which in running resulted more efficient than explosive [16] or resistance [17] training. However, when it was compared to plyometric [15] training, MST showed less efficacy in ameliorating running economy. This scarce effect of MST was evident also in cycling, as in all the studies reporting cycling economy values most of those resulted smaller than the overall effect [21; 23; 24; 25; 26; 28]. Therefore, it is reasonable to hypothesize that non-sport-specific strength training other than MST might be considered to improve work economy in cycling. By contrast, when cycling is considered as the component of triathlon discipline, its work economy seems positively affected by MST [30], as well as the running component [30]. Even if only one study on cross-country skiing was included in this meta-analysis, the MST was clearly able to improve work economy [31], suggesting that this type of non-sport-specific strength training might be effective in cross-country skiing athletes. As shown in Figure 3, the non-sport-specific strength training effects on endurance performance were variable. Among studies on running, only three reported data on endurance performance [15; 19; 20], considering pylometric or maximal strength training. MST resulted ineffective on endurance performance in running [20] as well as PST. This result of PST in endurance performance in running might be due to the low frequency [15] or time [19] of training protocols (see Table 3).