## 4 Discussion
Likewise, two athletes jumping from the same HF may not be exposed to the same relative intensity [6]. These findings are consistent with other studies [3; 23; 24; 25] that found no differences in changes in HJ and GCT variables for HFs between 30 and 60 cm. Similarly, during the assessment of the RSI in one study [26], the researchers instructed athletes to maintain the GCT below 250 ms; otherwise, the repetition should be repeated. However, it was not considered that the RSI is highly modifiable, and it depends a lot on the technique. Athletes that showed a decrease in the contact time may compromise jump height [12; 27]. This could explain the results of a study [12] with national and international sprimters that showed considerably good values for GCT but very poor values for JH and RSI performing a DJ30 (GCT = 0.170 \(\pm\) 0.028; HJ = 0.340 \(\pm\) 0.049; RSI = 2.06 \(\pm\) 0.043). In the current study, athletes were asked to maintain the correct relationship between JH and GCT, as it was reported by others [8; 27]. Following these indications, we found that both groups showed superior results in the RSI than previously reported with sprint athletes [12]. Although we did not find significant differences between the groups, the higher RSI value was founded in DJ50. We also report that, during DJ60, the 100 m group showed an RSI loss of only 0.3 m\(\cdot\)s\({}^{-1}\) relative to DJ50, while the 400 m group showed a loss of 0.14 m\(\cdot\)s\({}^{-1}\) for the same FH. Based on two issues raised in the previous paragraphs, the following aspects obtain: (A) \"To date, we have not found in the available literature a general accepted criterion to end the DJ tasks when the RSI drops in a determined value. (B) Furthermore, higher HF increases neuromuscular pre-activation levels and the speed that can be achieved in the eccentric phase, thus increasing the contractile potentiation mechanisms of muscles and tendons [9]\". In this sense, also, it may be implausible to accept that the reactive strength remains the same or relatively the same as the HF increases. The RSI orients us to stimulate the reactive strength on previously adapted zones and not on zones that could manifest a greater specificity for high-performance athletes [22]. Therefore, the integration of RC with the RSI allowed us to develop the IRSI, a more sensitive metric to detect changes in reactive force in relation to HF for the athletes (Figure 2). It is well-known that an athlete can manifest the same RSI for different drop heights [9], and this should be taken as something other than correct. The IRSI could be an alternative solution in high-performance sports, specifically to train and measure the reactive strength during the DJ. The IRSI has its foundations in the HJ, HF, and GCT. Therefore, this variable informs about the increase in reactive strength as a function of HF. Thus, athletes who increase HF while maintaining the same GCT could slightly decrease HJ, not seeing the IRSI affected, in opposition to what occurred with the RSI. Similarly, athletes who show a slight increase in GCT but maintain HJ will not see the IRSI affected, opposite to the RSI. Between DJ50-60, the RSI can show a difference of only a 0.3 m\(\cdot\)s\({}^{-1}\) decline, as in the 100 m group, or a 0.14 m\(\cdot\)s\({}^{-1}\) decline, as in the case of the 400 m group (Table 2). There may need to be a sufficient reason to end the task when the IRSI shows that it still maintains an acceptable level of reactivity considering the HF (Figure 2). Another important issue of the IRSI use is that coaches can observe the critical zone of the reactive strength work. The critical zone of the IRSI could be established when their values are larger than 0.10 m\(\cdot\)s\({}^{-1}\) with a small ES, compared to the adjacent incremental HF. In Figure 2, it is observed in each group that the DJ20-30, DJ30-40, and DJ40-50 sets present significant differences in the IRSI, and their effect sizes are large (ES \(>\) 0.80).