## 4 Discussion
### Countermovement Jump Performance  
Theoretically, it has been proposed that a high or faster eccentric load may allow for the selective recruitment of high-threshold motor units, leading to increased force production in the subsequent concentric muscle action [50]. In this regard, the application of AEL during jumping with minimal disruption to the natural mechanics of the movement may increase the rate of eccentric force production and the momentum of the SSC, thus improving power production [3]. The group analysis indicated significant post-intervention improvements for jump height (\(\Delta\) 15%) in the AEL group and in the CMJ group (\(\Delta\) 3%). However, the analysis of Rs and NRs indicated that nine boxers for the AEL group exceeded the technical error threshold for jump height, while for the CMJ group, only three boxers did so. This is consistent with abundant research that has shown increases in jump height using AEL via jumping. For example, [PERSON] et al. [12] reported increases in jump height via a four-week AEL drop jump intervention in rugby players, although the volume was not reported; the players used a 20 kg AEL load via dumbbells. This is similar to results reported in a second investigation by [PERSON] et al. [13], where they reported increases in jump height in trained athletes through a three-week intervention with AEL drop jumps using a 20 kg load via dumbbells. The higher number of Rs in the AEL group compared to the CMJ group could be physiologically attributed to a greater activation of the motor cortex that compensates for the spinal inhibition that occurs during eccentric actions [51]. In this sense, the additional weight used during the eccentric phase and released at the beginning of the concentric phase may have generated a greater accumulation of elastic energy in the lower body, potentiating the concentric phase of each jump [52]. Therefore, the effectiveness of AEL through jumps could be dictated by the magnitude of the differences between eccentric and concentric loads, which may explain the lower number of Rs in the CMJ group [6]. Regarding CMJ propulsive impulse, the group analysis indicated significant improvements post-intervention (\(\Delta\) 8.7%) for the AEL group, similar to those reported in an acute intervention by [PERSON] et al. [8] using CMJ with AEL via elastic bands with 20% and 30% of body mass, where they found increases in net lower body impulse (\(\Delta\) 16.65%) in the 30% AEL condition compared to the CMJ condition using only their BM. As mentioned above, during the execution of a AEL jump, the additional weight can increase the load of the hip and knee muscles by enhancing the pre-stretch of the muscle tissue, generating a greater force production in the propulsive phase of the jump [53]. Consequently, the improvements in propulsive impulse in the AEL group could be justified by the greater elastic energy stored in the parallel elastic component, the series elastic component and titin, the stimulated muscle spindle reflexes, type Ia afferent nerves, and the greater recruitment of high-threshold motor units [54]. The analysis of Rs and NRs indicated that nine boxers for the AEL group exceeded the technical error threshold for concentric drive, while only two boxers for the CMJ group did so. Based on these findings, it is likely that a larger stimulus is needed during the CMJ condition to improve concentric drive, since CMJ without additional load was only beneficial for 30% of the boxers in the CMJ group. Regarding peak power and relative peak power of the CMJ, the group analysis indicated significant improvements post intervention (\(\Delta\) 6.7%) for the AEL group. This is similar to that reported by [PERSON] et al. [14] in elite volleyball players, observing increases in lower body power after five weeks of training performing CMJ with AEL loads of 40 kg for males and 20 kg for females via dumbbells compared to the group that only performed the CMJ with their own BM. This is in line with research such as that of [PERSON] et al.