## 4 Discussion  
The objective of this study was to investigate whether the eccentric back-squat followed by 20 s and 2 min rest periods, as a preliminary conditioning stimulus, could improve vertical jumping performance. The findings of the present study indicated that the 2 min rest period after five eccentric half-squats at 85% did not improve SJ and CMJ performance variables. Multi-joint exercises, such as squats and half-squats, are considered effective conditioning stimuli to induce PAPE, leading to improved performance [2; 26]. In the present study, a half-squat leading the knee joint lower than 90\({}^{\circ}\) was used since it enables greater activation of the lower limb muscle groups [28]. However, our findings demonstrated that the jump tests were not improved after the conditioning stimulus. A previous study has shown that the optimal rest period between the conditioning stimulus and the test varies among individuals and seems to depend on factors such as age, sex, and training background [23]. In the present study, all the participants were male jumper athletes, with eleven of them being long jumpers and the rest of them being high jumpers. Moreover, the participants' age did not vary with a very small standard deviation. Therefore, age, sex, and training background could not be considered responsible for the observed findings. The sample size and experimental procedure used in the current study allow us to document the effects of the conditioning stimulus on this particular population. Given the above, it is reasonable to affirm that the effects of the specified protocol on jumping performance were recorded. Previous studies focusing on rest periods have demonstrated conflicting results. A rest period of 7-10 min is recommended to optimize power production following a conditioning stimulus. This is despite existing evidence that induction of post-activation potentiation (PAPE) can occur over a spectrum of rest durations, from short to long [23]. However, in the present study, a shorter rest period was used, which mimics the procedure during jumping events at the track and field. Our results showed that the 20 s and  
Figure 5: Representative (one subject) force\(-\)time curve of Countermovement Jump before (PRE), 20 s (POST1), and 2 min after the eccentric semi\(-\)squat (POST2). 2 min rest periods are quite short to lead to PAPE. The short rest period appears insufficient to induce a potentiation effect, as fatigue seems to prevail over potentiation during this time. However, after 2 min of rest, the emergence of a potentiation effect becomes increasingly evident, suggesting that a longer recovery interval is necessary to obtain the desired performance enhancement. While systematic strength training for jumping events may lead to resistance against muscle fatigue [19; 20], the present findings suggest that the 2 min rest period following eccentric half-squats is not sufficient to induce PAPE. Since subsequent power production and performance depend on the balance between potentiation and fatigue, one explanation for the lack of improvement could be the higher level of fatigue compared to the potentiating effect of the conditioning activity. A previous report [38] demonstrated that subjects with a higher percentage of fast-twitch muscle fibers induced a greater PAPE compared to predominantly slow-twitch subjects. However, they also experienced higher fatigue during maximal voluntary contractions [38]. These findings suggest that subjects with a high proportion of fast-twitch fibers may have greater PAPE but may be more prone to experiencing higher fatigue during conditioning when the rest period is short (2 min). Regarding the lower limbs and jumping ability, it was demonstrated that eccentric half-squats could induce the PAPE effect [29] but not after the specific stimulus and experimental procedure. According to the previous study, eccentric half-squats increased jumping height after the third minute of rest, with most participants maximizing their performance in the sixth minute [29].