## 4 Discussion  
The aim of the current study was to examine the acute effects of five DJs on leg and vertical stiffness and the related mechanical and kinematic parameters during treadmill running at a speed of 6.67 m\(\cdot\)s\({}^{-1}\). The main findings of the present study partially supported our hypothesis, indicating that the preconditioning activity of five consecutive DJs, 3 min prior to treadmill running at a velocity of 6.67 m\(\cdot\)s\({}^{-1}\), resulted in a significant improvement in the subsequent SL, FT, F\({}_{\text{max}}\), and \(\Delta\)y by 1.4%, 3.1%, 1.3%, 2.5%, respectively. This study confirms, in part, the findings of previous studies, in which kinematic changes [14] or positive changes were observed only in SF (1.7% [9] and 3.3% [15]) after a preload stimulus to elicit PAPE with pyometric exercises. However, there were methodological differences compared to the current study, since in the studies of [PERSON] et al. [9] and [PERSON] et al. [15], the running velocity was increasing over time during the acceleration phase. It should be mentioned that the running velocity is a function of the inverse interaction between SL and SF [29]. Therefore, in the current study, the increase in SL in the EC resulted in a decrease in SF, as the running velocity remained constant (6.67 m\(\cdot\)s\({}^{-1}\)) during the treadmill running. Additionally, the increase in SL together with the decrease in SF explains the longer FT which was achieved in the EC following the preconditioning exercise. Moreover, this study shows greater F\({}_{\text{max}}\) and \(\Delta\)y values, while the CT remain constant in the EC. The greater F\({}_{\text{max}}\) achieved during the mid-stance phase implies more efficient energy storage and release or more efficient motor unit force generation [2]. The increased F\({}_{\text{max}}\) was probably produced due to the longer FT together with the higher SL and \(\Delta\)y. According to [PERSON] et al. [30], the greater supporting forces at higher running velocities during the stance phase of the running gait were able to increase the forward displacement and achieve a longer FT to reposition the swing leg with the shorter CT. These findings are in line with previous research, which indicated that a greater vertical ground reaction impulse is affected by the FT and the distance covered by each step [29]. Furthermore, another previous study revealed an inverse relationship between SF and \(\Delta\)y [31]. The latter  
Figure 4: Pre and post percent differences in the experimental condition in individual running kinematic (**(A)**: FT; **(B)**: SF; **(C)**: SL) and mechanical ((**D**): F\({}_{\text{max}}\); **(E)**: \(\Delta\)y) parameters. may further explain the increase in the body's centre of mass excursion and the decrease in the SF in the EC. On the other hand, the \(\text{K}_{\text{leg}}\), \(\text{K}_{\text{vert}}\), \(\Delta\)L, and CT variables during the mid-stance phase were not significantly changed after the preconditioning activity. The latter may be explained by the fact that the CT has a substantial influence on \(\text{K}_{\text{leg}}\)[32]. However, it should be pointed out that CT is changing as the running velocity is increased, whereas \(\text{K}_{\text{leg}}\) remains nearly constant across various running velocities. In the present study, the running speed remains constant and thus may be considered as the critical factor which has affected the \(\text{K}_{\text{leg}}\) and CT. Furthermore, the \(\text{K}_{\text{leg}}\) in the present study was estimated during running performance. According to earlier investigations, athletes were able to adjust their \(\text{K}_{\text{leg}}\) in various conditions such as for different running surfaces [33] and different simulated gravity conditions [34] or after high-intensity sprint fatigue [35], maintaining an unaltered overall stiffness. The calculation of \(\text{K}_{\text{leg}}\) and \(\text{K}_{\text{vert}}\) is based on the spring-mass model.