## 4 Discussion
As mentioned, the present combined training regimen did not induce any significant effect on the parameters determining the lower limb explosive performance. However, the current findings showed a small (\(p>0.05\)) improvement in maximum force, maximum impulse and maximum power in the CMJ task (1.0-2.7%) and a greater non-statistically significant change in mean power (+8.7%) in the SJ task, whereas jump height was decreased (\(-3.5\) to \(-3.9\)%). Quite a large number of studies have focused on jump height, the reason being that it is probably the most frequently chosen parameter for assessing lower limb explosive performance. Evidence of a moderate-to-large effect size of polyometric training on jump height has been previously reported, regardless of athletes' sex [43], and a small-to-large effect size has been found for female athletes [44]. In contrast with the present findings, a positive effect of short-term (6-8 weeks) polymetric multidimensional training has been shown in male [9; 10] and female soccer athletes [11; 12] on CMJ and SJ height. Recently, it was argued that interventions ranging from 400 to 600 min of total duration are required in order to achieve the maximization of the polyometric training effect on vertical jump height during adolescence [45]. Even if these suggested optimal intervention durations were scaled to our preadolescent athletes, the total duration of the polyometric part of the current study's combined intervention would still be considerably shorter, as it amounted to approximately 88 min. It is highly likely that the lesser, by 1/6 polyometric, training stimulus here, as compared to the recommended optimal one, accounts for the observed decrease in CMJ and SJ height. On the other hand, the results showed a tendency towards an improvement in maximum force, impulse and power in the CMJ and in mean power in the SJ, respectively. It has been theoretically and experimentally argued and examined in vivo for the vastus lateralis muscle that jump height does not constitute the most appropriate parameter for the assessment of the maximal force and power-generating capacity of the lower limbs in systematically trained individuals [46; 47]. The production of mechanical power during the propulsion phase of the jump, which will potentially affect the achieved jump height, is determined to a considerable extent by intrinsic neuromuscular mechanisms and, in particular, by the force-length, power-velocity and force-velocity potential. The athletes received specific instructions about how to execute the CMJ and SJ tasks with regard to fully extending their lower limb joints during the propulsion phase [38]. Assuming that their lower limb muscles were maximally activated, as they were also instructed to perform maximum-effort jumps, the lower limb joints' excursion corresponds to the distance over which muscles will generate force and mechanical power, and, thus, it could be hypothesized that all athletes operated on a similar portion of their individual force-length curve. Based on the proposed maximum dynamic output hypothesis, the optimum loading condition for maximizing power output during jumping is one's own body [48; 49]. It has been suggested that due to individual neuromuscular characteristics and training history,some athletes will need a positive load (that is, an additional load to their body mass) and some others a negative one (that is, an assistance to decrease loading) in order to ensure optimal loading conditions [46]. The athletes had similar training history since they trained in the same sport club during the last 2-3 years. While the assessment of biological maturation did not present any statistically significant difference, the inspection of individual values showed that nine (five male and four female athletes) out of the twenty athletes had already achieved their peak height velocity by approximately 0.7 years at the end of the training intervention.