## 4 Discussion
Likewise, in the present study, the \"duty factor\" also shows a negative relationship with speed (Figure 2). Furthermore, almost perfect and very large correlations were found between this variable and performance in the pre-test, explaining 93% of the performance in VT1 and 72% in VT2; in the post-test, this variable explains 49% in VT1 and 40% in VT2 (Table 4). Small and trivial correlations were found between duty factor and performance in MAS, maybe because it is known that, from a certain level of stress, the step frequency increases markedly, and the amplitude flattens out [53]. Concerning CT, in the same way as in previous investigations [16], controversial results are found, depending on the speed at which the CT is analyzed, but emphasizing the large ESs in all physiological intensities. In VT1, CT is shorter in the post than pre-test. It is suggested that short ground-contact times incur a high metabolic cost because faster force production is required, meaning metabolically expensive fast twitch muscle fibers are recruited [54; 55]. In contrast, in VT2 and MAS, the CT is longer. Conversely, long ground-contact times may incur a high metabolic cost because the force is produced slowly, meaning longer braking phases when runners undergo deceleration [56], as in the post-test in comparison to the pre-test in VT2 (3.38 ms) and MAS (4.8 ms). Small and moderate ESs were found in stiffness, VO, cadence and FT. On one hand, it is known that fatiguing runs to volitional exhaustion have led to reductions in leg stiffness [57; 58], as in the post-test. Moreover, leg stiffness was higher in all physiological intensities in the pre-test than in the post-test. These non-improvements in leg stiffness are supported by some studies showing that stiffness has not increased and even decreases after 8 weeks of concurrent pyometric and running training in novice runners [19]. On the other hand, it is worth noting the negative correlation between VO and speed in both the pre- and post-test is because a lower VO could make the athlete more efficient [59] and decreases in VO leads to improving the running economy [60]. Hence, increasing VO leads to increases in VO\({}_{2}\)[61]. For this reason, a running technique that does not lead to VO\({}_{2}\) is important. For example, the running technique characteristic of high-performance triathletes could explain why they can maintain a good technique at high speeds similar to a competition pace. [PERSON] et al. [62] point out that this is what happens when individuals run barefoot, probably due to a smaller vertical displacement during stance. In the investigation of the differences in barefoot-shot [62], it has been shown that the higher oxygen consumption in shed running compared to barefoot running is attributed to the additional mass of the shoe, and that changes in VO\({}_{2}\) may be more attributed to the additional mass associated with shoes than to the mechanical alterations in the running pattern. However, in elite triathletes, this decrease in VO as speed increases could be explained by the good running technique at high speeds (competition pace) rather than by the type of shoes. Finally, the distance of the competition or the age matters little to affirm that the optimal cadence is above 170 spm [63]. This is supported by previous research in which trained runners have run at the speed of jogging, during volumetric training and not a competition, and although they decreased their cadences somewhat, they were maintained above 170 spm [64]. Specifically, this cadence is very similar to that of the competition pace (VT2), both in the pre-test (172.2 \(\pm\) 2.18 spm) and in the post-test (171.2 \(\pm\) 1.85 spm). It should be kept in mind that runners appear to naturally choose a step frequency or step length that is economically optimal, or at least very near to being economically optimal [16].