## 4 Discussion
### Physiological Effects  
The maximality of the trials was controlled by VO\({}_{2}\) levelling-off (i.e., three consecutive points with an increase \(<\)100 mL/min), blood lactate at the end of every trial (13.2 \(\pm\) 4.2 mmol\({}^{-1}\)), and peak RER (1.08 \(\pm\) 0.08). Despite the fact peak blood lactate and RER showed no significant differences between the three studied conditions, it must be acknowledged that \(\sim\)60% of participants failed to achieve all three maximality criterion (i.e., VO\({}_{2}\) plateau, [La+] \(\geq\) 8 mMol\(\cdot\)L\({}^{-1}\) or RER \(\geq\) 1.10) and a test was taken as valid if volitional exhaustion was reached under the other two conditions. Endurance training has been linked to a reduction in the RER at a given running speed, indicating an increased reliance on lipids as the energy source for the exercise [35]. Therefore, the somewhat low RER values reported here could be partially explained as an adaptation to endurance training.  
Strikingly, the VO\({}_{2}\) peak was significantly lower as the additional load increased (\(p=0.008\); ES = 0.29), particularly in the +10% BM condition. In this regard, [PERSON] and [PERSON] [36] raised the question about VO\({}_{2}\)max determination and its consideration as a gold standard measurement of cardiopulmonary-muscle oxidative function. Their main concern was about the arbitrarily selected cut-off values to confirm the maximality of the tests. Thus, many researchers opted for incorporating confirmatory tests and assumed the higher VO\({}_{2}\)max as the true maximal value [36]. Interestingly, these authors confirmed previous evidence pointing that shorter duration tests tend to underestimate VO\({}_{2}\)max calculation [37]. A possible explanation for this finding could be that the increase in workload produces an elevation in the RER for the same VO\({}_{2}\) level. In fact, when the work rate increases, the HCO\({}_{3}\)--buffering of H+ ions derived from lactic acid will become faster. This means that the rate of CO\({}_{2}\) output may be greater, because it tracks the rate of H+ buffering, without a necessary increase in VO\({}_{2}\)[37]. Another explanation would be a shift towards peripheral fatigue due to overloading [38], although volitional aspects related tovest discomfort could be a confounding factor.  
Of note, RER and rVO\({}_{2}\) (%) at VT1 and VT2 did not change significatively throughout the trials, whereas CoT experimented a slight increase at VT2 due to extra load (\(p=0.013\); ES = 0.27). Again, the increase in CoT was particularly observable in the +10% BM condition, and arguably it may be due to the lower peak VO\({}_{2}\) values reported under this condition. Previous evidence on the running CoT is inconsistent [39; 40; 35]. On the one hand, it is known that fatigue induces a degradation of CoT [40], while other authors argue for the existence of a quadratic trend with increasing speed [39], such that an optimal CoT is reached at intermediate speeds. Regarding overloaded running, a previous study [11] also found that running with light loads (i.e., +5% BM) increased leg stiffness, while moderate loads (i.e., +10% BM) prevented the optimization of elastic energy reutilization. In this context, greater leg stiffness has been shown to correlate highly with a lower CoT and improved performance [41].