## 4 Discussion  
The main findings of the current study suggest that the application of a 10% load of the participant's body mass during a sprint interval exercise consisting of two series of 3 \(\times\) 10 s all-out efforts (SIE\({}_{10\%}\)) provides a higher relative peak power output (Figure 3), achieved at a statistically significantly faster time, and a lower physiological cost (Figure 4), as measured by mean oxygen uptake and mean heart rate, compared to the control condition (SIE\({}_{7.5\%}\)) (Table 2, Figure 2). A further interesting observation is that a statistically significantly higher mean heart rate was achieved during both the first and second series at SIE\({}_{10\%}\), with significantly more effective restitution of HR (Figure 2A) and VO\({}_{2}\). After both the first and second series of SIE, the rating of perceived exertion was statistically significantly lower in SIE\({}_{10\%}\). In contrast, there were no differences between protocols for changes in lactate accumulation and changes in mean body temperature. Additionally, there was a higher fatigue index (FI) in the SIE\({}_{10\%}\) series compared to SIE\({}_{7.5\%}\). Similar observations were observed by [PERSON] et al. (2019) in FI when comparing different loads in Wingate tests, although unfortunately they did not measure cardiorespiratory parameters. According to [PERSON] et al. (2018) and [PERSON] and [PERSON] (2018), muscle oxygen availability and O\({}_{2}\) kinetics ([PERSON] et al., 2018) are thought to be important factors in ATP regeneration via phosphocreatine (PCr) resynthesis for energy supply during repeated short (<10 s) sprint  
Figure 4: Time to achieve PPO (tPPO) in subsequent SIE repetitions. # statistically significant difference (\(p<0.05\)) in the second repetition of the 2 nd series between SIE\({}_{7.5\%}\) and SIE\({}_{10\%}\). Figure 3: Relative peak power (rPPO) in subsequent repetitions during the SIE session. * statistically significant difference in the first repetition of the 1 st series (\(p<0.05\)) between SIE\({}_{7.5\%}\) and SIE\({}_{10\%}\). sequences. Therefore, the attainment of a higher rPPO while maintaining a faster restitution of oxygen uptake and heart rate may have resulted from a faster rate of phosphocreatine resynthesis. [PERSON] et al. [34] also acknowledge that crucial factors for maintaining and recovering power output include the availability and resynthesis of PCr. However, this requires in-depth analysis of muscle biopsies, which was not performed in the present study. Further research may verify whether different acute responses affect long-term adaptations of aerobic metabolism. Additionally, after both the first and second series of SIE, the rating of perceived exertion was statistically significantly lower in SIE\({}_{10\%}\). This assessment may have been due to a 25.5% shorter time to peak power output (Figure 4) and a significantly higher fatigue index in the first series (Table 3). This effect could also have been caused by the simultaneous recruitment of more type Ix fast-twitch fibers [35]. This is consistent with the findings of [PERSON] et al. [36], who reported that the achievement of higher power output and performance of greater work was associated with changes at the neural level (greater activation of type II motor units) and at the metabolic level after maximal anaerobic exercise. This may indicate greater activation of glycolytic processes and a more efficient ATP-PCr system [3; 37]. [PERSON] et al. [38] tested different loads during 30 s maximal efforts and reported that loads in the range of 9% to 11% of the participant's body mass resulted in greater energy production in physically active adult men compared to lower loads. [PERSON] and [PERSON] [39] suggested that the optimal load should be 8.7% of the participant's body mass. However, the results of a study by [PERSON] et al. [40] showed that the use of this load (8.7% of the participant's body mass) represented a large underestimation (\(\sim\)30%) in the achievement of PPO by highly performing individuals.