## 4 Discussion  
To our knowledge, this is the first study to investigate force output, power output, and FvP profiles during sprinting under overload conditions with a non-motorised HRD. The results show that the HRD underestimates F\({}_{\mathrm{r}}\), F\({}_{\mathrm{avg}}\), P\({}_{\mathrm{avg}}\), F\({}_{0}\), and P\({}_{\mathrm{max}}\). Both, FP and HRD showed that the force output, power output, and FvP profiles were significantly affected by the overload conditions. F\({}_{\mathrm{avg}}\), P\({}_{\mathrm{avg}}\), and F\({}_{0}\) were the highest under HiL, while \(\mathrm{v_{0}}\) and \(\mathrm{v_{max}}\) were the highest under MiL. These results, which shed light on the specific biomechanical  
Figure 3: Average force–velocity (solid line) and power–velocity relationship (dashed line), i.e., the force–velocity–power profile (FvP), with standard deviation (grey shading): (**a**) minimal overload (MiL), calculated with the force plate (FP); (**b**) MiL, calculated with the hydraulic resistance device (HRD); (**c**) moderate overload (MoL), calculated with the FP; (**d**) MoL, calculated with the HRD; (**e**) high overload (HiL), calculated with the FP; and (**f**) HiL, calculated with the HRD. The maximal theoretical force (F0), maximal theoretical velocity (\(\mathrm{v_{0}}\)), and maximal theoretical power (\(\mathrm{P_{max}}\)) are shown in Panel (**a**). properties of sprints with the HRD, are consistent with observations from studies on sled sprints. The HRD is a unique, non-motorised resistance device designed for sprint training and testing [22; 30]. While there are numerous options for resisted sprinting, few allow for the simultaneous measurement of resistance force and sprint velocity [14]. A study by [PERSON] et al. [16] compared a motorised device with ground reaction force plates embedded in the ground and showed a very high correlation (r = 0.99) but underestimated the F\({}_{\text{h}}\) by 5-23% with the motorised device. Similarly, our results showed a nearly perfect-to-perfect correlation between the HRD and the FP, although the HRD significantly underestimated F\({}_{\text{avg}}\), P\({}_{\text{avg}}\), F\({}_{0}\), and P\({}_{\text{max}}\). These results support the suitability of the HRD for monitoring the force output, power output, and FvP profiles in sprints with hydraulic overload. Using these parameters to optimise load during resisted sprints could improve the training efficiency [24]. However, due to the underestimation of Fr and the associated fluctuations in other variables, we recommend using the HRD consistently in the training process as its variables should not be used interchangeably with those of other resistance devices. As expected, \(\
u_{\text{max}}\) decreased significantly under MoL and HiL conditions, similar to the 12-43% reduction reported at 3, 7, and 14 kg overloads using a motorised device [33]. In a review of sled sprints, loading regimens were categorised as low (2.5-10% \(\
u_{\text{max}}\) decrease), moderate (11-30% \(\
u_{\text{max}}\) decrease), heavy (31-50% \(\
u_{\text{max}}\) decrease), and very heavy (\(>\) 50% \(\
u_{\text{max}}\) decrease) [4]. In our population, MoL (Fr \(\sim\) 35 N) represented a low-to-moderate overload, as the \(\
u_{\text{max}}\) decreased from a minimum of 2.43 to a maximum of 12.35%. HiL (Fr \(\sim\) 110 N), on the other hand, corresponded to a moderate-to-heavy overload, as the \(\
u_{\text{max}}\) decreased from a minimum of 12.76 to a maximum of 46.97%. However, as two higher overloads were compared with the MiL (Fr \(\sim\) 15 N) in this study, such interpretations should be treated with caution, and even greater decreases compared to the \(\
u_{\text{max}}\) of the normal sprint are to be expected. Nevertheless, the ability of the HRD to produce a significant reduction in \(\
u_{\text{max}}\) emphasises its versatility for achieving various resisted sprint training objectives, including improving the sprint technique, speed-strength, power, or strength-speed capacity [34].