## 4 Discussion  
The purpose of this study was to test the differences between explosive sustained (\"fast-and-hard\" instruction) contractions and ballistic (pulse-like) contractions on the RFD during isometric ankle and knee extension tasks. We hypothesized that the ballistic contractions in our study would elicit a higher RFD than the sustained contractions. Our hypothesis was partially confirmed, as the RFD was indeed higher in the ballistic contractions, but only during late time intervals. Thus, ballistic contractions seem to enable the participants to achieve a higher RFD compared to sustained contractions (even if an emphasis on the fastest possible start is given for the sustained contraction). The differences between contraction types were more evident in females, which is also in line with our hypothesis. The exception was the peak RFD, where males tended to show a higher difference between the contractions; however, this outcome had low reliability and a very high between  
Figure 3: Comparison of RFD outcomes between explosive sustained and pulse contractions for the knee extension task across time intervals for females (**top graph**) and males (**bottom graph**). \({}^{*}\) indicates statistically significant pair-wise comparison. participant variability. The intrasession reliability of the RFD was moderate to excellent, with very similar results for both contraction types. We also found moderate-to-strong correlations between the RFD in each of the contraction types in females, but only moderate correlations in males. Altogether, our results indicated that ballistic contractions should be preferred over sustained contractions, when RFD assessment is the primary aim, and that both contraction types produce reliable RFD outcomes, with the exception of the peak RFD and early (0-50 ms) RFD. When the aim is to assess both the RFD and peak force, it is suggested to use a combination of separate \"hard-and-fast\" sustained contractions and ballistic contractions in the protocol. In case of time constraints, \"fast-and-hard\" instruction can be a reasonably valid solution, considering that the RFD values between sustained and ballistic contractions are moderately to highly correlated. Similarly high correlations have also been reported for the RFD obtained with contractions between \"hard-and-fast\" and \"fast-and-hard\" [10]. The lower reliability of the early RFD is consistent with previous studies [23] and has been attributed to neural factors, as early evoked force is much less variable. The reliability of the RFD in ballistic contractions in later time intervals is comparable or even higher compared to the previously reported reliability for \"hard-and-fast\" contractions [16; 24; 25]. In sum, ballistic contractions exhibited acceptable intrasession reliability and elicited a higher RFD than sustained contractions. Several underlying mechanisms could be responsible for the difference in the RFD between sustained and ballistic contractions. Given that muscle/motor unit activity was not measured in this study, the discussion can only be speculative at the moment. The primary candidates for the underlying mechanisms include increased motor unit firing rate [12], speed of motor unit recruitment [13; 26] and increased presence of doublet discharges [27]. Indeed, [PERSON] and [PERSON] (2014) [12] have made a strong case that motor unit recruitment at the onset of contraction is the primary determinant of the RFD during ballistic contractions. While motor unit firing rate is considered as a primary determinant for the RFD in general [1], recent evidence also suggests a very important role for the speed of motor unit recruitment (i.e., the number of motor units recruited per second) [13]. Another possible determinant of the RFD is the co-activation of the antagonist muscles [28]. In sub-maximal ballistic contractions, the antagonist muscles are responsible for decelerating the force rise prior to peak force [29].