## 4 Discussion
Regarding maximal strength and RTD gains, it should be noted that the level of the electrically evoked torque achieved in this study ranged between 58% and 63% MVC (the average intensity across participants was 60% MVC). The intensity was selected following previous recommendations and knowing the limitations of EMS [24, 25, 36]. Specifically, to promote strength gains, EMS was previously most commonly applied at intensities of -60% MVC [24]. To ensure a comparable training volume between the two training modalities we matched the contraction intensity and duration for the EMS and VOL groups. Although higher voluntary exercise loads are recommended to promote muscle hypertrophy and strength gains [40], significant strength gains could be achieved even when using medium loads (50-60% MVC) [25, 41, 42, 43, 44]. In fact, [PERSON] et al. [44] demonstrated a 31% increase in MVC following isometric VOL training at 50% MVC, while [PERSON] et al. [24] reported gains in maximum strength of 32.6 \(\pm\) 17.6% in trained subjects and of 32 \(\pm\) 15.6% in elite athletes following isometric EMS. Nonetheless, the results obtained in the current study are in line with those reported previously [24, 42, 43, 44] and show no substantial difference in MVC torque gains between VOL and EMS training modalities. While MVC Tmax was among the most investigated variables in EMS studies [23, 24], changes in RTDmax have been scarcely reported. For example, [PERSON] et al. [45] demonstrated significant increases of up to 16% in the QF muscle during the early time sections of RTD (100-200 ms from the contraction onset) but not in maximal RTD after dynamic whole-body combination EMS training [24]. That is not surprising since it is well known that performing isometric exercises in a ballistic manner can maximize the improvement of RTD [40]. Nevertheless, as shown in the current study, an increase in RTD max (15% for both training modalities) is warranted even when isometric exercises are performed on a single muscle group and in a non-ballistic manner. Our second hypothesis and our final findings concern the cross-education to the non-trained leg. A significant change in RTD-SF of the non-trained leg was obtained only following EMS training, suggesting potential central adaptations. Moderate, non-significant change was also observed for the Intercept. Although the cross-education effect is well described in the literature [18, 19, 26, 32], this is the first study to investigate this phenomenon concerning RTD-SF and strength training. In the aforementioned study of [PERSON] et al. [12], the authors reported that their lower limb speed-cycling training program had a positive cross-education effect on RFD-SF of non-trained upper limbs. Those who compared the effects of voluntary and stimulated isometric exercises on the studied phenomenon report no difference in strength gains between the two training modalities [26; 32] or greater gains following EMS than VOL exercises [34]. In fact, similar strength gains in the non-trained leg were reported both after low-intensity (i.e., 20% MVC [32]) and medium-intensity EMS and VOL training (i.e., 65% MVC [26]). However, the size of the increase may depend upon the type of muscle action. Namely, [PERSON] et al. [19] reported greater strength gains in contralateral limbs with EMS than with VOL exercises, but their study investigated eccentric contractions. Based on [PERSON]'s d, our study indicates that changes in Tmax could be larger after EMS than VOL training. To our surprise and contrary to previously published studies [20; 26; 30; 32], it appears that training intensity of isometric VOL exercise applied in the current study (60%) was below the necessary threshold for altering functional properties of corticospinal pathways [19], unlike in the EMS group. Interestingly, a study similar to ours used five sets of eight contractions at 65% of MVC over a 4-week period and reported a similar cross-education effect (21%) following isometric and EMS exercises [20]. We can only speculate the reasons for such findings.