## 2 Materials and Methods
### Training Procedures  
The training procedures were applied unilaterally over the QF with lower MVC torque at the pre-test, regardless of leg dominance. As a note, there was a significant between-leg difference in Tmax in both groups (10 \(\pm\) 7%; \(p\) < 0.01 in EMS, and 12.5 \(\pm\) 5%; \(p\) < 0.01 in VOL group). Participants' positioning during training sessions was identical to that during testing procedures. The participant's trunk, waist, and thighs were firmly strapped to the chair with knee and hip angles fixed at 120\({}^{\circ}\) and 100\({}^{\circ}\), respectively. The distal part of the trained leg was affixed to a strain gauge force transducer (Figure 2, upper left panel). Each EMS and VOL training session consisted of 40 contractions each lasting 6.25 s and separated by 20 s inter-contraction intervals. The number of contractions and contraction-to-rest ratio (6.25/20 s) were in accordance with the work-to-rest pattern produced by the muscle stimulation device (Figures 1 and 2--upper and lower right panels). The stimulation intensity was determined during the familiarization when EMS was delivered at the individually maximal tolerable dose and was kept close to identical among the session, averaging in total \(\sim\)60% MVC per subject [24]. Specifically, EMS intensity varied between 58% and 63% across sessions and subjects, depending on their daily pain tolerance. Due to anticipated positive exercise effects on Tmax, participants were re-tested after the third week of intervention (mid-test), and these data served to adjust the exercise threshold (please see Figure 2, upper right panel). The subjects were instructed to stay relaxed during EMS sessions, thus stimulation was not superimposed onto voluntary contractions. A portable battery-powered commercially available stimulator (Compex SP2.0, Medicompex SA, Ecublens, Switzerland) was used in the EMS group. The motor points were identified by stimulating the skin surface with a pen electrode and a large reference electrode placed over the skin. Stimulatory current was gradually increased until a clear muscle twitch was observed. The positions of the motor points were marked with water-resistant markers, so the electrodes were applied at the same sites throughout the intervention period. The positive electrodes, measuring 25 cm\({}^{2}\) (5 cm \(\times\) 5 cm), which had membrane-depolarizing properties, were placed as close as possible to motor points of vastus medialis and vastus lateralis muscles. The negative electrode, measuring 50 cm\({}^{2}\) (10 cm \(\times\) 5 cm), was placed near the proximal insertion of rectus femoris muscle. The stimulator discharged biphasic rectangular pulses lasting 400 \(\upmu\)s. The stimulation frequency and duty cycle were 75 Hz and 6.25 s of stimulation followed by a pause lasting 20 s (duty cycle, 24%) [25; 37]. The selected stimulation parameters correspond to a maximum strength development program. The stimulation intensity was monitored on-line and determined by the subject at the start of each EMS session according to their pain threshold and to produce a torque corresponding to at least 60% of the pre-test Tmax score [24]. Regarding the VOL group, participants were required to reach the prescribed torque level only through voluntary activation of QF. To attain the same number of contractions and contraction-to-rest ratio as in EMS, automated audible signals were delivered in accordance with the contraction-rest pattern produced by the muscle stimulation device. Depending on the type of exercise, the subjects were asked to increase the current (please see Figure 2, lower left panel) or voluntary activation intensity throughout the training session to maintain target torque level of \(\sim\)60% MVC. The exerted torque during each contraction was measured with the same force transducer and software as described for testing procedures, and real-time feedback along with the individual 60% MVC threshold was provided on a computer screen.