## 2 Materials and Methods
### Testing Procedures  
All sessions were conducted on workdays and began with a standardized warm-up consisting of stationary cycling for 5 min and 5 min of active stretching. Following the warm-up, the participants were seated in a custom-made chair with knee and hip angles fixed at 120\({}^{\circ}\) and 100\({}^{\circ}\), respectively (Figure 2, upper left panel). The chair was adjusted for every participant with respect to their body size. The participant's trunk, waist, and thighs were firmly strapped to the chair and the distal parts of their legs were affixed to strain gauge force transducers (Dongguan South China Sea Electronic Co., Ltd., Dongguan, China; load cell S-Type CZL302; range 2 kN) using rigid cuffs. The cuffs were secured with hook and loop fasteners 2 cm above the malleolus lateralis.  
All testing procedures were performed separately for each leg. Maximal voluntary contraction (MVC) was always performed prior to RTD-SF testing to determine the greatest value of torque during the performed task [1]. Participants were instructed to extend the knee 'as fast and as hard as possible' and each maximal contraction was sustained for approximately 3 s [28]. Three MVCs were completed for each leg with a 60 s rest between successive trials [3,4]. The MVC with the highest value of torque (Tmax) over the three  
Figure 2: Testing and training set-up (upper left panel; 1: force transducer; 2: shanks; 3: rigid straps; 4: acquisition and analog-to-digital conversion unit; 5: monitor with visual feedback). Note that only trained legs were affixed to force transducer during the training sessions. Lower left panel shows positioning of EMS electrodes and subject increasing the current to maintain targeted training intensity. Upper and lower right panels depict training procedure across weeks and visual feedback given during the training sessions (first two contractions out of 40 are marked as “1” and “2”).  
consecutive trials was used for further analysis as well as to prescribe the subsequent submaximal fast contractions that were expressed as a percentage of Tmax.  
Following a 5 min rest, the RTD-SF testing protocol was conducted. Subjects completed \(\sim\)15 submaximal rapid contractions for familiarization purposes. They were instructed to contract their QF muscle as quickly as possible and to relax instantly. Three sets of \(\geq\)20 contractions were completed for experimental purposes. In each set, there were 5 consecutive contractions at 4 different intensities presented in ascending order (20%, 40%, 60%, 80% of Tmax, i.e., MVC) [7; 8]. The rest time between the sets was 60 s [1]. The experimenter gave a voice command to perform a new trial every 4-5 s. Visual feedback of torque as a target line was provided to the subjects on a computer monitor, but subjects were advised not to pay attention to accuracy.  
Data were collected and sampled at 1000 Hz using the commercially available software Isometrics (version 4.0.0, 'Sports Medical Solutions', Belgrade, Serbia). Signals were filtered with a low-pass (5 Hz), second-order Butterworth filter. The software automatically calculated the Tmax (peak value on the torque-time trace after reaching the plateau) and RTDmax (peak of first derivative of the torque-time signal overlapping 0.1 s intervals). The peak torque and peak RTD from the isometric torque pulses were also automatically calculated as the maximal value during the 1 s interval of each repetition and as a maximum of the first derivative of the torque-time curve, respectively. Each recording was inspected to ensure that no countermovement was present as this would influence RFD.  
Peak torque and peak RTD were expressed as a percentage of Tmax and plotted to obtain a regression line (slope and intercept) and R\({}^{2}\) values for each participant, respectively. The slope of the regression line provides the RTD-SF (for more details, please see Figure 2). The R\({}^{2}\) represents consistency in the performance rapid muscular contractions [1; 12].