## 2 Materials and Methods  
### Experimental Protocol  
The procedures are described with sufficient detail to allow other authors to replicate the protocols, while well-established methods are briefly presented and appropriately cited. **Subjects**. Ten right-handed healthy subjects, with a mean age of 34 (standard deviation (SD) 10 years, age range 27-52 years), participated. The hand dominance was determined by the Edinburgh Handedness Inventory [23], and the absence of neurological or psychological symptoms was determined by the TMS Adult Safety-Screening Questionnaire [24]. The inclusion criteria cover age, handedness, and health status. In the beginning, twenty volunteers gave their written informed consent to participate in this study. Four of them stopped the experimental procedure immediately after the first stimulus. The data of another six subjects were excluded during the offline processing of the experimental data. Exclusion criteria were: (1) the EMG activity did not correspond to the respective muscle condition task, and (2) the evoked responses were unclear to identify. Due to the above exclusion criteria, the group sample size became smaller than expected. Although the small sample size complicates summarizing the findings, may not properly represent the target group, and limits the generalizability for a larger set of subjects, the data processing followed all statistical and representation rules. **Transcranial magnetic stimulation**. The research complies with all ethical and regulatory issues related to TMS and all recommendations of the \"Safety of TMS Consensus Group\" [25]. The experimental procedure was approved by the ethics committee of the Institute of Biophysics and Biomedical Engineering at the Bulgarian Academy of Sciences (N* 487/17.05.2018). TMS was provided by a single-pulse monophasic MagStim200 stimulator (MagStim Co., Whitland, UK) connected to a BiStim module (MagStim Co., Whitland, UK) with a figure-of-eight coil (mean diameter 7 cm). The coil was oriented to produce an approximately posterior-anterior current flow perpendicular to the central sulcus. All TMS intensities were determined as a percentage of maximum stimulator output (peak magnetic field = 2 T). The stimulation coil was adjusted over the optimal area of the left motor cortex to produce motor-evoked potentials in the right first dorsal interosseous muscle. The motor hotspot was localized following the procedures recommended for motor threshold evaluation [3]. The motor threshold at relaxed muscle condition was detected by applying a threshold hunting paradigm [26], according to which the experimental procedure starts with higher TMS intensities and goes to lower intensities. All individual RMTs were determined as a minimum TMS intensity that produced 5 MEPs out of 10 consecutive stimuli. Responses with an amplitude of 0.05 mV (peak-to-peak) or greater were defined as MEPs [27]. TMS intensities selected for the current experimental protocol were 100%, 110%, 120%, 130%, and 140% of the individual measured RMT. **Muscle condition tasks**. TMS intensities were sequentially applied during three muscle condition tasks: relaxed muscles, isometric index finger abduction, and co-contraction of antagonist muscles. Each muscle task had 3 repeats with each stimulus intensity. **EMG recordings**. We used modular BIOPAC hardware items (BIOPAC Systems Inc., Goleta, CA, USA) connected to an unconventional computerized system for data acquisition, amplifying, recording, storing, and processing of the EMG signals. Surface electromyograms were recorded from the right FDIM by a pair of surface Ag/AgCl disc electrodes (8 mm diameter). The active pole of the electrode was fixed on the muscle belly, and the reference pole--on the distal tendon at the index finger base. EMG activity and force signal were continuously monitored for visual feedback on the correct execution of muscle condition tasks.