## 2 Materials and Methods
### Experimental Protocol
After amplification (gain increase of the input signal \(\times\)1000) and filtering (band passes 10 Hz-1 kHz), EMG signals were digitized (sampling rate 2 kHz)and stored on a disk for offline analysis. A land electrode was used to reduce unwanted outdoor signals. **Experimental procedure. ** Subjects were seated comfortably in a chair, with the right arm gently fixed in slight shoulder abduction from the trunk (20\({}^{\circ}\)) and flexion in the elbow (110\({}^{\circ}\)). The hand and forearm were pronated and relaxed on horizontal support. The right index finger was positioned in a non-movable manipulandum connected to a force transducer sensitive in all directions. The other right fingers were immobilized with Velcro straps (Figure 1). All experiments started with the determination of the individual maximum voluntary contraction (MVC) level without TMS. Each subject's force was measured as a maximal index finger contraction in the direction of abduction. Then, each subject released 20% of the individual MVC level in the direction of abduction. In all experiments, at the monitor of the experimental session, the indicator line (IL) was fixed according to the individually measured 20% MVC level. The IL was used to provide real-time visual feedback about the current MVC level, but it does not show the EMG activity and force level in detail observed in offline data processing. Both the researcher and the subject verified that any deviations would be handled immediately. Then, we determined individual RMTs as previously described. After these, TMS was applied during the three muscle condition tasks: (1) relaxed muscles: without voluntary muscle activity and zero force production; (2) isometric index finger abduction: voluntary activity of index finger in the direction of abduction as close as possible to 20% of individual MVC level in direction of abduction; (3) co-contraction of antagonist muscles: voluntary simultaneously activated antagonist muscles, matching level equal to 20% MVC in direction of abduction by increasing the angle stiffness and without producing of external force. TMS intensities during each muscle condition task were 100%, 110%, 120%, 130%, and 140% of individual RMT. If the individual measured RMT was high, the following TMS intensities were applied as far as the stimulator reached its maximal output, and it was impossible to produce a higher stimulus intensity. Three sample repeats were made to the subject for each TMS intensity at each muscle condition task. **Data processing. ** Epochs of 2 s duration (400 ms prior and 1600 ms after the stimulus) were stored on a disk for offline analysis. The measured parameters in relaxed muscle condition (R), isometric index finger abduction condition (A), and co-contraction of antagonist muscles condtion (C) were: (1) peak-to-peak amplitude of MEPs, further noted as R_Ampl, A_Ampl, and C_Ampl, respectively, measured in mV; (2) duration of latency of MEPs, named for convenience R_Lat, A_Lat, and C_Lat, respectively, measured in ms; and (3) duration of silent periods in the two active muscle conditions, abbreviated as A_SP and C_SP, measured in ms. Statistical analyses were made using the STATISTICA data analysis software system, installed version 8.1 (StatSoft Inc., Tulsa, OK, USA) [28]. The effect of different types of muscle conditions and the effect of different TMS intensities were evaluated by two-way repeated measures ANOVA with factors \"muscle condition task\" (relaxed  
Figure 1: Experimental hand manipulandum. muscles, index finger abduction, co-contraction of antagonist muscles) and \"TMS intensity\" (100%, 110%, 120%, 130%, 140% RMT). A test of sphericity was used to check the variances of the differences between sessions. The variances of the differences were considered to be equal if the test reported a significance level of \(p\geq 0.05\), and sphericity was assumed. If the test reported significance levels of \(p\leq 0.05\), sphericity was then not assumed, and Greenhouse-Geisser correction was applied.