## 2 Materials and Methods
### Protocol  
Three experimental paradigms were tested for different upper limb conditions (Figure 1B): stationary arm (Exp 1), non-voluntary rhythmic arm oscillations evoked by triceps brachii (TB) vibration (Exp 2), and voluntary rhythmic arm movements (Exp 3).  
Exp 1. Under stationary conditions, TMS-induced MEPs were recorded in the proximal (biceps brachii, BB; triceps brachii, TB; deltoideus anterior, DA; deltoideus posterior, DP) and distal (flexor carpi radialis, FCR; extensor carpi radialis, ECR) muscles of the left arm. About ten stimuli without vibration and ten stimuli during TB vibration were applied to the stationary relaxed arm (Figure 1B). The stimuli were delivered with a frequency of 0.4 Hz. The duration of the trial was about 60 s. All 13 subjects participated in this experiment. In the 7 subjects in whom vibration induced rhythmic arm movements (see below), it was not possible to administer 10 TMS stimuli in one trial, because the vibration-evoked non-voluntary movements had begun. However, the latency interval for the onset of evoked rhythmic movements (Figure 1B) varied between participants and trials, ranging from a few seconds to tens of seconds (2-25 s) [8]. Therefore, after applying 10 stimuli without vibration, 2 to 5 stimuli were applied against a vibration background until the onset of movement. Samples with vibration were repeated until a total of ten stimuli against the background of vibration were recorded.  
Exp 2 and Exp 3. In the second and third paradigms (Figure 1B), the motor cortex was stimulated when arm movements approached steady-state levels (constant frequency and amplitude). We recorded these steady-state oscillations for \(\sim\)10 s, followed by 40-60 s of low-frequency TMS, and finally \(\sim\)10-15 s without TMS. A computer program automatically defined the onset of the movement cycle (when the shoulder angle changed its direction) and was used to trigger the TMS stimuli in different phases of the cycle (Figure 1C) at a frequency of approximately 0.4 Hz (\(\sim\)T = 2.5 s), and the first stimulation time was alsochosen in a random manner across trials. Since the interstimulus interval was different and lower than the cycle duration of non-voluntary arm oscillations (\(\sim\)1.7 s, see Results), the TMS stimuli were applied at different moments of the individual arm movement cycles within the trial. MEPs were recorded in the same 6 muscles as in Exp 1. The duration of each trial was 60-80 s. Six to ten trials in Exp 2 and six to ten trials in Exp 3 were recorded. The subjects were given a rest of 30-60 s between trials. Since the stimulation time was 40-60 s, an average of 10-18 stimuli per trial was applied.  
First, we recorded the trials in Exp 1, followed by Exp 2 and finally Exp 3. The effects of muscle vibration are known to manifest in only a subset of subjects [9; 32; 35]. Exp 2 included 7 of the 13 subjects who participated in the previous experiment (Exp 1) and were prompted to move their upper limb in a rhythmic manner by TB vibration. In Exp 3, we recorded the same seven subjects as in Exp 2 in order to compare the effects of TMS during non-voluntary and voluntary arm movements.