## 4 Discussion  
As evidenced by the notable facilitation of non-voluntary rhythmic movements (elicited by sensory stimulation) by low-frequency TMS of the motor cortex (Figure 3)--as opposed to its statistically non-significant effect on voluntary upper limb movements (Figure 4)--our results demonstrate that motor responses to TMS are not limited to short  
Figure 4: Effect of low-frequency TMS of the motor cortex during voluntary upper limb oscillations. Same format as in Figure 3. (**A**) Example of the effect of TMS on voluntary arm oscillations (n = 7 subjects). (**B**) Effect of TMS on kinematic and EMG parameters (mean + SE). (**C**) Background EMG activity (**upper panels**) and MEPs (**lower panels**) during different phases of the cycle. Note a non-significant effect of TMS on arm oscillations (cf. Figure 3A,B with Figure 4A,B). BB, biceps brachii; TB, triceps brachii; DA, deltoideus anterior; DP, deltoideus posterior; FCR, flexor carpi radialis; ECR, extensor carpi radialis. Asterisks (*) denote significant differences. latency motor-evoked potentials (Figure 2). Rather, these responses are also reflected in the magnitude of induced non-voluntary limb oscillations (Figure 3A,B). This study has several limitations. For instance, non-voluntary upper limb oscillations were evoked in only \(\sim\)50% of participants. This could be due to the fact that muscle vibration stimulates a relatively small number of sensory fibers entering the dorsal roots and/or to individual differences in the excitability of the spinal pattern generation circuitry. These individual differences are not only interesting in their own right but may also be critical for understanding behavioral variability and for developing personalized therapeutic approaches. In our previous work, we showed that individuals with a higher responsiveness of the pattern generation circuitry to tonic sensory input--both in the upper and lower limbs--tended to have larger H-reflexes [32]. In the present study, we were unable to evaluate the relationship between non-voluntary movements and TMS-evoked muscle responses due to the inability to reliably normalize and compare MEPs (and TMS intensity) across participants. Nonetheless, regardless of the exact mechanism behind the individual differences in the appearance of non-voluntary rhythmic arm movements, it is important to highlight that their facilitation by low-frequency TMS (Figure 3) was consistently observed in all participants who exhibited such oscillations. Another limitation is the lack of a sham-TMS or control-coil condition. Non-specific TMS effects on MEPs--such as auditory or somatosensory coactivation--cannot be fully excluded. However, our previous study showed that electrical stimulation of upper limb skin receptors did not evoke non-voluntary arm oscillations (in contrast to the prominent effect of peripheral nerve stimulation), suggesting limited contribution of such non-specific factors [33]. Notably, the effect of low-frequency TMS was primarily observed during non-voluntary (Figure 3B) rather than voluntary (Figure 4B) arm movements, despite identical stimulation intensity. This supports the interpretation that the observed effects are mainly due to TMS-induced modulation of spinal circuits rather than sensory coactivation. The finding that motor cortex TMS significantly potentiated non-voluntary arm movements is of particular interest (Figure 3). This effect cannot be attributed solely to the direct \"mechanical\" influence of MEPs on muscle contraction, as stimuli were delivered at various phases of the movement cycle, and the timing of stimulation did not align with the natural movement period. Furthermore, the observed increases in arm oscillations were accompanied by heightened EMG activity in proximal muscles, which gradually declined after TMS cessation (Figure 3A,B). In contrast, the influence of TMS on voluntary hand movement parameters was considerably smaller, if present at all (Figure 4).