# Force Sensation Induced by Electrical Stimulation of the Tendon of Biceps Muscle
## 5 Discussion
Only a left-handed participant slightly underestimated the magnitude of no-stim, while the other left-handed overestimated it more than some right-handed participants. On the other hand, one right-handed participant, who reported he was a tennis player and his right arm's muscle was larger than the left, scored the sensation magnitude under the no-stim (as well as under the other conditions) slightly larger than the mean, that is, he had a typical score. Therefore, we could not confirm that handedness has a dominant effect; indeed, there were too few participants to conclude that. In future work, we have to conduct some multimodal experiments to confirm if this proposed method is useful in an applied environment such as VR or remote communications. This study has just shown that stimulation of the tendon can present force sensation without physical movement and is still limited to the basic research. Additionally, in order to use this proposed method adequately as a way to present one single sense modality, we have to reveal the mechanism of this in detail. The tactile sensation path can be considered as another candidate that could induce the force sensation, other than the Golgi tendon organ. In the condition of electrical stimulation of the tendon, the tactile sensation tended to occur in the forearm because nerve bundles reside in a relatively shallow area where the electrodes were placed. Even so, some participants felt no tactile sensation on the forearm, but they felt a force sensation as if the elbow was stretched. This suggests that the contribution of deep sensation was greater than that of cutaneous sensation. Indeed, since some studies have shown that cutaneous sensory nerves contribute to proprioception [44], we cannot rule out the possibility that the cutaneous sensory nerves on the ventral side of the elbow were stimulated and processed centrally only as force sensation without cutaneous vibration or pressure sensation. However, some participants reported that the force illusion did not occur when their posture changed largely from the initial posture, suggesting that the stimulation of deep sensory nerves, which are easily affected by posture changes, was the main factor. The muscle spindles' contribution to the force sensation should be considered. However, we estimate its contribution as small because none of the participants reported kinesthetic illusion like that induced when the vibration stimulation is applied. On the other hand, some participants felt they moved their arms more or faster when they voluntarily moved their arms while the electrical stimulation was applied to the tendon. This could be interpreted as a motion illusion caused by muscle spindles, but it could also be interpreted as the participants recalling acceleration from the perceived external force. Indeed, it has been reported that electrical stimulation of the tendons could produce a phenomenon similar to the motion illusion [45]. We observed another interesting phenomenon. At least three participants reported that they experienced involuntary extension movements of the elbow joint at the moment of electrical stimulation of the tendon of the flexor. If it had worked as EMS, the elbow would have moved in a flexing direction rather than the extension. This might be explained by the Ib reflex, which is regarded to be contributed to by the Golgi tendon organ. This reflex suppresses the excitation of motor nerves to protect the muscle when a massive force is applied to the muscle instantaneously. This inhibitory reflex has been confirmed to occur when needle electrodes stimulate the Golgi tendon organ or Ib nerve. So, in order to elucidate the mechanism of force illusion induced by electrical stimulation of tendons, measuring these reflexes with the setup of the present experiment should be conducted as future work.