# Force Sensation Induced by Electrical Stimulation of the Tendon of Biceps Muscle
## 2 Related Work
### Stimulation of Tendons and Their Effects  
2.2.1 Tendon Stimulation Focusing on Muscle Spindles and Observation of Reflexes and Motor Illusions  
[PERSON] et al. [21] observed that the tendons on the elbow side of the biceps and triceps muscles produced the illusion of motion when subjecting them to vibratory stimulation. In their experiment, a vibrating stimulus was applied to one arm of the participants, and they were instructed to reproduce the motion of the vibrating arm with the other arm. They confirmed the illusion of \"motion\" in the direction in which the vibrated muscle was stretched (e.g., the vibration of the biceps tendon makes the sensation as if the forearm was extended). The intensity of the motion illusion varied with the intensity of the vibration stimulus; in particular, a clear correlation was observed in the vibration frequency, suggesting that around 80 Hz is optimal [22]. Tonic vibration reflex (TVR), which weakens the efferent signal to the antagonist muscle, is also known to occur when the intensity of tendon vibration exceeds a certain level [23]. These methods, which provide the sense of movement without actually moving the body, can make clinical applications for rehabilitation or VR applications. [PERSON] et al. [24] confirmed that Ia nerve stimulation by tendon vibration improved motor performance in stroke patients. Several attempts have been made to improve the operability of an arm in VR space [25] or a robotic prosthetic hand [26] when operating a BCI. In addition, several augmented reality applications have attempted to modulate real body motion [27]. Furthermore, [PERSON] et al. showed the possibility of presenting kinematic sensations in arbitrary two or three dimensions [28]. In addition, [PERSON] et al. [29] reported that electrical stimulation from the skin surface electrodes aiming at the median nerve in the wrist, including Ia fiber, produced a similar kinesthetic sensation to needle electrodes. In addition, during EMS, not only the alpha fiber but also the Ia fiber can be stimulated; especially when stimulating the tibia, a muscle contraction caused by alpha nerve stimulation (M wave) was observed, followed by the one that seemed to be a reflex caused by Ia nerve stimulation (H wave) [30]. In addition, it has been confirmed that electrical stimulation of the muscle belly produces a sensation similar to a motion illusion [31]. .2.2 Tendon Stimulation Focusing on Golgi Tendon Organs and Observation of Reflex and Force Illusion  
Most studies using stimulation of the Golgi tendon organs or Ib nerves have focused on the inhibitory reflex called the Ib reflex. [PERSON] and colleagues used surface electrodes, confirming that stimulation of the gastrocnemius tendon caused inhibition of muscle contraction [32]. The cathode was placed in the middle of the gastrocnemius muscle (1 cm from the myotendinous junction on the heel side), and the anode was placed next to the cathode so that no muscle stimulation occurred. On the other hand, it is also known that signals from the Ib nerve can cause excitatory reflexes rather than inhibitory reflexes, depending on the situation, since they pass through many intervening nerves. For example, the signal of the Ib nerve in the tibia has a positive feedback effect when feet are hitting the ground [33; 34]. The Golgi tendon organ has a high threshold to muscle stretch or vibration stimuli during relaxation [35], while the optimal stimulus to the Golgi tendon organ is muscle contraction [17]. Therefore, if selective stimulation to Golgi tendon organs or Ib fibers is possible, force sensation due to virtual muscle contraction, such as a sensation of supporting a virtual object pushing the arm, can be generated. In our previous work, we reported that electrical stimulation to the wrist extensor's tendon produced a force sensation as if the arm was pushed from the back of the hand [12; 13; 14]. As an application to VR, [PERSON] et al. [36; 37] proposed to present the sensation of stepping forward by electrical stimulation above the tendons of the feet.