# Force Sensation Induced by Electrical Stimulation of the Tendon of Biceps Muscle
## 1 Introduction  
When a person supports an object, a force interacts between the object and the body. This force causes localized skin deformation and provides detailed shape and texture information. Additionally, a certain amount of force attempts to bend limbs around a joint, changing muscle length if the limbs are physically bent or making the motor nerves excited and then causing the muscles to contract if a person tries to resist the external force. These haptic sensations provide detailed information about the interaction between the body and objects that cannot be obtained through vision alone. In particular, movements around the joints are perceived by the receptors around the muscles. Such a haptic sensation is called proprioception.  
Many methods of artificially presenting proprioception have been proposed for remote control and work in VR spaces. Many of these methods have used electromagnetic motors to physically present force, making it possible to present precise force sensation and even skin sensation. However, the multiple degrees of freedom of motion require the same number of motors, causing an increase in the size of the device in order to have multiple degrees of freedom [1, 2]. Indeed, some studies showed that one actuator could cover a multi-dimensional haptic feedback motor system [3, 4]; however, the situation where the technique is available is limited, and a heavy motor is still required for a large degree of torque. On the other hand, a method to present force by controlling human muscles with electrical stimulation (EMS) has been proposed [5, 6, 7, 8, 9, 10, 11]. This method requires only a tiny electrical circuit board for power supply, one for stimulus control, and electrodes to beplaced on the human body, the net size of all of which is much smaller than the devices using electromagnetic motors. This method can be regarded as a method that uses motors in vivo (i.e., muscles) instead of the external electromagnetic motors, which is the main reason for the large size. On the other hand, EMS inevitably involves muscle contraction, which does not usually occur in natural situations, and thus also produces sensations that are different from the actual sensations.  
EMS stimulates motor nerves and therefore requires actual movement to produce sensation. On the other hand, the authors have proposed a method of presenting force sensation by stimulating sensory nerves related to proprioception. We especially focused on Golgi tendon organs, which are related to muscle tension and force. We targeted their nerves called Ib fiber from electrodes on the skin surface as a method that does not require the generation of actual movement [12; 13; 14]. In the previous report, we stimulated the dorsal wrist's tendon connected to the muscle that extends the wrist. We speculated that if we could stimulate the Golgi tendon organ, we could present the information that this muscle is tensed, while the muscle length does not change because no movement is physically generated. Additionally, such an electrical stimulation of afferents can ignore the mechanical structure of the mechanoreceptor to a certain extent; thus, this method has the potential to render haptics, keeping the footprint small even as EMS. As a result of the experiment, we confirmed it is possible to present a force of about 300 gf (\(\simeq\) 2.94 N) [12], and the effect was enhanced by multimodal stimulation with visual and cutaneous sensation [13]. Here, [gf] stands for the unit of gram force and 1.0 gf = \(1.0\times 10^{-3}\) kgf = \(9.8\times 10^{-3}\) N.  
However, it has not been investigated in detail whether this method also works in other upper limb joints. The muscles in the upper arm are mostly used for lifting and supporting objects, and many wearable interfaces have presented force to the upper limb and elbow joint [10; 11; 15; 16].  
This study investigates whether the force sensation in extending the elbow joint is generated by tendon part stimulation and compares it with the conditions of stimulating biceps brachii muscles.