# Force Sensation Induced by Electrical Stimulation of the Tendon of Biceps Muscle
## 3 Materials and Methods
### The Hypothesis of the Experiment and Experimental Conditions  
In this experiment, electrodes were placed above the tendon of the biceps brachii muscle (Figure 1a) aiming at the Golgi tendon organ and its nerve, Ib fibers, at the musculotendinous junction, so that the stimulation could give a user the impression that the biceps were contracting more strongly than they actually were. As the users also found that the angle of the limb did not change (because real force does not change), they could feel virtual external force opposing the virtual contraction. When seeking the muscle belly and tendon of a participant, we asked the participant to have and keep the same posture illustrated as Figure 1b and pulled the participant's forearm in the extending direction at a certain intensity to make the muscle and tendon stand out. Then, we considered the thickest part as the muscle belly, the part where the muscle gets gradually thinner as the musculotendinous junction, and the thin part as the tendon.  
Figure 1: (**a**) Electrodes’ position. A pair of electrodes is put above the biceps muscle of the left arm, and the other pair is put over the tendon and the junction between the muscle and tendon. (**b**) Experimental setup for comparison of force sensation. (**c**) A conceptual side view of (**b**).  
In the previous report [12; 13; 14], we presented only electrical stimulation. This study combined it with actual force presentation generated by weights to investigate how much the force sensation would be enhanced by electrical stimulation.  
On the other hand, the stimulus above the tendon might eventually become a muscle stimulus and cause a weak muscle contraction, affecting the Golgi tendon organ and muscle spindles and producing force sensation. However, according to the conventional EMS work [5; 6; 7; 8; 9; 10; 11] that stimulated the same muscle to mimic an external force, they produced a force sensation opposite to the one produced by stimulating the tendon organ. In order to investigate whether our proposing method is based on muscle stimulation or not, we placed electrodes not only on the tendon but also on the muscle belly of the biceps muscle (Figure 1a) as a condition that actively induces muscle contraction.  
Therefore, we measured and compared the magnitude of the force sensation in each of the three conditions: electrical stimulation of the tendon of the biceps, electrical stimulation of the muscle belly, and no electrical stimulation. The electrodes' positions were decided heuristically, which can be considered reasonable to a certain extent because the effect of electrical stimulation grows weak quickly if an electrode's position is moved from the position no more than 1 cm.  
Our hypothesis is as follows. Stimulation of the tendon will increase force sensation, while stimulation of the muscle belly will decrease it because the stimulation will act as a force driving in the direction of elbow flexion (in the opposite direction to the force due to the weight) as in the conventional method [5; 6; 7; 8; 9; 10; 11].  
The elbow was placed on the desk so that the other proximal joint was not involved in the movement (Figure 1b).