To Kiren, from my personal experience, I don't think the electrical shock like sensation is caused by metal. When I was a student, my classmate put the US on my elbow, I don't have metal inside, but I felt this kind of electrical like pain, very sharp. The intensity is very low, we even put it very low, but I still feel it. I can't explain why this happened. I just know that I don't have metal implant.