Before I ask my question, here's a little introduction of myself. I am a college student studying in Bioengineering, so I've been tasked to create an equipment to help rehabilitation for upper limb (any point of the upper limb, doesn't have to be the entire upper limb). So I have this initial Idea that I'd be creating a device that hugs onto your forearm and biceps, allowing for rotation at the Antecubital area (The opposite side of the elbow). Therefore allowing the user to do bicep curls, however I'm facing a problem, I need to give the device adjustable resistance. Here's what I came up with 1 - Inserting a huge torsion spring the middle of the equipment, using a sliding mechanism on the forearm & bicep pieces to adjust the active length of the torsion spring to adjust resistance 2 - Using cables or elastics to adjust resistance the pieces can give 3 - Scrap the idea of resistance and just give a piece that rotates for exercise Any advice on calculations, design ideas or insights into this would be appreciated. Thanks in advance.