I don't have muscular, or fat!, arms but the bindings that go round the arm barely make it. I trimmed the Velcro hooks back to expose more loops and that bought me one more centimetre, which helped.
I haven't tried it in action yet but the principle is good, a plastic strip is basically pinned against the point of your elbow, if you bend your elbow the strip "pops" and makes a clicking noise. (The strip is a "U" shaped channel, some rigidity but able to deform. Imagine a steel tape measure bending the wrong way.) Straightening your elbow (I have to flick my arm to slightly hyperextend) pops the strip rigid again. Plus the overall strapping effect encourages the arm to stay straight. My right arm collapses almost before I hit the ball and ends up wrapped round my neck so I'm hoping this works. Compared with the price of clubs and lessons (and even golf balls!) it's worth try, just be prepared to do a bit of DIY if it is tight.
Oh, no instructions included so I was rather surprised when it clicked the first time!