It’s a question I sometimes remember, when doing fitness. I always wanted to wear weights on my arms or legs. But they are always filled with sand and are very bulky and weird. I am wondering, why we’re not using more dense materials? Why not metal? The weights at the gym aren’t sand.
I’ve had several cuff weights over the years and a lot of them used what I assume is bird shot. So some do use metal, it’s probably based on weight and quality.