I notice that most anatomy books don't show you how the whole muscle looks by itself, only how they look all bundled together. And they also tend to show muscles colored in red, which makes it a little harder to discern them individually.
These Halloween skeletons are not truly anatomically correct, usually. They are fairly simplified. But their proportions are more or less close enough. I'm primarily interested in the superficial muscles, as they apply to drawing, so I'll be simplifying it too.
Step 1: Materials
bake and bend sculpey mixed into muted colors $7-$8
optional: pin vise, small drill bit and large jump rings for jointing the arms.
You could try this with Sugru if you're fast enough. Though I worried that the 30 minute working time just wasn't enough. Note: Sugru does seem to bond to these plastic skeletons reasonably well.