Max Lebled, one of the developers of s&box (a sandbox spiritual successor to Garry's Mod built on Source 2), shared a look at a cool animation test that uses the game's new first-person arms to simulate muscle jiggle. Check out the result below:
The developer was also kind enough to share details. For example, Lebled explained a trick for avoiding volume loss on the wrist.
The result is, of course, exaggerated a bit with muscle jiggle but the developer noted that he did that intentionally to test the rig. The whole thing is not automated meaning it's a manual setup.