One of the many things trapped in our Discord Showcase channel since before the move to this forum, an example of how one can achieve muscle simulation with Ragdoll.
The simplest case, a sphere connected at two points. You’d get jiggle in the translate channel, leaving rotate free and aligned with its parent, such as the upper arm.
Capsule instead of sphere, meaning you get both translation and rotation, but only along a single axis. This would be a little more detailed, but still perform very well.
Finally, for a full translate and rotate result, with collisions against the underlying skeleton, use 3 or more points to constrain a custom shaped muscle.
Whichever route you pick, you could end up driving a set of joints used to skin the character like this.