Synthetic musculature, hmm... I never really thought about it, but now I'm curious about Bubbles's inner workings.
well, from the joke at the bottom of the comic, we might infer that there are ball bearings to help facilitate the movement in her hip joints. I wonder how small they are.
EDIT: How small the ball bearings are.
I see two basic styles of chassis here with a whole range of variation in each category and some possible overlap.
Punchbot seems to be more industrial.
The chassis lacks any extraneous sensors or accessories like hair or ears.
Design seems to be very industrial with a mixture of internal framework and external shell both providing structural support.
Limbs have full freedom of movement based on how they are assembled.
Hydraulics would be the most practical for speed and power though fine motor control is also doable.
Most likely it is a mix of hydraulic and electromagnetic servos.
Full anthropomorphic chassis like Momo and possibly Bubbles, are modeled on a humans skeleton and musculature structure.
Ranges of movement and strength levels will therefore be similar or regulated to keep them scale-able to the human norm.
So dermal layers are more like organic skin and provide a protective layer as well as having the option of embedded micro-sensors of the required types like pressure, temperature, light, etc.
As for locomotion there are already in existence various technologies based on memory metals and plastics that can be assembled as fibers to look and behave like muscle tissue.
So no need to go all industrial with a rotating asymmetric ring design.
TL:DR
Full Anthropomorphic AIs have synthetic muscles and can be as sculpted as a Greek statue.
Warning - while you were typing a new reply has been posted. You may wish to revise your engineering speculation. Also a micro black hole drank your coffee.