The user has complete control over their own muscles, allowing them to freely alter and manipulate them, including altering muscle tissue itself, its development, increase/decrease muscle mass/strength and thus stamina, flexibility, reflexes, etc.
Gyanza Rujike (Black Cat) can increase his muscles.
Ikkaku Madarame Uses his muscles to fix his arm (Bleach)
Vilgax (Ben 10)
Humungousaur (Ben 10: Alien Force/Ultimate Alien)
Doomsday (DC Comics) has the power grow stronger every time that he is killed.
Bane (DC Comics)
Most Super Saiyans such as Trunks (Dragon Ball Z) can ascend into an ultra state when not only their powers increase, but their muscles too.
Younger Toguro (Yu Yu Hakusho) bulking his muscles to 20% power...
...and bulking his muscles to 120% power.
Suigetsu (Naruto) can manipulate his own watery body to increase muscle mass.
Shinnō (Naruto) using Body Revival Technique to activate cells, healing his body by manipulating the muscle's "Destruction and Rebirth" cycle, which also makes his body stronger in response.
Roronoa Zoro (One Piece) can use pure muscle to pump up his arms with his Ichi Gorilla and Ni Gorilla technique.
Charlotte Katakuri (One Piece)
Muscular (Boku No Hero Academia) posses a quirk called Muscle Argument which allows him to change and increase the size and mass of his muscle fibers and coat his body parts, modifying his strength and endurance.
Hulk (Marvel Comics)
Medaka Kurokami (Medaka Box) have absolute control over all her muscles, making possible to cancel involuntary reflexes.