Muscle Augmentation

Muscle Augmentation, or Kin'niku Zōkyō, is a Transformation Quirk built for close quarters, first appearing in Chapter 74 and Episode 42. Supplementary material has also listed it under the name Pump Up. It reaches print through Goto Imasuji, better known as Muscular, though the roster of holders is unusually wide for a power this destructive.

All For One carries a version of it, lifted at some point from a person the story never identifies. Muscular's documented history with Kyudai Garaki suggests the copies handed out to others are duplicates spun off from his own. In function it is nearly indistinguishable from Muscle Expansion.

The holder can bulk up, shape, and direct the muscle layers of their body, whether those layers stay under the skin or burst out over it. The result is strength, speed, and toughness far past human ceilings, enough that a user can go toe to toe with One For All running at 100 percent.

Brute force still has a ceiling, though. Anyone strong enough can simply out-hit it, which is what happens when a One For All user pushes past their own maximum output. Muscular's variant carries a sharper flaw: fibers that burn through their energy start shredding and peeling off him. Yo Shindo weaponizes that by running Vibrate through a mass of strands and wrecking them in a single stroke, which strips Goto of reliable access to his Quirk and leaves him open. Izuku Midoriya leans on the same weak point to take him apart quickly in their rematch.

Goto Imasuji sheathes his limbs in bulging fiber that ruptures out through his skin, and he picks and chooses which parts of himself to reinforce. At his peak he stacks upward of twelve thousand layers. The payoff includes tearing chunks out of a cliff face, kicking off shockwaves that trigger rockslides, and manhandling Izuku in a contest of pure strength until the boy is nearly crushed. Those layers double as armor, soaking damage well enough to shrug off a 100 percent Detroit Smash carrying All Might's own power; only Izuku's 1,000,000% Delaware Detroit Smash finally rips through. Loading the augmentation into his legs turns him into a blur that leaves 5 percent Full Cowl behind and lets him slip attacks entirely, a problem Izuku solves by wedging his hand into the exposed fibers so Muscular cannot dodge the next hit.

The Four Eyed Nomu of Hosu City swells its own mass with the Quirk and pairs it with Tongue Web against Endeavor and Gran Torino.

Wolfram receives the ability from All For One. His version keeps the fibers beneath the skin, which instead flare a vivid red and visibly change him, much the way One For All at 100 percent alters All Might and Izuku. Plugged into the Quirk Amplification Device, it briefly gives him the muscle to seize control of his fight with All Might, injure him, and paper over how poorly Metal Manipulation serves him up close.

Hood treats the fibers as scaffolding for his other powers. He sheathes his arms, reshapes the mass into whipping tendrils, and stacks a separate strength-boosting Quirk on top for even more output, which is enough to bury Endeavor under sheer force. Anything torn off him grows back courtesy of Super Regeneration.