The Quirk allows Muscular to augment the muscle fibers that are beneath his skin, increasing the power and speed of his movements. When using this Quirk, the enhanced muscle fibers protrude from and cover the skin of the area that is being augmented. This rises Muscular's physical Strength to the point that, when he is seriously using his power, can obliterate a large chunk of a cliffside with a single strike.
As an example of a more complex strength-enhancement Quirk, Muscle Augment can be used to attack and defend in a number of different ways. As well as simply augmenting Muscular's strength and speed, he is also seemingly able to control his muscle strands individually, utilizing them for more refined attacks, such as crushing Izuku's hand against Muscular's body by trapping it between him and his augmented muscles.
This Quirk also grants Muscular a very high level of protection. When taking attacks, he can manipulate his muscles to act as a full-body shield, making him capable of withstanding attacks from One For All, even when Izuku used 100% of his power, indicating this Quirk's extraordinary durability. However, it was not enough to protect him from Izuku's 1000000% Deleware Detroit Smash.
The absolute limits of this Quirk are unknown.