Strongarm allows Rappa to rotate and move his shoulders at extreme speed and power, giving him commensurately fast and strong punches.
He is able to throw a huge flurry of punches that are incredibly quick, and immensely powerful. This Quirk is strong enough to shatter Eijiro's Red Riot Unbreakable form with ease, as Rappa broke Eijiro's hardened skin with a single punch.
Kendo Rappa's Quirk works by constantly rotating his shoulders a superhuman speeds. Although he appears to ignore it, the drawbacks of his Quirk can tire out his muscles if overused. This power can also only be used for a few seconds at a time. In terms of raw power, it is unable to break through Tengai's Barrier Quirk.