na-adjective, noun which may take the genitive case particle 'no', noun
robust; brawny; muscular; sturdy
Describes a person, especially a man, with a powerfully built, physically strong body. Often used in descriptions of soldiers, athletes, or laborers.
彼は屈強な体つきの男だ。
He is a man with a robust build.
屈強の兵士たちが行進した。
The brawny soldiers marched.
頑丈 is more commonly used for objects (sturdy, solid) and can also describe a person's build, but 屈強 specifically emphasizes muscular, powerful physique, often with a nuance of toughness.
たくましい is an i-adjective meaning 'sturdy', 'brawny', or 'rugged', and is more colloquial. 屈強 is a more formal, literary term often used in written descriptions.
Compound of 屈 (くっ, 'bend' or 'yield') and 強 (きょう, 'strong'), suggesting a strength that does not yield. The exact historical derivation is uncertain, but the combination conveys unyielding physical power.