Of the three boys, one was big — muscled like a serious weight lifter, with dark, curly hair.

【翻译】三个男孩中，有一个大个子，肌肉发达得俨然像个举重运动员，一头乌黑的卷发。

muscled 是形容词，意为“肌肉发达的”，破折号后面的 muscled like a serious weight lifter，进一步解释或描述was big 的样子，介词短语with dark, curly hair 也是作描述性状语。with 前面加了逗号，说明它不是修饰weight lifter 的定语。介词短语没有动作概念，不作伴随状语。