省略了that的定语从句.
He ran as fast as his short legs (that) could carry him.
that could carry him 是his short legs的定语从句.
翻译：他用他的两条短腿尽可能快的奔跑.谢谢，请问carry him是什么意思呢？英语翻译不需要逐字译的。 his short legs that could carry him 可翻译为 “支撑着他的两条短腿。”