The old man was hobbling after he injured his knee in a fall.

hobbling 是动词 hobble 的现在分词形式。hobble 的意思是"蹒跚而行、跛行"，通常指因受伤或年老而走路不稳、一瘸一拐的样子。这个词形象地描绘了腿脚不便时走路的样子。