He cracked his knuckles before he started typing.
他打字前把指关节掰得咔咔响。
She skinned her knuckles climbing over the rough brick wall.
她翻过粗糙的砖墙时擦破了指关节。
crack one's knuckles 是掰响指关节的常见说法。脚趾的关节也叫 knuckle。