build up one's strengh 增强力气
He tried to build up his strength.
他试图增进体质。
You must build up your strength again.
你必须再次增强你的力量。
build up one's body 锻炼身体
例句
Exercise will firm up your muscles.
锻炼身体能使肌肉结实。
I've been trying to tone up at a health club.
我一直努力地在一家健康俱乐部里锻炼身体。
追问他们之间可以互换吧？
追答意思差不多。