The noun 力气 (lìqi) refers to physical strength or muscular effort. It is used when talking about the energy or force someone exerts to perform a task. Common collocations include 有力气 (to have strength), 没力气 (to lack strength), and 用力气 (to use strength).
搬了一天的家,我已经没有力气了。
Bānle yī tiān de jiā, wǒ yǐjīng méiyǒu lìqi le.
After moving all day, I have no energy left.
他力气很大,一个人就能搬动那张桌子。
Tā lìqi hěn dà, yī gè rén jiù néng bāndòng nà zhāng zhuōzi.
He's very strong — he can move that table all by himself.
你得吃饱饭才有力气干活啊。
Nǐ děi chī bǎo fàn cái yǒu lìqi gànhuó a.
You need to eat your fill before you'll have the energy to work.