4 sentences

1. 即使再苦再累，他依然挺直了脊。

   jí​shǐ zài kǔ zài lèi, tā yī​rán tǐng​zhí le jǐ.

   No matter how hard or tired he was, he still kept his back straight.

   Source: Dong Chinese Dictionary

2. 听到老师走进来，他立刻挺直了腰板。

   tīng​dào lǎo​shī zǒu​jìn lái, tā lì​kè tǐng​zhí le yāo​bǎn.

   Hearing the teacher come in, he immediately straightened his back.

   Source: Dong Chinese Dictionary

3. 医生建议他在坐着时挺直腰部，以减轻背部的压力。

   yī​shēng jiàn​yì tā zài zuò zhe shí tǐng​zhí yāo​bù, yǐ jiǎn​qīng bèi​bù de yā​lì.

   The doctor advised him to sit up straight to reduce the pressure on his back.

   Source: Dong Chinese Dictionary

4. 他坐得非常挺直，显得很有精神。

   tā zuò de fēi​cháng tǐng​zhí, xiǎn​de hěn yǒu jīng​shen.

   He sat very upright and looked very energetic.

   Source: Dong Chinese Dictionary