4 sentences

1. 坐着的时候挺直背部。

   zuò zhe de shí​hou tǐng​zhí bèi​bù.

   Keep your back straight when sitting.

   Source: CCMatrix

2. 保持背部挺直，抬头。

   bǎo​chí bèi​bù tǐng​zhí, tái tóu.

   Keep your back straight and head up.

   Source: CCMatrix

3. 最重要的是保持背部挺直。

   zuì zhòng​yào de shì bǎo​chí bèi​bù tǐng​zhí.

   The most important thing is to keep your back straight.

   Source: CCMatrix

4. 保持肩膀挺直。

   bǎo​chí jiān​bǎng tǐng​zhí.

   Keep your shoulders straight.

   Source: CCMatrix