Keep back straight and bend knees.

背要直，只弯两膝。

youdao

Lean on the ball as shown, keep the back flat or maintain its natural curve, with the legs straight.

身体俯卧于球体之上，动作如图所示。背部保持正直或保持自然弯曲状态，腿部伸直。

youdao

Keep your front shin vertical, back knee straight, back heel lifted, and chest lifted.

保持前面小腿垂直地板，后膝伸直，后脚跟离地，胸腔上展。

youdao

"It tickles a bit when it's applied, " Ms. Pavenooch said, leaning back and closing her eyes as she tried to keep a straight face.

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