士道は身体を硬直させた

悪質な白昼夢でも見ている気分だった。
だがーーそこに振り立った人影を見て、士道は身体を硬直させた。
xxxx
背には大きなスラスターがついており、手にはゴルフバックのような形状の武器を携えていた。
士道が身を凍らせた理由は単純だった。少女の顔に、見覚えがあったのである。


Hi. Could you please explain the underlined parts?
I know they should mean "Shidou's body stiffened/froze up". But why is causative form used here? The causative form seems to render the sentences into "Shidou made his body stiffen (or Shidou stiffened his body)" and "Shidou made his body freeze up (Shidou froze up his body)", which seems quite strange.
Thank you.

The causative form is often used for the subject's body parts in Japanese even if it's an intentional action. Those are the same as 士道の身体が硬直した and 士道の身が凍った in meaning, respectively.
e.g.
胸を躍らせる （＝胸が躍る）
目を泳がせる （＝目が泳ぐ）
身を震わせる （＝身が震える）

Toritoribe said:

The causative form is often used for the subject's body parts in Japanese even if it's an intentional action.

Click to expand...

Thank you, toritoribe-san. But for the underlined part quoted above, do you mean "unintentional action"?

Ooops! Sorry. Yes, I meant "unintentional action".