When you're feeling tired, it seems that by moving your body, it takes some of that tiredness out.
When you're feeling tired, I heard that by moving your body, it takes some of that tiredness out.
ご質問ありがとうございます。
「疲れている時こそ体を動かすと疲れが取れるらしい」は英訳すると「When you're feeling tired, it seems that by moving your body, it takes some of that tiredness out.」になります。
上記の英文の「it seems that」を言うとその知識は何処かで聞いたことを伝えられます。日本語だと「らしい」と同じ意味です。
また、「it seems that」の代わりに「I heard that」に変更すれば、大体同じが残ります。
When you're feeling tired, I heard that by moving your body, it takes some of that tiredness out.
ご参考になれば幸いです。