dig in one's heel

土日と卓球の試合が続いたので忙しかった。成績は３位と２位で中々優勝できませんがその内に勝てる時もあるでしょう。さて、英語の勉強を再開します。Net Forceは主に青少年を対象にしたSFだと思いますが、慣用句が頻出します。
The whole thing reminded him of a horseback riding incident long ago, when the horse had abruptly broken into a gallop and he had been able to do nothing but clutch at its mane and hang on, hang on until it stopped. That awful feeling of helplessness. Nothing you could do. Dig in your heels, hang on harder, yell, scream, it won't make any difference. You're going to be crazy now.
"Dig in your heels" は慣用句でしょう。辞書で調べます。
・Oxford English Dictinary: Resist stubbornly; refuse to give in: 'he has dug in his heels and refuses to leave’
・Merriam-Webster's Online Dictionary: to take or persist in an uncompromising position or attitude despite opposition
・Cambridge Dictinaries: to refuse to change what you believe is right or what you want to happen Be firm on important issues, but do not dig in your heels at every opportunity.