The Japanese is:>気づかない内に“振動”トレーニングでもしたか？
It sounds like the part that confuses people is the "shake training" part which is:>“振動”
"Shindou" (i.e. similar to the pronunciation of the character's name) which means oscillation or vibration, or as it's been previously localized before, shaking. And then:>トレーニング
Which is literally just "training" spelled out in katakana.
And generally speaking, shake training is a type of resistance training people will sometimes do I think?
The rest is just 気づかない内に which is the "without realizing" part, and でもしたか which is the "or something like that" part.
Basically Deku is saying that Shindou's big sacrificial shaking attack is what ultimately exhausted his muscles all at once, so it wasn't for nothing and all that jazz.