To repeat what I stated in the comment, 「体幹の見切りと筋肉の凝固弛緩その隙間」 is NOT a sentence by any standard. It is only a noun phrase in which everything else will eventually end up modifying the last noun 隙間. I repeated it because so many members here call little phrases "sentences".
隙間 is a key word here. If one thinks of a "spatial gap" just because a dictionary says so, one will be left in the dark with the whole phrase. The word is being used to refer to a "temporal gap" here. I shall return to this point.
体幹の見切り might be a difficult phrase to comprehend because 見切り is being used as a technical martial art term here rather than for a usual dictionary meaning. It means pulling back one's upper body to parry an attack.
筋肉の凝固弛緩 means what OP stated without a twist.
Now, in order to pull back one's upper body, 筋肉の凝固 naturally must occur, followed immediately by 筋肉の弛緩. That split second between the two sets of muscle activities is what the author refers to as その隙間. He is saying that that is the ultimate moment for an effective attack because that is the only moment that the opponent is defenseless.
If it helps the Japanese-learner, try inserting a 「それに伴う」 between 「体幹の見切りと」and「筋肉の凝固弛緩」with それ referring to 体幹の見切り. It MIGHT become a little easier to understand the phrase.