Muscular firmness (4)
I believe the answer is:
tone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">muscular firmness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for tone that I've seen before include "Sound signal on a telephone" , "Musical note; general character" , "A pitch of note" , "Musical sound of note" , "Pitch of a voice" .)