I find this advances very quickly to pretty much every muscle. The real trick seems to be finding the opportunities in when to relax.
Kicking leg-angle-foot: relax on down-kick, semi-flex to get toes pointed, and establish the controlled leg-rudder in the wake, but then relax it. Repeat.
Core: quick inhale then relax to let air expand into diaphragm and throughout recovery, then flex to hold streamline during kick and rotation to apply power to the anchoring arm without flopping the vessel all over the place. Repeat.
Neck: flex to turn to breathe and position chin to air, then relax ASAP during recover and before spearing. Repeat.
Arms: flex underwater when hands and arms are in the core zone: shoulder to hip. Relax ASAP as recovery begins throughout recovery and entry, only flexing again to increase spear length. Repeat.
I find in long swims in pool, that my thighs, calves and soles of my feet tire and want to cramp if I'm pushing off the walls hard into long streamlines and if I forget to relax them while swimming.
I find in long open water swims, that my butt muscles tire from holding my vessel and the lower side legs in position (keeping legs streamlined and not splaying) in that breathing phase. Breathing more to the right, my left butt gets tired. Breathing to the left, my right butt gets tired. So I know that I need to work on keeping things inline early, when they need to be controlled while holding the skate edge, then relax in transition when fully relaxed legs fall into the right position without energy (when I'm flat).
We can always flex everything all the time, and it makes for a great, fast, low SPL 50Y.
But swimming a full 45-75min session, for me means I have to find which can go when. It may seem like minutia but it pays off in longer swims. Recently I found (OK Coach Stuart found) my left forearm flexing during recovery which tucks my wrist into a funny looking claw. Fully relaxing straightens it out, but I have probably over a million strokes flexing that little forearm muscle, so it's taking me quite some effort and time to unlearn that muscle memory.
Good topic BTW.