Isometric exercises actually involve contracting the muscles.
I'm talking about not contracting the muscles, but simply imagining/visualizing them lifting/pushing/pulling etc.
Here's a fun experiment:
1) stand beside a wall with one arm flat against it, and try pressing the arm into the wall as hard as you can. Hold a few seconds.
2) Step away from the wall, relax that same arm completely and imagine that you are doing that same press without any actual muscular contraction. Hold a few seconds, then let go of the image.
3) Repeat step 2 a few times and see if you can feel any difference when you call upon the image/sensation and when u just completely relax
This is a small taste of how evoking sense memories is important mental exercise.
"We are not stuff that abides, but patterns that perpetuate themselves." - Norbert Wiener