Charlie often refers to the state of the muscle and muscle tone or tonus. I found a great description from a book by John Jerome - The Elements of Effort: Reflections on the Art and Science of Running. It’s basically a book for the recreational runner/jogger, but has some great info in it. Jerome has also written a couple of other good books that are a must for your library: Sweet Spot in Time - and - Staying Supple (discussing stretching and flexibility). He is a fabulous writer (he’s a writer by trade, not a physiologist or sports expert) that does not get overly scientific in his writings, but is very concise with his descriptions.
Anyways, here is the quote:
"Skilled movement is possible only if the muscle can read its own state of tension: it must be informed, in contact with itself. The contact is maintained, the muscle kept at the ready – cocked, in effect – by muscle tone, a low level contraction maintained by signals from a neural loop call the gamma system. Keep the signal turned on and tone takes up the muscle’s slack, keeping it purring away at idle, ready to respond without lurching.
Good muscle tone works as a buffer against the shock of sudden action, absorbing forces that can, over time, prove harmful to the less elastic tissue of tendons and ligaments. But it is more than a shock absorber: it is part of our very liveliness, the simple underlying tautness that keeps us quick and responsive to the world.
Of course maintaining muscle tone requires nothing more than that we continue to move. But runners have always suspected something like that, haven’t we?"