If you are reading this while in any position other than about-to-fall-asleep, your "core" is firing. These muscles--not so unlike the smooth, always-working muscle of the neighboring digestive tract--are in near-constant chorus as they hold us upright and help us navigate our worlds. The sum of all that activity, in real-time and in response to real and changing stresses, is called posture.
A conversation with a friend recently yielded a similar description, though it was referring to the kind of mental discipline that we see in the more authentic/successful/happy people whom we interact with. These people stand out most because they just seem to make the right social/time-allotment/happiness decisions most of the time. They don't struggle with too much deliberation, but they also weigh options objectively and judiciously. How?
It seemed to us that this everyday wisdom came from a kind of mental discipline. Without clenching, these people are loosely focused on what's important to them, more or less all the time. They consider the impact of their choice, and they choose according to how their next step fits with the larger context lives. But most importantly, they don't commit themselves to the emotional attachments of certain decisions before they have had a chance to think objectively, even for an extra moment or two.
These folks express life-fitness because of their ability to be mentally strong in a simple, low-level, but critical way, all the time. They have great emotional posture.