This is going to be Part 1 in a multi-part series on the feet as a doorway into the nervous system.
We know that having flat feet or collapsed arches has been touted as a risk factor for injury (albeit somewhat inconsistently). And it just doesn't pass the eyeball test... Flat feet just look unathletic. However, the static assessment of having feet flatter than Frodo is just that -- static. A more appropriate assessment of the feet should include a dynamic element that allows for pronation when they're supposed to pronate and supination when they're supposed to supinate. Which illustrates the point here: the foot is a dynamic system of sensory inputs and neuromuscular outputs. One way that we can upregulate and harness this dynamic system is thru maintaining the short foot position during movement. Or, to steal my own thunder, generation of the short foot may be an indicator of neurological readiness and capacity to safely tolerate high volume and load.
Developmentally, the foot has a few fixed points that were crucial when we learned to bear weight and eventually walk; points which we can reclaim as adults and undo our shoe-wearing, flat-footed foolishness. And most babies are not born with flat feet - we develop collapsed arches with our propensity for cushioned, heeled shoes and artificial arch supports robbing the foot of the demand to maintain the perfect balance of mobility and stability.
But I digress...
The three crucial points of the weight-bearing foot:
Essentially, this is the tripod or short foot position. This foot position is absolutely vital for powerful lifts, big jumps, and at the end of stance phase of gait when the foot re-supinates. Short foot puts the 33 joints of the foot on tension, centers the foot, and fires off the high density of proprioceptive nerve endings which are relayed to the brain. This stable position allows for a system-wide increase in force output and motor control. The strength of this stimulus probably improves motor learning due to the positive, feedforward stimuli. A centered foot position tells the brain that the position is a stable one and it is safe to generate a lot more force without risk of destabilization and injury.
It is no coincidence that the external cue to "screw the feet into the ground" helps to foster this short foot position. The system seeks joint centration for max output, so by bearing weight predominantly thru these 3 points the foot is most centered. The other thing you'll notice with this cue is that the foot is not a static, inanimate object! It is a prime source of neurological input and output (more on that with an upcoming post). Maintaining the short foot position can generate an arch and produce a monstrous feed-forward loop that maintains tension and stability throughout the system reinforcing to the brain that force can be safely produced. When the arch collapses the nervous system quickly downregulates in an attempt to avoid excess force thru the entire lower quarter while in an unstable position, yielding decreased output. This is not unlike how head/jaw position influences spine control or how chronically "tight" hamstrings are likely a protective response to prevent neurologic injury when in untenable positions.
Also, check out Charlie Weingroff and Andreo Spina for their take on this topic.