As much as “flat feet” sounds like a silly problem (Isn’t that why Pike was in the Home Guard in Dad’s Army?) it’s surprisingly serious in hypermobility type.
Flat feet, in real physical terms, is where the talus bone (right in the heart of the ankle) has fallen down and forwards, stretching the tendons and ligaments in the back of the leg. From the outside, this results in a foot which just appears to have a flat bottom, or a very low arch (Hence also being called a dropped arch). A “rigid flat foot” is one which doesn’t form an arch when standing on tip-toe, and a “flexible flat foot” is one which does form an arch upon standing on tip-toe. As far as I can tell, it’s flexible flat-footedness which tends to occur more in hypermobility (Because of the stretchy collagen letting the tarsus slip around), but I’m willing to be corrected on that.
Biomechanically though, it’s much more interesting. Or horrible, depending on how you look at it. The ligaments that hold up the talus and produce the arch basically create a mechanical shock absorber – The arch flexes and compresses throughout the stride, as weight goes from the heel to the toe, reducing how much force is transmitted up the ligaments and into the knees or the hips. Absent the foot arch though, and it’s basically the difference between driving a car with full, modern suspension, and none at all. Every ounce of force from planting the foot at the start of the stride is transmitted straight up the leg, into the knee and into the hip, doing damage to both the already-fragile hypermobile joints and fatiguing the muscles more quickly. As a result, walking and other weight bearing exercise can be prohibitively painful, or prohibitively difficult.
As well as the fallen arch itself creating more stress on the knee and the hip, it also changes the gait slightly – over- or under- pronation (Walking with too much or too little weight on the big toe, respectively) being common, and most noticeably, walking on the outer rim of the foot instead of the sole. This, in turn, places an uneven stress on the knee, and the hip, and eventually the spine, and causes uneven development of the muscles in the leg (Unless deliberate exercise is done to try to correct the unbalanced development) which can then make walking harder, make luxations in the hip and knee more likely due to the supporting musculature not being in the most useful places, and (anecdotally) cause back pain. Whether or not flat feet cause poor balance, or if it’s just that the plethora of other symptoms that come with flat feet in hypermobile people cause poor balance, is a bit of a chicken-and-egg question. Whichever way around it is, flat-footedness and poor balance seem to go hand in hand.
As usual – It’s a problem that, alone, isn’t really a problem (it won’t even get you out of military service these days), but when combined with hypermobility – Where every other joint is hypermobile and operating basically at the limits of its tolerance anyway, any extra pressure can be the straw that breaks the camel’s back.
(Notable runners-up to be “F is for…”; Fibromyalgia, feelings of failure, fatigue, food)