oh it's not weak muscles? walk/run barefoot for a few months and see if you continue to have this problem,
it took me a few weeks of gradually increasing barefoot walking/running until i was comfortable enough to go at it all the time,
i walk on pebbles, concrete, dirt paths, sand you name it, the human foot can handle it,
it's not unreasonable to say that pro nba ballers can have weak feet, you can be strong all over but if you're in cushioned shoes all the time...
weak foot muscles aren't something that will keep you from competing in your sport, at least not until you develop something like camby,
think of all the nba ballers with back pain, great example is toni kukoc, he had back pain later in his career because he had a weak back, if he strengthened his back he might have extended his career,
pro athletes are still humans and subject to all types of weaknesses, remember that sports are specialized, which means that they favor specific movement patterns,
if it's inflammation of a connective tissue, what exactly do you think the connective tissue connects? the foot musculature is very fine but important and a general atrophy brought on by an over-reliance on cushioned footwear can mess you up, mess you up bad,
you don't need an academic study to justify everything in life, start walking barefoot and stick with it, see what happens,
it's likely your plantar fasciitis symptoms will get worse in the beginning but will gradually improve as he weeks go by, eventually you will have a strong foot,
also, beach volleyball is different than regular volleyball, one is on sand and barefoot, the other is in shoes and on artificial surface,
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