In physical therapy and occasionally in sports coaching, there's often a precise description of the mechanics of what happens during the movement (which muscles are engaged, which joint moves where etc). I couldn't find any information on where that knowledge comes from - observation and "probing" for engagement alone would not be enough as some muscles are tiny and/or hidden under bigger ones. So, how was the theory developed?
Example: how do we know these are the muscled that perform external rotation (at the shoulder, for instance) ? https://en.m.wikipedia.org/wiki/List_of_external_rotators_of_the_human_body What tells us every single one is necessary? What tells us another one isn't involved?
(please let me know if this is better suited for a different StackExchange)
