Originally Posted by Greg Everett
You should get a better stretch reflex in the mid ROM because you're going to get the best actin-myosin coupling. More flexibility = longer resting length via sarcomere addition = mid ROM is farther out. So really you just need to decide 1) what the necessary ROM is and 2) where you need maximal tension in the pertinent movements. You may not be able to get the two to coincide well, so you want to minimize disruptions to each by the other.
But wouldn't that put say an unflexible person (whos limit is maybe a bit more than ATG) near the edge of their ROM like an arbitary length of 10cm close to about 1cm from their length which is ~4 cm from midpoint whereas if you had someone with the same biomechanical anatomy at 14 cm flexibility then they would be 5cm from the edge of ROM which would put them only ~2 cm from the midpoint.
Well, whether you're looking at sarcomeres or the whole muscle length it's still similar in the respect of near the middle you're the strongest and at the edge of ROM you're the weakest contraction-wise. So the "longer" the muscle is it would seem that the closer it would be to the midpoint if you had to make a cutoff for squat depth.