Testing what exercise actually does to your butt

I'm about two years late to this party and am not sure if anyone already mentioned this. But, to elaborate on the sprinter large booty vs distance runner flat booty. A sprinting athlete has different mechanics during a sprint than a distance runner on a long run. The variable mechanics causes a different demand for the muscles recruited. Secondly, because of the distance and time spent active during these different movement modalities the energy demand on the muscles will be different, aerobic vs anaerobic. This variance in energy demands causes muscle fibers to adapt differently and become more fast twitch dominant vs slow twitch dominant. Typically, a fast twitch muscle fiber dominant athlete will have more muscular hypertrophy as a byproduct than an athlete that is slow twitch dominant and higher aerobic capacity.

 Also, it's worth mentioning that during compounding movements like squats, muscle fiber recruitment can be higher if load/resistance is added and the mechanics/form of the exercise is executed properly.  Also, there exist other factors during the executed exercise that can alter muscle fiber recruitment like angle/ROM of movement, variance in speed of execution of exercise, time under tension (holding position), and focus on concentric or eccentric phases. 

Great video!