Strecthing will most likely not chance fascicle length. And increased fascicle length will most likely not improve perfomance.
Their are alot of theoretical problems with the glut bridge. And also more pragmatik problems like Technic.
1. The loading is horizontal, most of the load to overcome in sprinting is vertical in natur.
2. The limiting faktor for increasing weight in the excercise is concentrics strength in inner lane of the gluts. In sprinting the primarie load, is mid and outer range.
3. The systemic load on the body is low, and while the exercise shows High EMg activity i predict the micro trauma in gluts is a lot less than lunge and squatting due to the lack of overload in the hole length of the muscle. EMG does not measure load on the passiv structures and due to ssc their is a load on these in squatting exercises