Understanding the strength curve of an exercise is an important consideration that (hopefully) your coach takes into account. It's a challenging concept to get a feel for, but you can think of it as which part of the exercise that's the hardest, which often comes down to where gravity is working against you the most. So for these 45° back extensions, it's the mid-range of the movement, where your body is parallel to the floor. But with a standard back extension, your body is parallel to the floor at the end range. And with a good morning, at the beginning. Make sure your program touches on each spot so you're strong and healthy at every part of the curve.