It was stated by an observing coach that the girl, leading in the race, slowed because her core muscles are not as strong as her lower back. Are there other areas as well?
I don’t believe she lifts much, if at all. However, after discussion with the coach, she was led to believe that ab workouts are the next step. Any suggestions that can be forwarded?
And he determined this so-called imbalance how?? By “core” muscles are you referring to the musculature on the anterior side of the torso? I only ask this because “core” seems to mean different things to different people. Is this yet another “isolationist” solution to a problem that might be multi-factorial? Maybe the more observant coaches on our board can chime in regarding her form, but I’m not sure any could determine by observation alone that she has an imbalance between her “core” muscles and lower back. If anyone can please enlighten us!
might not just be core might be speed endurance, fitness level
That’s the point. It could be any number of things. Even events not directly related to her physical training could be contributers.
Well, I think she slowed down because she got tired 
Seriously though, I wouldn’t say she slowed because of her core strength. That may just be one of a dozen factors as others have said.
I appreciated the input. The person in question is a local college coach that possibly is recruiting her for his school. She is a sub 25 sec 200m sprinter outdoors and split 25.5 in the 300m video.