Why is that such a good exercise? I have always thought of sissy squat as a body building exercise. Frankly knowing what I know today, I would not have had those girls squat. Basically I am not so sure that that a middle distance and distance runner needs to do much more than bodyweight squats if that. Today I would have them doing lunges and step-ups. To answer your question – no none of them had an injury history. They were all athletes who were very active, very different from today’s kids.