I think that it’s down to the change in sprinting posture brought about by constant lactate tolerance training. Too much of this stuff and the athlete tends to sit and shuffle, much more like an 800m runner. It can be subtle, but when the athlete tries to sprint at something approaching full effort, the hamstrings go. I know that this was the experience of many in the college program that I was in.
The other factor, and Kevin Tyler spoke about this as it related to one of his athletes, is the loss of muscle mobility/viscosity brought about be too much work in this middle range. He might say any, but I think that there’s room for it in reasonable doses. Could there be some actually changes in the muscle chemistry? I’m not that scientific.
The other option is to try to sprint in this position. It’s safer, but totally ineffective.