Is this a wind up??
The straight leg sit-up is a good exercise for runners, because it works the pulling muscles of the upper leg as well as the abdominals.

The musccles used fro this one have very little to do with "CORE STABILITY".

See Fellrunner Magazine article ?? not sure when, but Denise Parks explains the whole thing very well........NOT to do with the superficial muscles or pure strength, more a deep postural thing...beefcakes beware, you have to think whilst you do any core muscle work!!