Where does the movement performed in sit-ups occur in a runner’s mechanics?

Or crunches?

Or back extensions?

The fact is the concentric contraction, i.e. the actual shortening of the abdominal and erector spinae muscles, occurs only at a very micro level while running. Instead, these muscles are used, along with their synergist muscles, primarily isometrically as they work to keep the hip and body stable. To get your core strength to a solid foundation ensure you are symmetrical and can hold each of these single leg positions for one min per leg!

With this symmetry of strength in place, you can now add mobility and dynamic movements to your core routine knowing that you are stable, balanced and strong enough to handle more athletic movements.

Here was an initial investigation into the Bunkie Test:

www.ncbi.nlm.nih.gov/pmc/articles/PMC4339703/

Note the added bent-leg version for the posterior hold!