"Excuse me, I meant is this research with the force being directed
downwards, in the direction as in skiing (a chopping or pulling
movement), or with the force upwards?



I checked the sources I was thinking of ("Ultimate Back Fitness and
Performance") , and it's definitely the crunch motion against a load
that also caused an increase in herniations. And it seems it was
originally thought that full flexion was required to cause herniation,
but recent paper in "Spine" said repeated flexion in the "mid-range"
also causes problems (I didn't see this directly--my chiropractor told
me about it).

In terms of what's going on with a traditional double pole, I always
thought the hip flexors played a big role, while the abs just
stabilized things. This seems similar to using an ab wheel at the
point you pull the wheel back to you--abs get an amazing workout but
spine is still perfectly straight.

Jon