Variation is the key to positive changes when it comes to our fitness. Sometimes our bodies grow too familiar with the same movements or our growing strength needs more intensity and challenges.

Core strength testing has come a long way from the straight-legged, twisting sit-ups used before World War II. We've moved on to bent-knee sit-ups, to crunches and now to the plank pose or hanging ...

Add Yahoo as a preferred source to see more of our stories on Google. Marlo Alleva demonstrates a reverse plank knee up. Variation is the key to positive changes when it comes to our fitness.