No, not sit-ups — try the runner's plank to strengthen your abs without hurting your lower back

The runner's plank is an alternative I use as a certified trainer when sit-ups aren't possible, and this is exactly how to do ...
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3 Abs Exercises You Need to Shred Your Six-Pack

ARE YOU SPENDING your summer crunching away, trying to get visible abs for the remainder of the sunny season? You can pile up all the situp reps you want, but you won't be much closer to the core ...