A recent ACE study found that crunches are the most effective exercise for activating the rectus abdominus, which is the main, outermost core muscle. But this is only one of the muscles you need to tone in order to score a cut core. "These findings do not mean that the crunch is the best ab exercise overall," says Bryant. "There are other abdominal muscles, such as the external obliques and rectus femoris, that play a powerful role in spinal and core stability. Side planks and other exercises are better at activating those muscles." Bryant says. "No single ab exercise is going to address all that you need." That means you need to combine exercises that target your core from all angles, including your lower back.
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