Crunches are the go to exercise for abs. Class instructors, PTs and fitness instructors love throwing in a crunch as it's a relatively simple exercise..... Right? Well, not necessarily.
Crunches are usually used as a time top exercising, meaning "45s of crunches go go go" but when the objective of the exercise is to ensure proper engagement of the rectus abdominus. When it's done for time as as quickly as possible (which most people adopt for class work) activation of the correct muscles lessens and we're left with a large degree of spinal flexion.
Now, despite what people think flexion of the spine is natural, its supposed to flex and extend. To an extent. When you take into considerarion the average amount of stress put onto the lumbar section of spine during a crunch we're looking at roughly 3300 N of force. So you can see why after the average person performing their 50 crunches can end up with a sore back and discomfort.
When coming up, aim to keep the spine as neutral as possible with minimal flexion, doing this engages the rectus abdominus fully and stops the risk of repeated spinal flexion. Additionally, for those with a weak or untrained core, I’d recommend partial crunches where you don't come all the way up, just use your abs to pull your shoulders off the floor and hold for 3-5 seconds. It should still burn and will help develop strength for those abs.
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