We go hard in Krav and ouchies like big bruises and sore muscles are just the cost of doing business if you want to train. As one of my early trainers told me when I was whining about being sore:
***Everybody has aches and pains. You can have the aches and pains of an athlete or the aches and pains of an overweight couch potato. Pick one.***
However, there's a difference between an ouchie and an injury. An injury to the shoulder or knee (I'm icing my knee as I write this) can significantly inhibit your training. A back injury can take you out of the game permanently.
So it's extremely important not only to train smart and know when to attack and when to ease off, but also to protect your spine by strengthening your core. It may not be fun to do core exercises, but you only get one spine. You're never going to get another one.
Does it sound like I'm lecturing you? Yeah, I kinda am.
Here's a video of some core exercises suggested by Stuart McGill, world renowned Professor of Spine Biomechanics at the University of Waterloo in Canada. McGill is the guy professional sports teams, governments, and rich dudes fly from all over the world to be treated by.
You will find that he dislikes crunches, and we do loads of those, don't we? So does every other gym I've ever stepped foot in, so take his advice and make it work for you. (Spoiler alert - here's your little change.) When you do sit ups or crunches, raise up from the chest & maintain a neutral spine and neck. Imagine there's a string attached to the ceiling and to your sternum and it's drawing your sternum straight up. It's hard, which is awesome, and saves your back.