Luckily the problem for both of them is the same. You’re not using your back, which is causing you to use a lot more back. I bet your lower back probably feels a little worked after a session like this.
In the start position you need to really pull your shoulders down towards your ass and straighten your mid back more. In the videos you can see it is slightly rounded to start and gets worse as you go through the set. This leads to the next point. Stop between each rep with the bar on the ground. Don’t stop and stand up, but stop, make sure you are tight again, fix you back, straighten it lock the shoulders down, then do the next rep.
