When it comes to big compound lifts like deadlifts and squats, many people think they know the correct technique and posture for them, but in most cases they are a bit off.
The most common problem is slouching your back over (like the guy on the left in the illustration below). People new to deadlifting seem to almost always start out like this if they are unsure on the proper alignment. When I first started I too did this.
You tend to also see guys do this when trying to lift extremely heavy weights that are in most cases too heavy for them. Hence the bad posture begins to come out to try and compensate and get the weight up. Doing this with heavy weight is just asking for an injury to happen, so make sure to learn the proper technique, and increase weight progressively for you.
So after some time, people may see your bad form and some may try to explain to you how to deadlift and squat properly. They usually then tell you to do the complete opposite of the slouching dilemma. They tell you to arch your back and stick your butt out (kind of like a much more exaggerated version of the guy on the right in the image below also known as Anterior Pelvic Tilt (APT)). I thought this was the correct way to do things for a very long time, as it did make more sense than the slouching version.
While it was a step in the right direction, the technique is still not 100% correct. Ideally you want your back to be completely flat and have a straight line from your head all the way to your butt. The way to do this is to squeeze your glutes and push your belly button in as much as you can (think of trying to touch your belly button to your back). Give it a try now. You can stand up and practice it. Don’t worry if you can’t get it right away, it takes some practice to change what you have been doing for so long.
The perfect exercise to master this and get used to it is the hollow body hold or hollow body rocks (you can see how to do them in the video below). These are the core exercises (excuse the pun :p) in gymnastics. A lot of times when people train their core they forget this very important point, and that is to keep a flat back and squeezed glutes while training core. For instance, the plank that you see everyone perform. If you just lazily get into a plank and hold it without activating your glutes and bringing your belly button in to create that straight line from your head to your butt, you are working more of your arms and shoulders than your ‘core.’
The video below explains and shows all the above points quite nicely if you are still unsure: