Hi Kieran! Nice practice drawings!
My advice to you is that when doing your underdrawing, resist the urge to draw the torso as one long rectangle. The ribcage and the hips are actually distinct structures with a fair amount of space in between them, allowing the human body to bend, hinge and twist in the middle, almost like another joint.
You can gain a better understanding of the structures you're looking at, and work toward more dynamic poses, if you practice drawing the torso as two different structures rather than one long rectangle.
Hope that helps. Happy practicing! :)