I've heard that there are two sorts of skeletal muscles-- little muscles near the bones which specialize in support, and bigger muscles in a layer farther out which specialize in movement. Biology being the complicated thing that it is, I expect that there's some overlap.
If one's body isn't organized in a way which lets your support muscles really do their work, then some of it has to be done by the movement muscles, which means they're overworked (support muscles are optimized for endurance) and less free for movement.
What has just occurred to me is that support muscles aren't very voluntary compared to movement muscles. That's why it's a standard piece of advice to imagine a string pulling you up-- using your imagination contacts your support muscles in a way that deciding to stand up straight doesn't. (The string thing might contact your support muscles-- it doesn't do anything for me. I'm willing to cut the common wisdom of the human race some slack and assume that it works for a lot of people.)
 In other words, I've heard it from people I trust, it sounds reasonable to me, and I don't have a convenient way to check it.