Do you have chronic lower back pain? Neck and shoulder pain? Do you sit all day at your job? It could just be that the way in which you "sit up straight" is a bigger contributing factor to your discomfort than you realize.
I worked with a woman who'd had chronic neck and back pain for 10 years. In our first private Somatics session I taught her to release and relax the tight muscles of the back. She felt much better. Then I asked her to do a simple awareness exercise to further increase her awareness of her back muscles: She sat profile in a chair in front of a full length mirror. I asked her to "sit up straight." "Just pretend you're at work," I said. Then I had her close her eyes and gently arch, then slowly round her back, letting her pelvis and tailbone go along with the movement. I asked her to slowly "sit up straight," with her weight sitting on top of her "sitbones" - the bones at the bottom of the pelvis. When she was sitting with her back relaxed and tall, I asked her how she felt. The answer wasn't surprising: She felt "slumped."
She had become so habituated to sitting with her back contracted, slightly forward of her pelvis, that sitting without the back contracted actually felt odd and unfamiliar! When she realized that all these years at work she'd been sitting arched and contracted, with her weight pitched forward, rather than on top of her spine, she was shocked. Did that have something to do with her neck and back pain? She decided that it was a big factor. I sent her home with instructions to do this "mirror exercise" at work whenever she felt her back get tight. She reported back to me that this simple exercise had a profound impact on her ability to reverse the pain she'd suffered with for so many years.
Here are the two variations of "straight" that I see most often:
This isn't really straight. The back is arched and the weight is pitched forward. We call this the "green light reflex." It's a reflex that tightens the back in response to the need to get things done. It pulls the spine into a bow rather than allowing the spine to be vertical and relaxed. The back muscles contract from the tailbone all the way to the base of the neck. It's very common in those who work on computers all day long.
This is the exact opposite of the "green light" posture. It's also not straight; the belly is tight, the weight in back of the pelvis and the neck juts forward. This can create a tight, painful neck.
Arched forward or rounded down, either extreme can make sitting uncomfortable and laborious. Whichever way you sit, however, will become "the norm."
Try this exercise at home and see if you can retrain your brain so you can sit tall and effortlessly:
Close your eyes and sit up to your normal sense of straight. Look in the mirror. Are you arched? Rounded? Notice how you feel, but don't try to change it. Now close your eyes. Inhale and arch, then exhale and round gently. Repeat 2-3 times. Let your pelvis move. Let your head go back as you arch back. Let your head drop down as you round forward.
Slowly come up to what your brain tells you is "straight." Sense your weight. Is it on top of your sitbones? In front? In back. Do some self adjustment. Rock side to side on your sitbones. Feel your hip flexors at the groin line. Are they relaxed?
When you feel relaxed and balanced, open your eyes. See if your internal sensations agree with what you see in the mirror. If you're still arched or rounded, close your eyes and do it again. The goal is to get your internal sense of your back muscles to agree with your visual sense. This exercise can permanently change the way in which you sit. You'll be able to sit effortlessly without fatigue.