Shorten your rectus abdominis and lengthen your erector spinae—this is pretty much my prescription for the world. Culturally we have a leaning backwards problem– almost everybody does it and it serves no one well. Leaning backwards does terrible things to the lower body forcing the pelvis to tuck under and the thigh bones to move forward from under the hips. In the upper body leaning backwards compresses the lower spine and throws the muscular relationship of the lower back and the lower belly into disarray.
The front and the back of the body are meant to be equal in length so that the distance between the top of the pelvis and bottom of the rib cage should be the same. This is rarely what I see. People tend to be splayed open in the front, too long in the long abdominal muscle the rectus abdominis and way too short in the muscles of up the back such as the erector spinae and the quadratus lumborum.
These all important relationships but they do need to change if you to live to a ripe old pain-free age, which should be what we are focused on. I think it helps to have a general understanding of anatomy when you are beginning to work on bringing balance to your musculature.
Most of these ideas are basic and simple. The title statement is true. You need to shorten your rectus and lengthen your erectors. But there are two important questions to ask before you begin.
Building proper tone in the transverse abdominis before you take on the rectus abdominis is of paramount importance because while these muscles are meant to be even in tone the rectus abdominis, though too long, tends to do a great deal of the work that is required of the abdominal muscles and therefore the deeper transverse abdominis muscle is often lagging behind in strength.
If the psoas are tight and you shorten the length of the rectus abdominis you will effectively be downsizing the room available for the organs to hang out in the abdominal cavity If your psoas are tight you need to release or stretch them to create more room for the rectus abdominis to shorten.