Why changing posture is not the same as moving
Standing changes the position of your body, but it does not automatically change what your muscles are doing. When you stand still, muscles in your back and legs remain gently active to keep you upright. Holding that position for a long time can tire them, much as remaining seated can leave muscles and joints feeling stiff. A desk that lets you stand can offer a break from sitting, but it is not a complete solution to prolonged stillness.
What movement adds
Walking, stretching, or simply shifting position asks your body to do something different. Muscles alternately contract and relax, joints move through a wider range, and the load on any one area changes. Those small changes interrupt the sustained effort of maintaining a fixed posture. They need not be strenuous: a short walk to refill a glass or a few gentle stretches can help break up a long stretch at the desk.
Treat the desk as an option, not a cure
The useful feature of a sit-stand desk is that it makes changing position easier. It does not make one position ideal for hours, and standing all day can create its own discomfort. Alternate between sitting and standing, and build in brief opportunities to move rather than waiting for your back to hurt.
The practical goal is not to find a perfect posture and hold it. It is to avoid asking your body to hold any one posture for too long.
