Recently, I've been researching ways to increase my core strength and hip flexibility. Over the past year, I've seen my seat position deteriorate. I was starting to develop a "chair seat".
A "chair seat" is a position in which your legs aren't directly underneath you in the correct "shoulder, hip, heel alignment". Your legs are slightly in front, which makes it look like you would be sitting in a chair. Your pelvis is rotated slightly forward which causes your hips to be slightly closed and your upper body to be slightly forward. This position not only hinders you, in your effectiveness and contact, but it causes your horse to be tense in the back.
One of the biggest factors can be saddle fit. If the stirrup bars are too far forward for you this can cause your hips to close and your seat to fall apart.
Another factor can be a sedentary lifestyle. I work 9-5pm in an office and I sit all day long at my desk. I have almost no opportunity to move and walk around throughout the day. Over time this can cause your hip flexors to tighten, back pain, and poor posture. In turn, this can cause a poor riding position.
You can help combat this by working out, lifting weights, stretching, or walking the recommended 10k steps a day. But it doesn't completely offset the effects of sitting all day long.
I have found some solutions for those of us equestrians that have no choice but to be at a desk all day everyday. All of these chairs include some sort of motion and balance characteristic that encourages movement throughout the day. These not only improve your posture, but it helps build your core and open up your hips.
Below are some active chair options at all price points.
These last two can be added to your current office chair: