Sitting in a chair should be considered harmful. This is a very recent practice that we have not evolved to handle well. I know few adults that can still sit comfortably in a squatted position. Almost everyone has sore hip flexors and poor hip mobility. Try pressing in on yourself in areas around your hips- it is not supposed to hurt.

Alternatives: Standing. Standing still in one place for a long time can have its own problems. But when standing it is much easier to have some amount of movement- the minimum is just shifting your weight from one leg to the other. The treadmill desks are an interesting concept to keep you moving.

Lying down. These positions are usually difficult to work in.

Sitting (not on a chair). Sitting on your bed or on the floor. There are a few possible positions here.

Squatting. This has to be done with assistance (raised heel and butt) until you gain the strength and flexibility required to do this.

Sitting and squatting tend to put you in a hunched over position though.

Ultimately the best thing is going to be to move around between different positions as much as possible, along with taking as many breaks as you can and making them as physical as you can.

I am trying to combine these concepts with some amount of working outside also. I would love to hear from anyone else working at this.