fetters
桎 centers on physical restraint, specifically wooden shackles or fetters used to bind the feet.
桎 combines 木 (wood) with 至, which likely contributes the sound. The character originally referred to wooden fetters for the feet.
A tree 木 grows around your feet, reaching 至 them and holding you in place like wooden fetters.
For シツ, imagine the fetters are so tight they make you sit still: sit -> シツ, and the wooden shackles keep you seated.