Unless you moonlight as a professional wrestler, the typical office worker isn't likely to get hit over the head with a chair. Nevertheless, a seat can do long-term musculoskeletal damage if not set up and sat in properly.
To prevent injury, use a chair with an adjustable height, seat back and arm rests. When sitting in the chair, OSHA advises that your feet should rest flat on the floor, your thighs should be parallel to the ground and the seat front should not press against the back of your knees or lower legs.
It's not just about where you sit; it's also about how you sit. Office safety experts at James Madison University advise employees to resist the urge to lean back in a chair to the extent that its wheels or legs leave the floor (or risk falling backward) and to take the time to check that the seat is beneath them before actually sitting down.