Short answer is “it doesn’t!” People sit for years without any back problems. What now hurts your back is having sat for extended periods of time for many years without moving much. Your body is not well adapted for long hours spent sitting in a restrictive or constrained posture.
It’s been alright up until now…
At the same time your body is also amazingly strong. So, initially you will get away with it for years and years. At some point, however, your body has to let you know that you need to change your ways. It communicates through the language of pain – often severe pain! 80% of the adult population in western countries receive this “message from the body” at some point in their lives.
That is why your back hurts when you sit. Explanation from an expert
So, why is the body eventually unable to cope with the continued abuse of prolonged sitting? Personal Trainer, Derran Langston explains:
“When you don’t move and actively target muscles you get into trouble. Gluteus, hamstrings, etc, don’t strengthen by accident – ever! They just get weaker and weaker.
The load these muscles are supposed to support is then transferred into smaller, far less capable muscles. And in worst case the load gets transferred into the spine. The spine is not designed to carry your entire body weight. Purpose of the spine is to give structure to your body and support your muscular system. So, your body is forces to rely on small ineffective muscles, and joins that are used in a way they were not designed for.
Eventually these small muscles strain or spasm. When that happens, even more workload gets transferred to the remaining muscles.”
And that is why your back hurts when you sit … Now, it’s up to you whether you listen to the body’s powerful pain message, or just ignore it. The precise causes and contributing factors to your pain are individual to you. So, you need to discover your own back pain solutions and claim your life back.
Part of that solution could be to find a better and more dynamic way of sitting.