Slumping in front of the television or a computer for hours at a time may damage important back muscles, according to scientists.
Many people suffer from lower back pain
Australian researchers studied a group of 19 young men who spent eight weeks in bed.
They found that the lack of movement weakened the muscles that support and protect the spine, New Scientist magazine reported.
The researchers said that hours sat at a computer could have the same effect.
The researchers said the pain was as bad as having a physical injury.
Previous studies have shown that in most cases of lower back pain, the muscles that keep the vertebrae in place, or those that hold the pelvis together, are inactive.
In some cases, both sets of muscles are not working.
In about 15% of cases, this is caused by heavy lifting, whiplash or other injuries that damage these support muscles.
However, for most people the cause of their lower back pain is a mystery as doctors are often unable to find a cause.
This latest study suggests that long periods of inactivity may be to blame.
Researchers from the University of Queensland found that the support muscles of the men who spent eight weeks in bed were inactivated in a very similar way to those of lower-back pain patients.
Using magnetic resonance imaging, they showed that after eight weeks the muscles that support the vertebrae had wasted and become inactive.
"This is the first study to show that these muscles that protect your spine are switched off," said Julie Hides, one of the researchers.
She said slumping in front of the television or computer could have exactly the same effect.
The researchers also found that switching these muscles back on is not simply a matter of getting up and walking around.
Some of the volunteers have been monitored for six months and their back muscles have still to recover, despite exercise.
Robert Moor, of Australia's Adelaide Centre for Spinal Research, said the findings made sense.
"We know that bones and soft tissues need physical stresses to maintain vitality," he said.
The UK charity BackCare said many people forgot to think about their back when the get home.
"Lots of us are very careful to look after our posture and how we lift things at work and then forget all about it at home," said Nia Taylor, its chief executive.
"Use cushions to maintain the s-shape of your spine, avoid twisting your spine, change position often and stretch and move about from time to time."