If you stand on a flat surface with your feet fully supported, your ankles do the work of keeping your body centered over your feet. But if you stand on a surface that is smaller than your feet, then the hips work extra hard to keep you upright on your feet.
Postural control is the term used to describe what goes on in the body to keep us up and centered. Scientists think this postural control is affected in people with low back pain (LBP).
It's possible that pain causes changes in postural activity of the trunk muscles. This leads to a backwards shift of weight on the feet in the quiet standing position. It's like standing on a surface that is smaller than the feet. This is called a short base of support.
In this study, researchers started out with the idea that people with LBP have trouble keeping their balance because of this weight shift. A short base of support forces them to use the hips more than the ankles to stay centered in the upright position.
They compared the postural control of people with and without LBP. This was done while standing on a short and a long base of support with eyes opened and eyes closed. Several different positions were used with both feet and standing on just one foot. Each subject was marked successful if he or she could hold the position for 70 seconds (both feet) or 30 seconds (one foot).
The results showed that patients with LBP have poor balance compared to adults the same age and gender without back pain. Balance is worse with eyes closed and when standing on a short base of support. The authors think this is caused by decreased shear force from front to back at the hip. They suggest a closer look at improving balance during rehab. Specific training should focus on the hip.
Nicola W. Mok, MphtyStud, et al. Hip Strategy for Balance Control in Quiet Standing is Reduced in People with Low Back Pain. In Spine. March 15, 2004. Vol. 29. No. 6. Pp. E107-E112.