Without muscles and ligaments, the spine would collapse. When the stomach muscles and the diaphragm contract, there’s an increase of pressure inside the abdomen. This is called intra-abdominal pressure (IAP).
What’s the effect of IAP on the spine? According to physical therapy researchers in Australia, IAP increases the stiffness of the spine. Increased stiffness means greater stability and more control of the spinal motion. Pigs were used to measure how much motion occurs between the vertebrae when the diaphragm or abdominal muscles are stimulated. The diaphragm is a dome-shaped muscle that separates the chest from the abdomen. It moves up and down when breathing.
Only one of the abdominal muscles was stimulated (the transverse abdominis, or TrA). The TrA goes across the belly, rather than up and down. By contracting and applying downward pressure, the TrA helps us force air out of the lungs, pass stool, and vomit. The muscles were tested in three ways: (1) both sides of the diaphragm contracting at one time, (2) both sides of the TrA at the same time, and (3) just one side of the TrA at a time (left, then right).
First the researchers measured how much movement occurs between the third and fourth lumbar vertebrae (L3 and L4) when there are no muscles contracting. They compared this control value to what they found with each of the three tests. The results show that contracting the diaphragm or the TrA does indeed reduce motion between L3 and L4. This increases the stiffness (and thus the stability) of the spine.
Increasing spine stiffness relies on three things. The muscles must be attached to the spine properly. The muscles must contract normally. And there must be an increase in IAP. These findings are important because other studies show TrA contraction may be delayed in people with chronic low back pain.
The authors suggest weakness of the transverse abdominal muscle may lead to decreased spinal stability. At the same time, when breathing is increased and the diaphragm is weak, the lumbar spine is affected. This information is helpful for physical therapists working with back pain patients. A program of strengthening exercises for the diaphragm and the TrA may be needed to improve spinal stability.