You may be referring to real-time ultrasound (RT-US). Sound waves are bounced off tissues deep in the body. An image of the muscle can be seen this way on a TV screen. This shows the size and shape of the muscle at rest and as it moves.
Researchers are using RT-US to find out what is normal muscle activity. They are comparing these results with patients who have low back pain (LBP). For example, RT-US has been used to monitor the size of the multifidus muscle when changing position from prone (face down) to an upright position. The tests are also being done as the patient moves from standing upright to a forwared bent posture.
The multifidus is a deep muscle of the back that helps stabilize the spine. A recent study from Hong Kong showed a reverse pattern of muscle contraction of the multifidus for patients with chronic LBP.
Such a test could be helpful in identifying the cause of nonspecific LBP. A treatment program of exercise may help bring muscle control back to normal. Right now, RT-US is not used routinely. It may be some time before this diagnostic tool is used on a daily basis by most doctors or physical therapists.