Effect of extensor muscle activation on the response to lumbar posteroanterior forces

The purpose of this study was to examine the responses of normal subjects to the application of cyclical lumbar posteroanterior forces which simulated a manipulative therapy technique known as mobilization. The specific aim was to determine whether increases in spinal extensor muscle activity could modify the stiffness of lumbar posteroanterior movements. The lumbar posteroanterior stiffness was measured in eleven asymptomatic subjects in the prone position, both in the relaxed condition and during maximal voluntary isometric muscle contractions. The electromyographic activity of lumbar extensor muscles was measured in the relaxed and maximal contraction conditions during the application of mobilization. The posteroanterior stiffness was found to be significantly greater during maximum activation of the extensor muscles. The results indicate that muscle activity can significantly alter lumbar posteroanterior stiffness. Clinicians often apply posteroanterior forces over a spinous process of a vertebra to assess the resistance to movement. Information about the degree and nature of perceived resistance to posteroanterior movement is used to help make a diagnosis and select treatment techniques. This study has shown that increased activity of the spinal extensor muscles can increase the stiffness of lumbar posteroanterior movements, compared with the case where the subject is relaxed. When interpreting the posteroanterior responses of patients, clinicians should be aware that spinal extensor mucle activity can influence resistance to posteroanterior movement.