ISSLS Prize Winner: A Novel Approach to Determine Trunk Muscle Forces During Flexion and Extension

Study Design. Disc pressure and fixator load were measured in an in vitro setup and compared to in vivo measurements with the identical transducers from the two groups participating in this study. Objectives. The goal of this in vitro study was to determine the magnitude of trunk muscle forces during flexion and extension. The loading conditions in this study accounted for body weight, local and global muscles, and forces resulting from the support of the abdominal soft tissue in different postures. Resulting intersegmental motions and intradiscal pressure in each segment and the six load components in both rods of an internal fixator were determined. Summary of Background Data. The spine is primarily stabilized by muscle forces, which greatly influence spinal loads. However, little information exists on the magnitudes of trunk muscle forces during postures like flexion and extension of the upper body. Methods. Seven human cadaveric lumbar spines were mounted in a spine tester and adjusted to different degrees of flexion and extension of the upper body with different hip flexions. For each specimen, a total of 124 load cases were studied. They included combinations of a vertical compressive load, a follower load and forces pulling with cables at a plate fixed at the cranial end of the specimen to simulate rectus abdominis, erector spinae, and a supporting force of the abdomen. The muscle forces were varied until the external moment, necessary to keep the lumbar spine specimen in the examined posture, was zero. This was achieved with different muscle force combinations. Loads on internal fixators as well as intradiscal pressure and intersegmental rotation at all levels were measured. The muscle force combination that caused intradiscal pressures and loads in the internal fixator closest to those measured in vivo were assumed to be the muscle forces which can be expected in vivo. Results. Generally, intradiscal pressure was closer to in vivo measurements than the fixator loads. The force in the m. erector spinae increased with the flexion angle but was only slightly influenced by extension. The estimated forces in the erector spinae were 100 N for standing, 130 N for 15° extension, and 520 N for 30° flexion of the upper body. Little influence was found on the intersegmental motion. Conclusion. In vitro loading conditions can be approximated closely to in vivo conditions with the simulation of an axial preload, local, and global muscles. This novel approach can help to estimate muscle forces, which can usually not be measured. The results from this study provide important input for FEM models, which may then allow the investigation of different load cases.