Relative efficiency of abdominal muscles in spine stability.

Using an iterative kinematics-driven nonlinear finite element model, relative efficiency of individual abdominal muscles in spinal stability in upright standing posture was investigated. Effect of load height on stability and muscle activities was also computed under different coactivity levels in abdominal muscles. The internal oblique was the most efficient muscle (compared with the external oblique and rectus abdominus) in providing stability while generating smaller spinal loads with lower fatigue rate of muscles. As the weight was held higher, stability deteriorated requiring additional flexor-extensor activities. The stabilising efficacy of abdominal muscles diminished at higher activities. The difference in critical loads in frontal and sagittal planes computed in the absence of abdominal coactivity disappeared under prescribed coactivities suggesting an optimal system in stability. The central nervous system may settle for a less stable spine in favour of lowering the risk of injury. Findings could help introduce stability criterion in optimisation models.