Lumbar spine biomechanics during the forward-bending of the upper body (flexion) are well investigated by both in vivo and in vitro experiments. In both cases, the experimentally observed relative motion of vertebral bodies can be used to calculate the instantaneous center of rotation (ICR). The timely evolution of the ICR, the centrode, is widely utilized for validating computer models and is thought to serve as a criterion for distinguishing healthy and degenerative motion patterns. While in vivo motion can be induced by physiological active structures (muscles), in vitro spinal segments have to be driven by external torque-applying equipment such as spine testers. It is implicitly assumed that muscle-driven and torque-driven centrodes are similar. Here, however, we show that centrodes qualitatively depend on the impetus. Distinction is achieved by introducing confidence regions (ellipses) that comprise centrodes of seven individual multi-body simulation models, performing flexion with and without preload. Muscle-driven centrodes were generally directed superior–anterior and tail-shaped, while torque-driven centrodes were located in a comparably narrow region close to the center of mass of the caudal vertebrae. We thus argue that centrodes resulting from different experimental conditions ought to be compared with caution. Finally, the applicability of our method regarding the analysis of clinical syndromes and the assessment of surgical methods is discussed.

体内和体外实验都对上身前屈（屈曲）过程中的腰椎生物力学进行了很好的研究。在这两种情况下，实验观察到的椎体相对运动都可以用来计算瞬时旋转中心 (ICR)。ICR（中心极）的及时演变被广泛用于验证计算机模型，并被认为是区分健康和退化运动模式的标准。虽然体内运动可以由生理活动结构（肌肉）引起，但体外脊柱节段必须由外部扭矩应用设备（如脊柱测试仪）驱动。隐含地假设肌肉驱动和扭矩驱动的中心是相似的。然而，在这里，我们表明质心在质量上取决于推动力。区别是通过引入置信区域（椭圆）来实现的，该区域了解七个单独的多体仿真模型的中心，在有和没有预紧的情况下进行屈曲。肌肉驱动的中心通常指向上方 - 前部和尾部形状，而扭矩驱动的中心位于靠近尾椎质量中心的相对狭窄的区域。因此，我们认为应该谨慎比较由不同实验条件产生的质心。最后，讨论了我们的方法在临床综合征分析和手术方法评估方面的适用性。肌肉驱动的中心通常指向上方 - 前部和尾部形状，而扭矩驱动的中心位于靠近尾椎质量中心的相对狭窄的区域。因此，我们认为应该谨慎比较由不同实验条件产生的质心。最后，讨论了我们的方法在临床综合征分析和手术方法评估方面的适用性。肌肉驱动的中心通常指向上方 - 前部和尾部形状，而扭矩驱动的中心位于靠近尾椎质量中心的相对狭窄的区域。因此，我们认为应该谨慎比较由不同实验条件产生的质心。最后，讨论了我们的方法在临床综合征分析和手术方法评估方面的适用性。