Systems Neuroscience Group, School of Psychiatry, University of New South Wales Sydney, NSW, Australia.
Front Neurorobot. 2012 Jan 11;5:5. doi: 10.3389/fnbot.2011.00005. eCollection 2011.
Computational models of neuromotor control require forward models of limb movement that can replicate the natural relationships between muscle activation and joint dynamics without the burdens of excessive anatomical detail. We present a model of a three-link biomechanical limb that emphasizes the dynamics of limb movement within a simplified two-dimensional framework. Muscle co-contraction effects were incorporated into the model by flanking each joint with a pair of antagonist muscles that may be activated independently. Muscle co-contraction is known to alter the damping and stiffness of limb joints without altering net joint torque. Idealized muscle actuators were implemented using the Voigt muscle model which incorporates the parallel elasticity of muscle and tendon but omits series elasticity. The natural force-length-velocity relationships of contractile muscle tissue were incorporated into the actuators using ideal mathematical forms. Numerical stability analysis confirmed that co-contraction of these simplified actuators increased damping in the biomechanical limb consistent with observations of human motor control. Dynamic changes in joint stiffness were excluded by the omission of series elasticity. The analysis also revealed the unexpected finding that distinct stable (bistable) equilibrium positions can co-exist under identical levels of muscle co-contraction. We map the conditions under which bistability arises and prove analytically that monostability (equifinality) is guaranteed when the antagonist muscles are identical. Lastly we verify these analytic findings in the full biomechanical limb model.
神经运动控制的计算模型需要肢体运动的前向模型，该模型可以在不考虑过多解剖细节的情况下，复制肌肉激活和关节动力学之间的自然关系。我们提出了一种三连杆生物力学肢体模型，该模型强调了在简化的二维框架内肢体运动的动力学。通过在每个关节两侧设置一对可以独立激活的拮抗肌，将肌肉共同收缩效应纳入模型中。已知肌肉共同收缩会改变肢体关节的阻尼和刚度，而不会改变净关节扭矩。使用包含肌肉和肌腱并联弹性但省略串联弹性的 Voigt 肌肉模型来实现理想化的肌肉执行器。使用理想的数学形式将收缩性肌肉组织的自然力-长度-速度关系纳入执行器中。数值稳定性分析证实，这些简化执行器的共同收缩增加了生物力学肢体的阻尼，这与人类运动控制的观察结果一致。通过省略串联弹性，排除了关节刚度的动态变化。该分析还揭示了一个意想不到的发现，即在相同的肌肉共同收缩水平下，可以共存不同的稳定（双稳定）平衡点。我们绘制了出现双稳定性的条件，并通过分析证明了当拮抗肌相同时，单稳定性（等价性）是有保证的。最后，我们在完整的生物力学肢体模型中验证了这些分析结果。
