Univ Paris-Sud., UR CIAMS, EA 4532, Motor Control & Perception Team, Orsay F-91405, France.
Hum Mov Sci. 2013 Apr;32(2):290-300. doi: 10.1016/j.humov.2013.02.006. Epub 2013 May 22.
The velocity-dependent change in rotational axes observed during the control of unconstrained 3D arm rotations may obey the principle of minimum inertia resistance (MIR). Rotating the arm around the minimum inertia tensor axis (e3) reduces the contribution of muscle torque to net torque by employing interaction torque. The present experiment tested whether the MIR principle still governs rotational movements when subjects were instructed to maintain the humeral long axis (SH-EL) as closely as possible to horizontal. With this view, the variability of 3D trajectories of the minimum inertia axis (e3), shoulder-center of mass axis (SH-CM) and shoulder-elbow axis (SH-EL) was quantified using a VICON V8i motion capture system. The axis for which the 3D variability displacement is minimal is considered as the one constraining the control of arm rotation. Subjects (n=15) rotated their arm in two elbow angular configurations (Elb90° vs. Elb140°), two angular velocity conditions (slow S vs. fast F), and two sensory conditions (kinaesthetic K vs. visuo-kinaesthetic VK). The minimum inertia axis e3 is angled 5.4° away from SH-CM axis, and varied from 27° to 15° away from de SH-EL axis, for Elb90° and Elb140°, respectively. We tested whether the participants would be able to maintain the instructed SH-EL rotation axis or if increasing the frequency of the arm rotations would override the initial rotation instructions and cause the limb to rotate around an axis closely aligned with e3. We expected that VK inputs would minimize the variability of the SH-EL axis and that K should facilitate the detection and rotation around e3 at the faster velocity. Taken together the results showed that the initial instruction, favoring rotation around the SH-EL axis, prevented the velocity-dependent change towards the minimum inertia (e3) and/or the mass axis (SH-CM), i.e., use of the MIR principle. However, the variability of the SH-EL axis was significantly increased in the F condition, confirming that arm rotations around the SH-EL axis produces larger mechanical instabilities in comparison to when the arm is rotated around a mass/inertial axis (Isableu et al., 2009).
在控制无约束的 3D 手臂旋转时，观察到旋转轴的速度依赖性变化可能遵循最小惯性阻力（MIR）原则。通过使用相互作用扭矩，围绕最小惯性张量轴（e3）旋转手臂可降低肌肉扭矩对净扭矩的贡献。本实验测试了当受试者被指示尽可能将肱骨长轴（SH-EL）保持水平时，MIR 原则是否仍然支配旋转运动。为此，使用 VICON V8i 运动捕捉系统量化了最小惯性轴（e3）、肩质心轴（SH-CM）和肩肘轴（SH-EL）的 3D 轨迹变异性。被认为限制手臂旋转控制的轴是 3D 变异性位移最小的轴。受试者（n=15）在两种肘部角度配置（Elb90° 与 Elb140°）、两种角速度条件（慢 S 与快 F）和两种感觉条件（动觉 K 与视觉动觉 VK）下旋转手臂。最小惯性轴 e3 与 SH-CM 轴成 5.4°角，对于 Elb90° 和 Elb140°，分别从 SH-EL 轴偏离 27°到 15°。我们测试了参与者是否能够保持指示的 SH-EL 旋转轴，或者增加手臂旋转的频率是否会覆盖初始旋转指令并导致肢体围绕与 e3 紧密对齐的轴旋转。我们预计 VK 输入将最小化 SH-EL 轴的变异性，而 K 应该有助于在较快速度下检测和围绕 e3 旋转。结果表明，初始指令有利于围绕 SH-EL 轴旋转，防止了向最小惯性（e3）和/或质量轴（SH-CM）的速度依赖性变化，即使用 MIR 原则。然而，在 F 条件下，SH-EL 轴的变异性显著增加，这证实了与围绕质量/惯性轴旋转手臂相比，围绕 SH-EL 轴旋转手臂会产生更大的机械不稳定性（Isableu 等人，2009 年）。
