Velocity-dependent changes of rotational axes during the control of unconstrained 3D arm motions depend on initial instruction on limb position.

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).