Differences in the control of unconstrained 3D arm motions due to gravitational torque

BACKGROUND: The selection of rotational axes during unconstrained 3D arm movements is influenced by limb dynamics. Previous research suggests a velocity-dependent shift toward the minimum inertia axis (e3), but it remains unclear how changes in gravitational torque, induced by different arm elevation angles, affect this axis selection. AIM: This study aimed to examine whether the previously described velocity-dependent shift toward the e3 axis occurs independently of arm posture and variations in gravitational loading. METHODS: Participants performed cyclic internal-external arm rotations at slow and fast velocities across four shoulder elevation angles (0 degrees, 45 degrees, 90 degrees, and 135 degrees). This experimental paradigm systematically altered gravitational torque while preserving the geometric relationships among three candidate rotational axes: e3, the shoulder-center of mass axis (SH-CM), and the shoulder-elbow axis (SH-EL). Angular variability of the axes' displacement was evaluated.<br /> RESULTS: A repeated measures ANOVA revealed a significant main effect of shoulder elevation angle on angular variability (p < 0.001), with significantly higher variability observed at 90 degrees and 135 degrees compared to 0 degrees and 45 degrees. Furthermore, a significant Axis & times; Velocity interaction was observed (F(2,18) = 4.32, p < 0.05). Post-hoc analyses indicated that during the slow condition, the SH-EL axis exhibited significantly lower variability than the SH-e3 axis (p < 0.05). In contrast, during the fast condition, the variability across axes converged and differences between the axes did not reach statistical significance (p = 0.08). The comparison of SH-e3 variability between slow and fast conditions revealed a marginal trend (p = 0.052), but contrary to our hypothesis, no consistent shift toward e3 was observed across all elevation angles.<br /> INTERPRETATION: Axis selection is jointly shaped by movement velocity and limb configuration. The findings indicate that the motor system does not uniformly adopt a single rotation axis (such as e3) across conditions but instead exhibits a context-dependent organization where gravitational torque modulates, but does not fully determine, the observed kinematic patterns.