Latissimus-dorsi-inspired trunk torsion mechanism for quadruped robot spine design

The body structure of living organisms provides a useful insight into sophisticated designs for robots. Incorporating a spine into quadruped robots has become a trend, with implementations of trunk flexion and torsion through the spine aimed at improving locomotion performance. However, the integration of multidirectional spine shape change requires a streamlined design to minimize mechanical complexity. This study introduces a spine structure inspired by the latissimus dorsi muscles that wrap around the trunk of quadruped animals, connecting the dorsal trunk to the forelimbs. The proposed design achieves two functions: generating trunk torsion through differential contraction of left and right latissimus dorsi muscle actuators, and converting spine flexion into forelimb propulsion motion via the latissimus dorsi muscle. We develop a forward kinematics model that incorporates a three-dimensional muscle path to estimate the design parameters achieving the intended functions in a real robot. Experiments with a quasi-quadruped robot based on the kinematic model show that the proposed design effectively integrates these functions using the latissimus dorsi muscle actuator.