Flexible-torso quadruped robots: biomimetic principles, structural designs, and torso-limb coordination – a review

Conventional quadruped robots are commonly modeled as single rigid-body systems, which simplifies mechanical design and control but limits the representation of torso deformation, energy exchange, and whole-body coordination. Inspired by the functional role of the spine in animal locomotion, flexible torsos have been introduced to improve stride regulation, maneuverability, impact attenuation, and terrain adaptability. Unlike previous reviews that mainly emphasize flexible-spine dynamics, animal-robot anatomical comparison, or amphibious and sprawling locomotion, this review links biological mechanisms, structural realization, torso-limb coordination, and task-level performance within a unified framework. This paper presents a systematic review of flexible-torso quadruped robots from three interconnected perspectives. First, biomimetic principles are analyzed with emphasis on feline spinal kinematics, musculoskeletal compliance, energy transfer, and posture adjustment. Second, existing structural designs are classified according to actuation mode and connection mechanism, including active, passive, and hybrid actuation, as well as revolute-joint, elastic-element, and flexible-material connections. Third, torso-limb coordination strategies are reviewed, covering rhythm-based, optimization-based, learning-based, and hierarchical or fusion control methods. Finally, open challenges in quantitative benchmarking, deformation-aware state estimation, sim-to-real transfer, durability, and real-world deployment are discussed.