Akira Fukuhara

Biomimetics replicates biological functions and helps investigate mechanisms and movements. This study focuses on mimicking canine limb structures, particularly the flexible forelimb connection. Due to vestigial clavicles, dogs’ forelimbs attach to the trunk via muscles, creating a suspension system, unlike the skeletal hip joint connection of hindlimbs. Zoological studies highlight this structure's role in movement, but robotics has largely overlooked it. This research aims to clarify its function in locomotion by constructing a musculoskeletal canine‐limb robot using thin McKibben muscles, known for their lightweight, flexibility, and contraction ability even when bent. Experiments demonstrated that adjusting pneumatic pressure in the shoulder muscles influenced stroke length, confirming that the hammock‐like structure enhances movement. Increased pressure resulted in longer strokes, contributing to efficient locomotion. Walking experiments verified successful straight‐line progression using a walking gait, demonstrating the feasibility of this biomimetic approach. These findings suggest that incorporating flexible shoulder structures in quadrupedal robots could improve mobility and adaptability. Future research could further optimize muscle control and gait patterns to enhance robotic performance. By better understanding and implementing biological musculoskeletal features, this study contributes to both zoological biomechanics and robotic advancements, bridging the gap between biology and engineering for more efficient locomotion.

During turning maneuvers in the galloping gait of quadruped animals, a strong relationship exists between the turning direction and the sequence in which the forelimbs make ground contact: the outer forelimb acts as the “trailing limb” while the inner forelimb serves as the “leading limb.” However, the control mechanisms underlying this behavior remain largely unclear. Understanding these mechanisms could deepen biological knowledge and assist in developing more agile robots. To address this issue, we hypothesized that decentralized interlimb coordination mechanism and trunk movement are essential for the emergence of an inside leading limb in a galloping turn. To test the hypothesis, we developed a quasi-quadruped robot with simplified wheeled hind limbs and variable trunk roll and yaw angles. For forelimb coordination, we implemented a simple decentralized control based on local load-dependent sensory feedback, utilizing trunk roll inclination and yaw bending as turning methods. Our experimental results confirmed that in addition to the decentralized control from previous studies which reproduces animal locomotion in a straight line, adjusting the trunk roll angle spontaneously generates a ground contact sequence similar to gallop turning in quadruped animals. Furthermore, roll inclination showed a greater influence than yaw bending on differentiating the leading and trailing limbs. This study suggests that physical interactions serve as a universal mechanism of locomotor control in both forward and turning movements of quadrupedal animals.

A locomotor system that can function across different environmental conditions and produce a range of performances is one of the most critical abilities needed for extant and extinct animals in order to survive and maximise their competitive fitness. Recent engineering-inspired paleontological studies have reconstructed feasible locomotor patterns in extinct animals. However, it is still challenging to describe how extinct animals successfully adjust their locomotor patterns to new situations (e.g., changes in locomotor speed and morphology). In this study, we develop a novel reconstruction method based on a bio-inspired control system. We focus on plesiosaurs, an extinct aquatic reptile group which has two pairs of flipper-shaped limbs, and demonstrate that a highly optimised, flexible locomotor pattern for all four flippers can be reconstructed based on a decentralized control scheme formulated from extant animals’ locomotion. The results of our robotic experiments show that a simple, local sensory feedback mechanism allows the plesiosaur-like robot to exploit the fluid flow between the flippers and generate efficient swimming patterns in response to changes in locomotor conditions. Our new method provides further evidence how decentralized control systems can produce a pathway between extinct and extant animals in order to understand the how extinct animals moved and how these movement patterns may have evolved.

Dynamic human movements are achieved by appropriate constraints on the degrees of freedom of the complex and flexible human body. The anatomy trains (ATs) theory explains such constraints with whole-body muscular connections called ATs. This paper proposes the design of a quasi-passive dynamic walker with whole-body viscoelastic connections inspired by the ATs theory and investigates the contributions of these long-distance connections to the achievement of gait. We designed a biped robot with a trunk and head, whose passive joints were supported by rubber fiber bands. The robot, named “PEARL III,” is equipped with an antagonistic pair of McKibben pneumatic actuators for each leg at the human hamstring and rectus femoris positions. The most important feature of this robot is that fabric wires mechanically connect its rubber bands and actuators on the back side from the head to the foot, modeled after one of the human ATs, the superficial back lines (SBLs). In an experiment, PEARL III achieved 2D quasi-passive dynamic walking on an inclined plane by contracting and relaxing its actuators using periodic feedforward control. This result suggests that in both the robot and human cases, when a controller contracts the SBL only in the stance phase during passive dynamic walking, the SBL can achieve whole-body posture control and weight support. In addition, the SBL appears to achieve this function depending on their mode of attachment to bones and the presence or absence of antagonistic muscles (or ATs). In the future, by introducing various ATs into robots while recognizing the importance of the appropriate attachment of ATs and the presence of their antagonistic muscles (or ATs), we can expect similar effects in various 3D movements.

Quadruped mammals can control the movement of their center of gravity when turning by skillfully utilizing their bodies to achieve adaptive turning movements. Interestingly, the low-speed turning behavior also changes depending on the animal’s morphology. Therefore, this study aims to understand the control algorithm of low-speed turning, which can reproduce the turning behavior according to the location of the center of gravity. Specifically, we constructed a control algorithm based on the knowledge that animals steer with the leg closest to the center of gravity and verified it with a quadruped robot whose center of gravity could be adjusted. Consequently, the behavior observed in animals was successfully reproduced, with a stable and large turning angle per time when the proposed control algorithm was used.

Animals can instantly exhibit versatile behaviors in response to their circumstances to survive in harsh natural environments. For example, many animals chase prey, escape from predators, forage, utilize tools, build nests, and breed, in accordance with their niches with limited resources. To gain a deeper understanding of the source of this versatility in animals and apply the principles of their body plans to robotics, the authors, who work in the robotics and anatomy fields, jointly review and compare the essential body structure differences between quadruped robots and animals. This review provides a comprehensive discussion of the various functions of animal bodies by particularly focusing on the anatomical structure and gives examples of how these functions have been implemented in robotics. Conventional biomimetic robots have taken the approach of realizing animal functions as simple single-functional modules in order to achieve specific animal functions in the most efficient way. In contrast, animals have situation-dependent multi-functionality, in which one part of the body exhibits multiple capabilities depending on the situation. This new ‘polysemantic body’ design principle would transform the traditional processes for understanding and mimicking animals and make the interaction between biology and robotics more fruitful. GRAPHICAL ABSTRACT