生体力学モデルと機械モデルによる絶滅哺乳類デスモスチルスの歩行復元

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  • RESTORATION OF EXTINCT MAMMAL DESMOSTYLUS BY COMPUTER SIMULATION AND MUSCULOSKELETAL ROBOT

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The Desmostylus was an ungulate mammal as large as a present hippopotamus, that lived approximately 15 million years ago. Although fossil bones of the whole body have been discovered, various postures have been restored because of the creature's unusual skeletal structure. In this study, we attempted to judge the reliability of each proposed restoration and also to investigate the creature's most probable walking style, using three different approaches: biomechanical analysis by computer simulation, comparison with living animals, and synthesis of natural motion by a quadrupedal walking machine. A three-dimensional musculoskeletal model of Desmostylus with 98 muscles and 16 bone segments was developed from the measured data for the whole body's skeletal structure and marks of muscle insertion. Fundamental standing posture was determined so as to minimize change in muscle length while walking and to maximize stride length. The sloth-type posture proposed by Domning and the crocodile-type posture proposed by Inuzuka were simulated using the musculoskeletal model. These were chosen since they are the only two restoration models that avoid dislocation of leg joints. The calculated results for muscle loads and stride length show that the crocodile-type posture is superior to the sloth-type. The results also showed that both postures must be supported by the abdomen because of the lack of muscle force. We concluded that the crocodile posture was the basic posture of the Desmostylus. The toe locus during walking was determined while maintaining an efficient range of muscular length so as to minimize body swaying in the stance phase and to maximize the toe lift in the swing phase. The cycle of leg motion was estimated from pendulum motion of the leg in the swing phase. Restored walking velocity was 36 m/min at a trot gait supported by the abdomen. The posture and walking were visualized using computer graphics techniques in order to confirm the validity of the restoration. In order to verify the restoration method, the posture and walking of an alligator (Caiman) were calculated using the same method, and the results were compared with the actual posture and walking pattern. The results obtained by computer simulation showed good agreement with those of an actual alligator, indicating that this restoration method is valid. The posture and walking of Desmostylus were also evaluated using a musculoskeletal robot. The developed 1/5-scale robot (580mm, 1.8 kg) can generate natural leg motion according to the innate musculoskeletal structures of Desmostylus. Referring to the muscle functions of an alligator, we classified the muscles of Desmostylus into the following three types: driving muscles, such as proximal big muscles; two-joint muscles, which are almost isometric during walking; and distal spring-like muscles with a long tendon. The isometric muscle and the bones parallel to the muscle act as a four-link mechanism and automatically generate coordinated leg motion. The four-link mechanism is extended by proximal driving muscles and flexed by distal spring-like muscles. The leg can therefore be driven only by two actuators for propulsion and by lifting of the toe. The restored posture and walking using the musculoskeletal robot agreed with the simulated results. Consequently, the restored Desmostylus has crocodile-type posture and trot walks supported by its abdomen. The proposed biomechanical restoration method can be easily applied to other extinct animals because it requires only the animal's skeleton and common locomotion principle. In addition, the mechanically imitated musculoskeletal robot is a useful tool for understanding the importance of body construction.

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