Musculoskeletal lower-limb robot driven by multifilament muscles
© The Author(s) 2016
Received: 9 February 2016
Accepted: 3 September 2016
Published: 13 September 2016
This paper presents a redundant musculoskeletal robot using thin McKibben muscles that is based on human anatomy. The purpose of this robot is to achieve motions and characteristics that are very similar to a human body. We use a thin McKibben muscle, which is compliant and flexible, as the actuator of a musculoskeletal robot. Using a bundle of thin McKibben muscles, we develop a multifilament muscle that has characteristics similar to those of human muscles. In contrast, the actuators of conventional musculoskeletal robots are very heavy, not densely attached and have poor backdrivability. Because multifilament muscles are light and can be densely attached, we can attach them to the musculoskeletal robot as skeletal muscle and achieve a redundant system that is equivalent to a human drive mechanism. In this paper, we report a method for fabricating multifilament muscles that imitate various muscles, the development of a lower-limb muscle mechanism for the redundant musculoskeletal robot with thin McKibben muscles and experimental results showing that the proposed musculoskeletal robot achieves humanlike motions that have not yet been reported for other robots.
Currently, research on humanoid robots that imitate human drive mechanisms is vigorously carried out worldwide. Our research group believes that we can achieve humanlike behavior by imitating human mechanisms and structure perfectly using muscle placement, redundancy and tendon-driven systems. “Humanlike” mechanisms imply particular mechanisms that conventional robots do not have but a human has. These mechanisms make our robot more similar to a human than other robots. There are motions that are achieved by imitating human motions perfectly, for instance, knee rotation that appears only when the knee is bending or the complex bending motion of an ankle with its many degree of freedom. Many kinematic differences exist between present-day robots and human bodies. For example, (1) robotic knees typically consist of a revolute joint with a fixed rotational axis, while human knees consist of a rolling joint with a shifting rotational axis; (2) robotic knees typically have one degree of freedom that supports bending, while human knees have two degrees of freedom at bending position that support bending and rotation; and (3) robotic ankles typically consist of a ball joint and a non-deformable foot, while human ankles consist of extrinsic muscles that not only support ankle motion but also deform the foot to a curved shape with inversion and eversion. These functional differences between present-day robots and human bodies also result in characteristic and physical appearance differences.
The final goal of this research was to further develop humanoid robots with human-like characteristics by imitating the human drive mechanism, including the number and arrangement of muscles in the human body. Human–like characteristics such as a deformable foot plays an important role in walking and knee rotation at a bending position contribute to operating the pedals of a car. Taking advantage of such human-like characteristics, our robot can be used to test hypotheses related to human motion as well as compare the performance of the robot to that of humans and work in the real world, e.g., as human interactive robots, amusement robots and medical training robots in the future. With this goal in mind, this paper describes efforts to verify the potential for human-like robotic mechanisms by building a similar drive mechanism using our unique thin muscles.
Musculoskeletal robots that have tendon-driven systems, mainly using motors, can better imitate the motions and characteristics of a human than those that have other drive mechanisms. Kenshiro [1, 2] is driven by motors and tendons. The body of Kenshiro is similar to that of a human because its muscles, bones and joint structures are based on human anatomy. For example, its knee joint is designed to imitate a human one; thus, Kenshiro has the functionality of a kneecap, cruciate ligament and screw-home mechanism in the knee joint using link mechanics, which allows for humanlike motion. ECCEROBOT (Embodied Cognition in a Compliantly Engineered Robot) [3, 4] is also driven by a tendon-driven system. It consists of a skeleton made from a polymorph that has a bone-like appearance and elastic actuators that include motors and elastic tendons to realize motions that are similar to a human. ECCEROBOT is used to test hypotheses about human motion as well as compare its performance with that of humans. However, the actuators of these conventional musculoskeletal robots with motor-driven tendon mechanisms are very heavy, not densely attached to the muscles and have poor backdrivability with respect to the motors, gear wheels and belts. Therefore, these robots do not have a redundancy that is as good as that of a human.
A McKibben artificial muscle is an actuator that has an elasticity and compliance that is similar to human muscle and can also be used in a tendon-driven mechanism. According to , the Shadow Biped Walker, which is a pioneering redundant robot with pneumatic muscles, has been developed by Shadow Robot Co. in 1988. The muscle arrangement in the robot is similar to that of human muscles; however, the number of muscles on its one leg is only 14 . Koh Hosoda et al. have developed musculoskeletal infant robots with pneumatic artificial muscles . These robots have a humanlike musculoskeletal structure and McKibben pneumatic muscles that imitate human muscles. Compared with motors, these actuators have some advantages regarding their mechanical softness and compliance. As a result, such robots are a good platform for investigating motion development. In addition, the same research group also developed a biped robot powered by antagonistic pneumatic actuators . The design concept of this robot is basically the same as that of the robots in . This robot suggests that joint compliance contributes to the realization of various types of locomotion. The humanoid muscle robot torso called “Zwei-Arm-Roboter” has been developed based on an idea that is similar to ours . Developers insist that biologically inspired robots embody no rigid movement, which are made possible by special joints or actuators. However, these pneumatic actuator robots do not perfectly imitate the redundancy of the human drive system with respect to the number of muscles for the same reason that Kenshiro and ECCEROBOT do not.
We have developed a thin McKibben muscle  with a flexible shape that is the thinnest McKibben muscle ever reported. This actuator is also lighter and more compact than other actuators such as conventional pneumatic muscles, motors and cylinders. We believe that our unique thin muscle is currently the only actuator capable of realizing this purpose: it is thin, deformable and light enough to be used inside a body with limited space; it generates a comparable contracting force and ratio as human muscles; and it can be easily bundled to form various shaped multifilament muscles, e.g., a muscle that has two ends on one side, such as a biceps muscle, or a flat muscle such as the pectoralis major and deltoid muscles. Kenshiro [1, 2] has a flat shaped (or planar) muscle ; however, this planar muscle requires several pulleys and associated supports. Comparatively, the shape and acting points of a multifilament muscle can be flexibly changed and adjusted during implementation in a robot body. As a result, these muscles can be densely attached in the robot. By replacing conventional actuators with thin McKibben muscles on a musculoskeletal robot, we can use them more densely and build a musculoskeletal robot with a perfectly humanlike redundant system, resulting in characteristics that are more similar to a human than other systems.
Thin McKibben muscle
Design of the multifilament muscles
Structure of the multifilament muscle
Characteristics of the multifilament muscle
Various shapes of multifilament muscles
Design of a lower limb musculoskeletal robot
In this research, we used a skeletal specimen that imitates a boy 1.6 m in height, usually used as an anatomical model of the human skeleton in a hospital or science room, as the body of the musculoskeletal robot. We obtained several advantages by using it as the body of the musculoskeletal robot. For example, most conventional musculoskeletal robots have a shaft in the knee joints with a fixed rotational axis; however, the human knee has no fixed rotational shaft and rolls with a shifting rotational center. Kenshiro realized this motion by using a linkage mechanism, which is a very different structure than that of the human knee. The skeletal specimen we used has the same structure in the joint as a human, where the bones roll in contact with each other. Additionally, the skeletal specimen has ligaments made of elastic cord. With such features, the skeletal specimen can move more like a human than conventional robot linkage mechanisms.
Design of knee motions
Design specifications of the multifilament thin McKibben muscles for the knee
Number of thin McKibben muscles
Length of constriction (mm)
Biceps femoris (long one)
Biceps femoris (short one)
Design of ankle motions
Design specifications of the multifilament thin McKibben muscles for the ankle
Number of thin McKibben muscles
Length of constriction (mm)
Flexor digitorum longus
Extensor digitorum longus
Flexor hallucis longus
Extensor hallucis longus
Development of a lower limb musculoskeletal robot
Motion evaluation results
Knee motion evaluation
Range of motion of the proposed musculoskeletal robot and human knee joints 
Range of motion of the proposed musculoskeletal robot and human knee joints
Musculoskeletal robot (90°)
Musculoskeletal robot (100°)
Internal rotation (deg)
External rotation (deg)
Ankle motion evaluation
Range of motion of the proposed musculoskeletal robot and human ankle joints 
Dorsal flexion (deg)
Plantar flexion (deg)
Another distinct point of the prototype is its foot deformation. As shown in Figs. 15 and 16, the prototype foot deforms in a curved shape, very similar to a human foot. This comes from the bone structure, which, like that of a human, consists of many bones, like a cubic puzzle.
We established a method for fabricating multifilament muscles using the thin McKibben muscle that we previously developed. We have also successfully imitated various-shaped muscles that are found in the human body. Using the multifilament muscles, this report shows that we can create redundant and compact tendon-driven systems suitable for the development of a lower-limb musculoskeletal robot. As a result, our robot contains the same number and arrangement of muscles in the human body.
The experiments show that the musculoskeletal driven mechanism achieves motions that are similar to those of a human. The prototype robot demonstrated nearly the same range of motion for both the knee and ankle as a human by using the same muscle arrangement and bone structure. Regarding the knee joint, our robot realized the same degrees of freedom at bending position as a human knee joint by using the same mechanism; two degrees of freedom support both bending and rotation. Our robot also demonstrated the use of an extrinsic muscle to deform a foot like a bow with inversion and eversion (a foot consists of many bones acted upon by an extrinsic muscle). It is the first robot in the world that has realized these human-like characteristics by imitating the human drive mechanism, including the same number and arrangement of muscles in the human body.
While this study evaluated these new motions from a kinematics perspective, we are planning to evaluate this musculoskeletal mechanism in terms of force, stiffness and dynamic characteristics in a subsequent research effort. In addition, this study focused only on knee and ankle motions; future research will consider the application of this method to the entire human body, including the hip and establish a control method for a redundant system to perform practical movements.
All authors equally contributed. All authors read and approved the final manuscript.
The present study was supported by JSPS KAKENHI Grant Number 26249028, “Realization of a Next-generation McKibben Artificial Muscles.”
The authors declare that they have no competing interests.
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