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Soft-bodied locomotion refers to the locomotion patterns and gaits exhibited by invertebrates like worms, caterpillars, etc. Unlike animals with articulated (stiff, jointed) skeletons where the range of motion are prescribed^[1], soft bodied locomotors have more flexibility (more degree of freedom) and maneuverability, especially over rough terrains or inside limited space. However, cost of transport (defined as the energy spent to move a given distance, per unit body mass) is generally higher for soft-bodied locomotors.

Hydrostatic skeleton

Many soft-bodied locomotors actuate their bodies through hydrostatic skeleton^[2], a structure quite distinct from the jointed skeletons. It generally consists of a soft body wall with fluid filled inside the cavity. The soft wall tissue could deform under external pressure or with muscle contraction, yet the volume of the body is maintained constant since the fluid is incompressible. The hydrostatic skeleton plays a similar mechanical role as the jointed skeletons during locmotion: they maintain the body shape, re-extend shorted muscles, and transmit forces between the locomotor and subtrates^[3].

Peristaltic locomotion

Earthworm and many other soft-bodied locomotors (sea cucumbers, snails, polychaetes and leeches, etc.) use peristalsis. During locomotion, waves of alternating longitude and circumferential muscle contraction propagate from head to tail. One cycle of locomotion has only one wave of each muscle activation. When longitude muscles of a body segment contract, that part of body expands in circumferential direction. The setae on the skin of the expanded body also extends to anchor the segment against the substrate. When the circumferential muscles of a segment contract, the body elongates in longitudinal direction; The setae retracts so that segment advance over the substrate ^[4].

Polychaete burrow kinematics

Polychaetes burrow in muddy sea sediments by cracking the media. Some species like Cirriformia moorei generates waves of persitalsis its their head and exerts a dorso-ventral force against the walls of the burrow to extend a crack and move the body forward^[5]. Other species like N. virens everts its pharynx to crack the wall and moves forward with a combination of undulation and peristalsis^[6]. The role of skin filaments(hair like structure on the side of body) in burrow locomotion is currently unclear.

Peristalsis in robots

Robots that use peristaltic locomotion has been developed by several groups^[7]^[8]. This type of robot is built with continuously braided mesh-tube and actuators that can contract longitudinally or circumferentially. The robot can maneuver inside a small tube with very limited spaces.

Undulatory locomotion

Some soft-bodied locomotors such as nematodes travel on substrates using undulatory locomotion. Nematode like C. elegans has a layer of muscle cells to the interior of the cuticle. These muscles can only contract along the long axis of the worm; There are no circular muscles and circumferential contraction is not possible (unlike the earthworms). The musculature is divided in two separted groups which are controlled by dorsal and ventral neurons separately. By activating the two muscle groups in a certain pattern the worm could undulate in the dorsalventral plane^[9].

Crawling and inching locomotion

Soft animals do not all have the same body structure. Caterpillars' body contains air (non-standard hydrostatic skeleton) and is no longer constant volume. Most caterpillars use a crawling gait that features a anterograde waves of muscle movement (opposite to the retrograde activation of muscles in peristaltic locomotion). Prolegs are used to anchor the worm and transmit forces between the soft body and stiff substrate(environmental skeleton)^[10]. When the muscles near the tail segments contract when a locomotion cycle starts, the prolegs in that region releases gripping to the surface so that the tail of the worm is pulled forward. The tail prolegs grasp the substrate again as soon as the muscle activation leaves the anterior segments and rest of the body forward is pulled forward. Inchworm lacks the prolegs in the middle segments of their body and their inching locomotion can be understood as an extreme case of crawling.

Other soft-bodied locomotion

Octopus is able to walk on land^[11] or bipedal walk^[12] at the sea bottom. They actuate their bodies using a different mechanism (muscular hydrostats) than hydrostatic skeleton^[13]. Lacking the fluid-filled cavity, this structure consists of a dense arrays of muscle and tissue fibers. The muscle fibers generally allow all three dimensions of movement. It has been reported that octopus could generate a series of bends on their arm that function as joints^[14].

A group in Harvard has developed a multi-gait soft robot composed of elastomeric polymers and driven by pneumatic actuator. The soft body of the robot has multiple chambers. When pumped with air, the chambers expand and deform the local body wall. The robot exhibits a hybrid crawling and undulation gait, and is able to crawl under thin gap^[15].

References

^ P. Holmes, J. Robert, D.E. Koditschek and J. Guckenheimer. The Dynamics of legged locomotion: models, analyses, and challenges. SIAM Reviews 48, 207-304(2006)
^ G. Chapman. The hydrostatic skeleton in the invertebrates. Biol. Rev. 33, 338-371(1958).
^ W. M. Kier. Hydrostatic skeletons and muscular hydrostats. In Biomechanics (Structures and Systems): A Practical Approach (ed. Biewener, A. A.), pp. 205-231(1992). New York: IRL Press at Oxford University Press.
^ Kim J. Quillin. Kinematic scaling of locomotion by hydrostatic animals: Ontogeny of peristaltic crawling by the earthworm lumbricus terrestris. J Exp Biol, 202, 661–674 (1999).
^ J. Che and K.M. Dorgan. It’s tough to be small: dependence of burrowing kinematics on body size. J. Exp. Biol. 213, 1241-1250(2010).
^ K.M. Dorgan, P.A. Jumars, B.D. Johnson, B.P. Boudreau and E. Landis. Burrow elongation by crack propagation. Nature 433, 475(2005).
^ E.V. Mangan, D.A. Kingsley, R.D. Quinn and H.J. Chiel. Development of a peristaltic endoscope, IEEE International Conference on Robotics and Automationbotics and Automation 2002 .
^ S. Seok et al. Peristaltic Locomotion with Antagonistic Actuators in Soft Robotics. IEEE International Conference on Robotics and Automation 2010 (ICRA).
^ E. Niebur and P. Erdös. Theory of the locomotion of nematodes: dynamics of undulatory progression on a surface. Biophys. J. 60, 1132–1146（1991）.
^ Huai Ti Lin, Barry A. Trimmer. The substrate as a skeleton: ground reaction forces from a soft-bodied legged animal. J Exp Biol 213, 1133-1142(2010).
^ http://www.youtube.com/watch?v=FjQr3lRACPI
^ C.L. Huffard, F. Boneka, R.J. Full. Underwater Bipedal Locomotion by Octopuses in Disguise. Science 25, 5717(2005).
^ W.M. Kier. The diversity of hydrostatic skeletons. J Exp Biol 215, 1247-1257(2012).
^ G. Sumbre et al. Octopuses Use a Human-like Strategy to Control Precise Point-to-Point Arm Movements. Current Biology 16, 767(2006).
^ R.F. Shepherd et al. Multigait soft robot. Proceedings of the National Academy of Sciences November 28, 2011

[1] P. Holmes, J. Robert, D.E. Koditschek and J. Guckenheimer. The Dynamics of legged locomotion: models, analyses, and challenges. SIAM Reviews 48, 207-304(2006)

[2] G. Chapman. The hydrostatic skeleton in the invertebrates. Biol. Rev. 33, 338-371(1958).

[3] W. M. Kier. Hydrostatic skeletons and muscular hydrostats. In Biomechanics (Structures and Systems): A Practical Approach (ed. Biewener, A. A.), pp. 205-231(1992). New York: IRL Press at Oxford University Press.

[4] Kim J. Quillin. Kinematic scaling of locomotion by hydrostatic animals: Ontogeny of peristaltic crawling by the earthworm lumbricus terrestris. J Exp Biol, 202, 661–674 (1999).

[5] J. Che and K.M. Dorgan. It’s tough to be small: dependence of burrowing kinematics on body size. J. Exp. Biol. 213, 1241-1250(2010).

[6] K.M. Dorgan, P.A. Jumars, B.D. Johnson, B.P. Boudreau and E. Landis. Burrow elongation by crack propagation. Nature 433, 475(2005).

[7] E.V. Mangan, D.A. Kingsley, R.D. Quinn and H.J. Chiel. Development of a peristaltic endoscope, IEEE International Conference on Robotics and Automationbotics and Automation 2002 .

[8] S. Seok et al. Peristaltic Locomotion with Antagonistic Actuators in Soft Robotics. IEEE International Conference on Robotics and Automation 2010 (ICRA).

[9] E. Niebur and P. Erdös. Theory of the locomotion of nematodes: dynamics of undulatory progression on a surface. Biophys. J. 60, 1132–1146（1991）.

[10] Huai Ti Lin, Barry A. Trimmer. The substrate as a skeleton: ground reaction forces from a soft-bodied legged animal. J Exp Biol 213, 1133-1142(2010).

[11] ttp://www.youtube.com/watch?v=FjQr3lRACPI

[12] C.L. Huffard, F. Boneka, R.J. Full. Underwater Bipedal Locomotion by Octopuses in Disguise. Science 25, 5717(2005).

[13] W.M. Kier. The diversity of hydrostatic skeletons. J Exp Biol 215, 1247-1257(2012).

[14] G. Sumbre et al. Octopuses Use a Human-like Strategy to Control Precise Point-to-Point Arm Movements. Current Biology 16, 767(2006).

[15] R.F. Shepherd et al. Multigait soft robot. Proceedings of the National Academy of Sciences November 28, 2011

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