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Trunks perform complex three-dimensional motions: elephants


Anatomical drawing of elephant trunk / Smith, K; Ki.. / LicenseCC-by-nc-sa - Attribution Non-commercial Share Alike

The trunk of an elephant does in work in 3-dimensional movement due to a muscular hydrostat that provides volume constancy and reversable torsional force.

The trunk of an elephant is highly dynamic, able to not only move in a variety of directions but also able to do so with immense strength and precision. What is particularly remarkable about this movement, however, is that the trunk is able to perform such tasks without the presence of skeletal support or fluid displacement throughout the muscle. Antagonistic movement controls the movement of most muscles; meaning that while one muscle group contracts an opposing muscle group elongates thus providing a bending movement (think of an arm bending). What is unique about the trunk of an elephant, however, is that there is no skeletal structure present in its trunk to create antagonistic muscles. 

The trunk is made up of an incompressible "fluid" (i.e., tightly packed muscle fibers) that maintains its volume to remain constant through a variety of movements. Structures composed of this incompressible fluid are known as muscular hydrostats. These muscles are arranged in three patterns (perpendicular to the long axis of the organ, parallel to the long axis, or wrapped helically, or obliquely, around the long axis) and provide versatility to the movement of the trunk. “Bundles of longitudinal muscle are located around the periphery of the structure. This location provides greater leverage for bending than a more central location near the axis of the organ. With a whole array of bundles around the periphery, movement in virtually any direction is possible” (Smith et al. 1989: 33). In addition, the helical muscle is observed in layers. The direction in which the layer of muscle is wound dictates the direction in which the trunk will twist. Being fixed at one end, the free end twists in the direction that the helical muscle contracts. This twisting, or torsion, provides force that is insurmountable by regular bending muscles because of the energy stored in the helical structure. By observing this torsional pattern and applying it to biomechanics, it is possible to create a small motor of immense strength- whose volume would remain constant throughout movement. 
About the inspiring organism

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Organism/taxonomy data provided by:
Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist

Bioinspired products and application ideas

Application Ideas: Fine movement of motors in compressible, constant-volume size

Industrial Sector(s) interested in this strategy: Manufacturing; Medical

Smith, K; Kier, W. 1989. Trunks, Tongues, and Tentacles: Moving with Skeletons of Muscle. American Scientist. 77(1): 28-35.
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Liang, Y; McMeeking, RM; Evans, AG. 2006. A Finite element simulation scheme for biological muscular hydrostats. Journal of Theoretical Biology. 242(1): 142-150.
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