William M. Kier

Kier’s research centers on the biomechanics and functional morphology of musculoskeletal systems in marine invertebrates,^[11] particularly cephalopods such as squid and octopus.^[12]

A major component of his work concerns the concept of the muscular hydrostat, a biological structure composed of densely arranged three dimensional muscle fiber arrays that function simultaneously as force-generating and support systems. Unlike vertebrate endoskeletons or arthropod exoskeletons, muscular hydrostats lack rigid elements and do not rely on fluid-filled cavities for structural support. Examples include cephalopod arms and tentacles, mammalian tongues, and elephant trunks.^[13] Kier’s analyses demonstrated that these structures are capable of complex and coordinated movements including bending, elongation, shortening, and torsion often occurring simultaneously along different regions, thereby enabling precise manipulation and grasping.^[14]

He has also collaborated with engineers and biologists to apply principles derived from muscular hydrostats to the development of soft robotic systems.^[15] These bioinspired designs replicate the flexibility and adaptability of cephalopod limbs and other hydrostatic organs, allowing manipulation of objects with varying shapes and mechanical properties in unstructured environments.^[16]

In studies of squid muscle, he demonstrated that high shortening velocities are achieved through structural modifications in myofilament arrangement rather than biochemical changes typical of vertebrate muscle, challenging assumptions regarding the generality of vertebrate muscle models.^[17]

In 1985, in collaboration with Kathleen K. Smith, he co-introduced the term muscular hydrostat, which has since become adopted in comparative and functional biology.^[18]

• Muscular hydrostat