Morphology, biomechanics and physiology of muscle, tendon and skeleton:
Integrated function of muscle and tendon in movement.
Muscles are the motors powering movement, but also the brakes that decelerate movement. To operate as brakes, muscles must lengthen actively, leaving them susceptible to damage. We have discovered that the elasticity of tendons connecting leg muscles to bones is critical during deceleration. For instance during a landing, tendons act as springs by temporarily storing impact energy, and releasing it at a slower rate to stretch the muscle. The series elastic shock absorber effect of tendon springs may protect muscles from damage when they operate as brakes. Moreover, the delayed and slowed transfer of energy from tendons to muscles may limit peak forces, thus protecting both the muscle and surrounding tissues from damage. We also examine adaptive changes in muscle contraction and mechanical properties of elastic structures resulting from training. Turkeys run either up- or downhill on a treadmill for ten weeks to isolate the effects of lengthening or shortening muscle contractions on muscle sarcomerogenesis, and stiffness of tendon and titin.
Collaborators: Thomas Roberts & Nicholas Gidmark, Brown University; Manny Azizi, U. C. Irvine.
Neuromotor control of movement:
Integration and coordination of rhythmic movement.
Cyclic and rhythmic movements are omnipresent in the animal kingdom. However, the causal relationships between cortical feedback and -forward control, brainstem central pattern generators and behavioral modulation remain unclear. We study muscle activity and bone kinematics in locomotion, and in the three vertebrate jaw systems, to map the evolution of cyclic and rhythmic movement-patterns. One aim is to determine how proprioception influences the rhythmicity of cyclic movements differently in amniotes and anamniotes.
Evolutionary shifts in muscle activity patterns.
A standing paradigm in evolutionary biology is the conserved nature of integrated muscle activity-patterns in animal movement: Shifts in muscle activity patterns are surprisingly rare, even when major evolutionary transitions occur. We probe the phylogeny of jaw-bearing vertebrates, to detect shifts in the activity pattern of four homologous muscles that drive chewing. We have discovered a muscle activity pattern shift associated with the rise of tetrapods, and we anticipate that other shifts coincide with changes in living realm, structural changes in the feeding apparatus, and ecological transitions into e.g. herbivory and durophagy.
Collaborators: Anthony Herrel, French Natural History Museum; Callum Ross, U. Chicago; Susan Williams, Ohio University; Rebecca German, Johns Hopkins University; Allan Thexton, Kings College; Fuzz Crompton, Harvard University; Chris Sanford, Hofstra University.
Testing engineering model use in biomechanics.
Biomechanical research relies on engineering models of lever-systems and 4-bar linkages to measure force-velocity tradeoffs in complex natural mechanisms like fish jaws. It is rarely considered that biological form and function potentially violates requirements in these engineering models, e.g. of link stiffness and linkage planarity. Moreover, fish heads often contain several mechanisms that should be modeled together, e.g. using integrated four-bar linkages of hyoid depression via head elevation, mouth opening via gill cover rotation and anterior jaw protrusion. We attempt to evaluate the biomechanical usefulness of 4-bar linkage models, test for linkage integrity and deviation from planarity, and calibrate for the discrepancies that we find. The aim is to develop tools to test for coupling or decoupling of linkages that govern a behavior, and explore if existing models, alone or in combination, explain the behaviors observed in vivo. Measurements and analyses include morphometrics, motion kinematics, computer modeling and XROMM.
Collaborators: Mark Westneat, University of Chicago; Ariel Camp, Elizabeth Brainerd & Nicholas Gidmark, Brown University.
Structural innovation, functional disparity and evolutionary ecology.
My Ph. D. examined the consequences in biting coral reef fishes from having evolved an intramandibular joint (IMJ) in the lower jaw, and my first postdoc examined how the tongue-bite apparatus (TBA) in basal bony fishes is used in food processing. These analyses made me wonder how structural innovation influences the evolution of functional disparity, or the range of functional diversity within a clade. These questions are addressed in several fish groups that have acquired intramandibular joints, tongue-bites and/or pharyngeal jaws (PJA). Interspecific variation in feeding apparatus function during different behaviors is measured at several biological determinant levels, including morphology, muscle activity, biomechanics and motion kinematics. The data are also used to revisit Liem’s hypothesis; that behavioral flexibility, or alternatively stereotypy shapes the trophic status of animals as either generalists or specialists.
Nutritional ecology and ecosystem function of biting reef fishes.
Biting reef fishes are derived and taxonomically diverse, but considerably less studied than their free water feeding sisters from a functional ecological perspective. We study the physiology of how biters gain nutrition from their novel niches, notable of which are corallivory and herbivory. In the field, we also examine how biting action modifies microhabitat topography. The idea is that that angelfishes and a few other groups are unique in their ability to excavate concavities in the reef matrix, while other biters mainly create convex surfaces. The relative abundance and species composition of these two functional groups may influence the resulting topographic complexity and ultimately the resilience of coral reef ecosystems.
Collaborators: Peter Wainwright, U. C. Davis; David Bellwood, JCU; Darrin Hulsey, U. Tennessee; Lara Ferry, Arizona State University, David Raubenheimer, U. Massey; J. Howard Choat, James Cook University.