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Team Members

  • Marina Dimitrov

General Objective

  • Further understand the mechanism of energy transfer between the tuna’s internal locomotor structure and the surrounding fluid medium.

Research Questions

  • What is the “natural frequency” of the tuna locomotor system?
  • Is this frequency comparable to the tailbeat frequencies and swimming speeds most commonly observed in wild and captive tuna?
  • [future work] Is this frequency comparable to that of vortex shedding from a tuna’s body at different speeds and scales?


I built the following simple model in OpenSim:


This represents the last set of musculotendon actuators in the tuna locomotor system (see Figure 5, later, for actual anatomy).



  • keel (ellipse) is currently fixed to the ground - in reality it moves back and forth driven by the rest of the fish (Figure 3, later), but I am focusing on the tail flick
  • tail is pin joint - in reality a set of several vertebrae with higher flexibility than the fairly rigid keel made up of several almost-fused vertebrae
  • two linear musculotendon actuators inserting on either side of the tail - in reality, some parts of the tendon also attach to the skin
  • origin of actuators is fixed - in reality, the muscles anterior pull the others forward as they contract


Things to improve model:

  • fill in correct mass and inertial properties for tail
    • maybe get a cool shape
  • correct coordinate limits and coordinate limit force values incorporating vertebrae bending research
  • correct muscle parameters to represent tuna
    • select values to feed into existing musculotendon model (some research indicates that shark red muscle operates similar to mammalian, and that tuna tendons are mammal-like)
    • do some geometry for initial muscle conditions so not pre-stretched (or are they pre-stretched in tuna? hm)


Approach 1 - Frequency Sweep



Approach 2 - Impulse Response



Approach 3 - Determine Stiffness



Compare to Observed Behavior



Challenges and Future Work






Extra Background for Reference


Tuna, one of the top predators in the ocean, swim with an incredible mix of power, speed, and efficiency. In this “thunniform” swimming, the front part of the body remains fairly straight. Bending the last part of the spine swings the stiff tail region right before the fin, called the peduncle, back and forth. The tail fin, in turn, bends relative to this peduncle, adding a characteristic flick to the tail strokes. Figure 1 shows this combined mode of oscillation.


(sorry, not published yet, but here is a video where you can get the idea)

Figure 1 – Tail motion of a swimming tuna (Dimitrov et al. in preparation). Note the two primary motions of interest: that of the peduncle at the end of the spine, and that of the tail fin relative to the peduncle.


This gives the tuna fine control over the interaction between vortices coming off the body and those generated by the tail, as modeled with computational fluid dynamics (CFD).1,2 Vortices are rotational patterns of fluid motion that a fish creates while swimming, generating force and leaving the equivalent of footprints in the water (Fig. 2).



Figure 2 – 3D fluid structure behind an oscillating tuna tail fin modeled using CFD (left), and a 2D slice of that (right), showing how the vortices form almost like “footprints”.2


During cruising, tuna swim by almost exclusively using a system of red muscle myomeres, ribs, and posterior oblique tendons (POTs), shown in Figure 3, which transmit muscle forces to the spine.3 The horizontal ribs coming out of each vertebra act as struts, allowing the muscle attached to each tendon to pull on a vertebra farther down the spine. Figure 4 shows how these muscles contract along the body throughout the swimming “stride.”


(sorry, not published yet, but here is a figure mashup from another paper that illustrates it)3

Figure 3 – A 2D engineering model (left) of the red muscle and posterior oblique tendon (POT) system in tuna (right), with the normally stacked muscles separated and folded out (Cromie et al. in preparation).


Figure 4 – Muscle activation patterns of a swimming yellowfin tuna. From the head towards the tail, the muscles on one side start contracting, until they are all contracted. This then repeats on the other side of the body.4


Posterior to this POT system, a bony keel jutting out horizontally from the vertebrae acts to increase the moment arm of the great lateral tendon (GLT), which attaches to the tail fin (Fig. 5). Passing over the keel rather than lying close to the backbone provides the GLT and associated muscle myomeres with greater mechanical advantage as they pull on the tail. The GLT also attaches to the skin at various points on the way back to the tail.


 (sorry, not published yet, but here is a photo from another paper illustrating the anatomy)3

Figure 5 – The great lateral tendon (GLT, blue) and associated muscle myomeres (red) of a yellowfin tuna (Dimitrov et al. in preparation).


  1. Yang, L. & Su, Y. CFD simulation of flow features and vorticity structures in tuna-like swimming. China Ocean Eng. 25, 73–82 (2011).
  2. Zhou, K., Liu, J. & Chen, W. Numerical Study on Hydrodynamic Performance of Bionic Caudal Fin. Appl. Sci. 6, 15 (2016).
  3. Westneat, M. W., Hoese, W., Pell, C. A. & Wainwright, S. A. The horizontal septum: Mechanisms of force transfer in locomotion of scombrid fishes (Scombridae, Perciformes). J. Morphol. 217, 183–204 (1993).
  4. Altringham, J. D. & Shadwick, R. E. in Fish Physiology (ed. Stevens, B. B. and E.) 19, 313–344 (Academic Press, 2001).