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First published online August 31, 2004
Journal of Experimental Biology 207, 3495-3506 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01125

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Neuromuscular control of trout swimming in a vortex street: implications for energy economy during the Kármán gait

James C. Liao

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA

View larger version (78K): [in a new window]   Fig. 1. (A) Posterolateral view of a trout Kármán gaiting behind a D-cylinder with fine wire electromyography electrodes inserted into the red and white axial muscles along the body (see Materials and methods for details). A map of electrode insertion sites for superficial red (R) and deep white (W) axial muscles along four longitudinal locations of the body in ventral (B) and lateral (C) view are shown. From the snout, insertion sites 1–4 correspond to 0.23, 0.40, 0.56 and 0.73 L down the body from the snout, respectively, where L is total body length. s denotes the placement of a suture loop, used to hold individual electrode wires in place against the body. Asterisks (i.e. R1* and R3*) indicate electrodes inserted on the right side of the fish. The center of mass is represented by a gray circle.

View larger version (17K): [in a new window]   Fig. 2. Representative electromyograms from a trout swimming steadily in uniform flow at three speeds, along with the corresponding lateral (z-axis) excursions of the center of mass (COM; gray circles). At a flow speed approximating the reduced velocity behind the cylinder (A), only the posterior red muscles are active. At a flow speed of 3.5 L s–1 (B), there is a propagating wave of red muscle activity down the entire body with no white muscle activity. When trout swam at the highest speed (C), anaerobic white muscles were recruited. At each swimming speed, staggered onset times of muscle activity down the body reflect the presence of a propulsive body wave. Scale bars for time (black, horizontal), electromyography intensity (black, vertical) and COM amplitude (gray) are shown.

View larger version (26K): [in a new window]   Fig. 3. Red and white axial muscle electromyography traces of a locomoting trout shown with corresponding ventral video images taken at evenly spaced intervals, for one oscillation of the center of mass. When swimming in uniform flow at 3.5 L s–1 (A), waves of red muscle activity propagate down each side of the body. Open rectangles provide an arbitrary, fixed reference point to assess lateral body amplitudes. The same fish Kármán gaiting behind a cylinder placed in a 3.5 L s–1 flow (B,C) activates its red muscles (R1) on one side of its body but not the other (R1* or R3*). B shares the same scale as A to facilitate comparison during one center of mass (COM) cycle. B is a subset of C, which shows the rhythmic nature of R1 activity. Open rectangles are aligned with the center of the cylinder, located approximately 1.5 L upstream (not shown). Scale bars are given for time (black, horizontal), muscle intensity (black, vertical) and lateral amplitude of COM (gray).

View larger version (26K): [in a new window]   Fig. 4. At times when the paired fins are actively abducted during the Kármán gait, there is no appreciable axial muscle activity on either side of the body. Pectoral and pelvic fin silhouettes were digitized and enhanced to illustrate the pattern of abductions as the fish is buffeted from side to side by cylinder vortices. Open rectangles are aligned with the center of the cylinder, located approximately 1.5 L upstream (not shown). Scale bars are given for time (black, horizontal), muscle intensity (black, vertical) and the lateral amplitude of the center of mass (gray).

View larger version (12K): [in a new window]   Fig. 5. (A) Average, rectified area of muscle activity (product of muscle intensity and burst duration) along the body for all five trout swimming in uniform flow at 3.5 L s–1 (light gray bars) and Kármán gaiting in a vortex street (dark gray bars). Vertical lines indicate S.E.M. Note that there is no white muscle activity (W5 and W6) during either flow treatment. Compared with trout behind a cylinder, axial muscles of trout swimming in uniform flow have a significantly greater average rectified area for all locations down the body except R1 (n=20). Kármán gait R1 muscle activity has a significantly lower intensity (B) but longer burst duration (C) than R1 muscle activity during swimming in uniform flow.

View larger version (35K): [in a new window]   Fig. 6. Normalized, average rectified electromyograms of anterior (R1) and posterior (R4) red muscle activity (solid black line) along with the upper limit of the S.E.M. (solid gray line) for all five trout swimming in uniform flow (A and C; n=20) and Kármán gaiting (B and D; n=14). Data are shown relative to one oscillation cycle of the center of mass (COM; gray circles). Mean onset and offset times of muscle activity (horizontal gray bar) along with their S.E.M. are shown above each graph, where applicable. When trout are behind the cylinder (B), R1 onset and offset times (vertical broken lines) occur relatively earlier in the COM cycle than for trout swimming in uniform flow (A). (C) Rectified area for R4 during swimming in uniform flow (n=20) shows delayed onset and offset times relative to A, indicating the presence of a propagating body wave. (D) By contrast, no posterior red muscle activity is present when trout Kármán gait.

View larger version (15K): [in a new window]   Fig. 7. Multiple linear regression model showing the relationship between lateral head amplitude and R1 muscle activity variables for trout swimming in uniform flow (black circles) and Kármán gaiting (blue circles). Circle diameters are scaled to illustrate the relati ve magnitude of muscle intensity. During swimming in uniform flow, head amplitude has a significant slope when plotted against burst duration and relative onset time (r2=0.68, P<0.05, n=20). Actual three-dimensional relationships of the data are plotted, along with corresponding two-dimensional deconstructions, on either side to facilitate interpretation. By contrast, during the Kármán gait, head amplitude is not correl ated to muscle activity (r2=0.22, P=0.65, n=14).

View larger version (21K): [in a new window]   Fig. 8. Body outlines of a dead trout towed behind the cylinder (A), a live trout Kármán gaiting behind the cylinder (B) and a live trout swimming in uniform flow at 3.5 L s–1 (C). Within each experimental treatment, body outlines have been displaced upstream at evenly spaced intervals to facilitate visualization of kinematics. Similar to a live trout Kármán gaiting, a dead trout towed in the wake of a cylinder oscillates laterally with high amplitude and has a tail-beat frequency similar to the expected vortex shedding frequency. The body wavelength is also longer than the expected wake wavelength, confirming that live fish can synchronize to the vortex street in a largely passive manner. Dead and live trout behind the cylinder adopt the same mean head angle. However, live trout head angles have a wider range of values and a higher variance, indicating a larger change in head angle during the Kármán gait compared with dead trout.