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Research Article
How cockatiels (Nymphicus hollandicus) modulate pectoralis power output across flight speeds
Tyson L. Hedrick, Bret W. Tobalske, Andrew A. Biewener
Journal of Experimental Biology 2003 206: 1363-1378; doi: 10.1242/jeb.00272
Tyson L. Hedrick
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Bret W. Tobalske
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Andrew A. Biewener
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Figures

  • Fig. 1.
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    Fig. 1.

    (A) A depiction of the pectoralis muscle, its attachment to the humerus at the deltopectoral crest (DPC) of the humerus, and implanted transducers. A pair of sonomicrometry crystals was implanted in the proximal portion of the pectoralis, and an EMG electrode was placed between the two crystals. A metal-foil strain gauge was attached to the dorsal surface of the DPC of the humerus. Wires from all these transducers were passed subcutaneously to a customized miniature plug attached to the bird's back. (B) Sonomicrometry and electromyogram (EMG) recordings are shown for three successive wingbeats at 7 m s-1. The time course of pectoralis shortening (downstroke) is shaded in gray. (C) Pectoralis force recorded by the strain gauge and (D) z-axis (vertical) motion of the wrist and wing-tip obtained from a 3-D reconstruction of digitized markers based on 125 Hz dorsal and lateral camera views.

  • Table 1.

    Morphometric data for the cockatiel (Nymphicus hollandicus) and experimental conditions

    VariableMean ± S.D.
    Body mass (g)83.0±5.0
    Total pectoralis mass (g)16.9±0.6
    Fascicle length (mm)34.9±5.0
    Wing span (mm)463.0±31.0
    Wing chord (mm)70.2±3.0
    Air temperature (°C)26.4±1.7
    Air pressure (kPa)100.6±0.3
    Air density (kg m-3)1.17±0.01
    • N=5 in all cases. Measurements were made with the wings spread as in mid-downstroke.

  • Fig. 2.
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    Fig. 2.

    A timing histogram relating the timing of electromyogram (EMG) activity, muscle length change, muscle force production and kinematic data for an individual bird flying at 7 m s-1. The zero time is set to the time at which maximum force occurred, which corresponds to mid-downstroke. The bars on each rectangle indicate standard deviation. Additional data on the variation across speeds in the relationship between EMG onset time, muscle shortening and force production are given in Table 2.

  • Fig. 3.
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    Fig. 3.

    Whole wingbeat in vivo work loops obtained from an individual cockatiel flying at (A) 1 m s-1, (B) 7 m s-1 and (C) 13 m s-1. The direction of each loop is counter-clockwise, resulting in positive work. Pectoralis mass-specific power output, wingbeat duration and work `shape factor' are noted in the upper left for each loop. The shape factor quantifies work loop shape by dividing the actual loop area by the theoretical maximum area for the observed peak force and length change.

  • Table 2.

    Muscle power output and associated subcomponents during the flapping flight of the cockatiel (Nymphicus hollandicus)

    Flight speed (m s-1)
    Variable135791113FP
    Mass-specific pectoralis power output (W kg-1)125.1±24.196.1±21.973.5±10.176.9±11.396.6±22.5118.1±23.4155.6±29.26.3080.0004*
    Positive work per wingbeat (J)0.23±0.040.18±0.040.14±0.020.16±0.020.21±0.030.26±0.040.32±0.056.9720.0002*
    Total pectoralis length change (L)0.41±0.060.38±0.040.34±0.050.36±0.080.39±0.080.42±0.050.44±0.067.3080.0002*
    Peak pectoralis force (N)31.8±9.127.0±8.222.3±5.223.3±4.228.3±4.534.6±5.338.9±4.97.5440.0001*
    Wingbeat duration (ms)109.0±6.0118.1±8.4119.5±3.8130.9±6.4133.9±8.6135.0±7.8122.5±5.87.9270.0001*
    Downstroke duration (ms)67.2±2.570.2±4.266.6±3.166.9±4.367.7±6.867.4±5.266.1±3.90.8210.5645
    Upstroke duration (ms)41.8±4.047.9±6.352.9±3.264.0±4.666.3±4.267.5±6.656.4±6.114.180.0001*
    EMG duration (ms)†48.1±7.950.3±8.754.5±12.659.5±19.861.7±17.957.4±8.650.2±4.41.720.2624
    EMG onset to force production (ms)†9.4±4.98.7±4.38.6±3.86.6±1.07.2±2.27.6±2.76.6±1.041.020.4888
    EMG onset to pectoralis shortening (ms)†10.7±1.49.7±1.312.4±1.515.2±3.116.0±4.214.7±4.613.9±4.75.450.0291
    Work loop shape factor0.52±0.080.53±0.060.55±0.060.56±0.060.55±0.050.53±0.050.54±0.051.010.4398
    Pectoralis shortening velocity (L s-1)6.11±0.875.42±0.775.19±0.815.43±1.075.77±0.986.20±0.806.73±0.9813.120.0001*
    • Values are inter-individual means ± S.D.; N=5 († indicates N=2).

      Significant effects of flight speed at the P<0.05 level after a Bonferroni correction for the table are marked with an asterisk (repeated-measures ANOVA with individual and speed; d.f.=6).

  • Fig. 4.
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    Fig. 4.

    (A) The relationship between mechanical power output and flight speed expressed as inter-individual means ± S.D. (N=5 for each speed; mean and minimum wingbeat sample sizes were 46 and 17, respectively, across all individuals). Pectoralis force recordings were calibrated using an aerodynamic power analysis at two intermediate flight speeds (7 m s-1 and 9 m s-1). The broken lines indicate the possible range of variation in muscle power output given varying aerodynamic assumptions. (B) A power curve for an individual cockatiel showing within-individual means ± S.D. (mean and minimum sample sizes were 50 and 19 wingbeats, respectively, per animal for each speed). The cockatiels tended to repeatedly gain and lose potential energy while flying in the wind tunnel, leading to large variation in per-wingbeat power output at all but the fastest speeds. We restricted our analysis to sequences of wingbeats with no net change in potential energy but allowed individual wingbeats that resulted in a change in potential energy.

  • Fig. 5.
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    Fig. 5.

    Variation in pectoralis power, pectoralis work and wingbeat frequency across flight speeds. All values are inter-individual means (N=5 for each speed) normalized as a percentage of the overall mean for each parameter. Standard deviations are not shown to improve clarity (these are presented in Table 2). These data show that pectoralis power is determined mainly by changes in muscle work per wingbeat and not by wingbeat frequency.

  • Fig. 6.
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    Fig. 6.

    (A) Variation in whole wingbeat, upstroke and downstroke durations across flight speeds (inter-individual means ± S.D.; N=5 for each speed) as measured via muscle lengthening and shortening. Consistent with the modest change in wingbeat frequency versus speed (Fig. 5), wingbeat duration varies slightly but significantly (P<0.05; Table 2) with speed. Changes in wingbeat duration across speeds is due entirely to changes in upstroke duration, as downstroke duration does not vary significantly with speed (P>0.1; Table 2). (B) Variation in wingbeat duration measured via a 125 Hz, three-dimensional kinematic reconstruction. Although the general pattern is similar to that shown in A, the relative durations of upstroke and downstroke have shifted such that downstroke is shorter than or equal to, rather than longer than or equal to, upstroke.

  • Fig. 7.
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    Fig. 7.

    (A) Variation in peak muscle force and muscle strain rate as a function of flight speed. (B) Changes in the amplitude of muscle strain with speed (P<0.01; Table 2). Values represent interindividual means ± S.D. (N=5 for each speed). Pectoralis force, strain rate and strain amplitude vary similarly with speed, and the pattern of variation matches that found for muscle power output versus speed (Fig. 5).

  • Fig. 8.
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    Fig. 8.

    A partial regression component model of the factors underlying pectoralis power output among individual birds as a function of flight speed with a least-squares multiple linear regression of power against selected factors. The arrows indicate the proposed relationships between variables, together with their partial regression coefficients, which indicate the relative strength of the relationship. Pectoralis power output as a function of flight speed is determined by the combination of work per wingbeat and wingbeat frequency, but work per wingbeat exerts the strongest effect. Pectoralis work per wingbeat is influenced by several factors, but the two most important are muscle force (r2=0.77) and muscle length change (r2=0.45; also see Fig. 6). The double-headed arrow between muscle length change and work loop shape factor indicates that increases in length change are correlated with decreases in the shape factor, and vice versa. The correlation also influences the total effect of the variables; for example, the total effect of muscle length change is 0.80-(0.63×0.40), or 0.55. There were no significant correlations between model variables aside from those indicated in the model via arrows. Statistical tests were conducted based on a set of individual means across speeds (5 individuals × 7 speeds: N=35).

  • Table 3.

    Additional multiple regression results for muscle power output against wingbeat duration, muscle force and muscle length change

    r2FPN
    All individuals across speeds0.8247.9<0.0001*35
    One individual across speeds0.99246.2<0.0001*7
    One individual within 1 m s-10.94344.6<0.0001*69
    One individual within 7 m s-10.88204.4<0.0001*86
    One individual within 13 m s-10.87116.6<0.0001*57
    • ↵* Indicates significance at P<0.05 after a Bonferroni multiple test correction for all electromyogram (EMG) regression tests.

  • Fig. 9.
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    Fig. 9.

    (A) A least-squares regression of muscle force versus electromyogram (EMG) amplitude within one individual across speeds. (B) A least-squares regression of muscle shortening velocity during downstroke versus EMG amplitude within the same individual across speeds. EMG amplitude was quantified as the integrated rectified EMG signal, divided by its duration. We performed the regression against mean values for amplitude, force and shortening velocity at each speed to give a balanced data set. However, we plotted all points included in the mean values as small `x' symbols to show the full range of variation; the mean values used in the regression analyses are shown as large diamonds. Table 4 reports additional analyses of EMG amplitude with respect to muscle force.

  • Table 4.

    EMG regression results, EMG amplitude versus peak muscle force

    r2FPN
    Individual 1 across speeds0.8020.00=0.0066*7
    Individual 3 across speeds0.9148.64=0.0009*7
    Individuals 1 and 3 across speeds
    Actual amplitude and force0.7230.44<0.0001*14
    Normalized amplitude and force0.7739.40<0.0001*14
    Individual #1 within 7 m s-10.6992.21<0.0001*43
    Individual #3 within 7 m s-10.63142.83<0.0001*86
    • ↵* Indicates significance at P<0.05 after a Bonferroni multiple test correction for all electromyogram (EMG) regression tests.

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Research Article
How cockatiels (Nymphicus hollandicus) modulate pectoralis power output across flight speeds
Tyson L. Hedrick, Bret W. Tobalske, Andrew A. Biewener
Journal of Experimental Biology 2003 206: 1363-1378; doi: 10.1242/jeb.00272
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Research Article
How cockatiels (Nymphicus hollandicus) modulate pectoralis power output across flight speeds
Tyson L. Hedrick, Bret W. Tobalske, Andrew A. Biewener
Journal of Experimental Biology 2003 206: 1363-1378; doi: 10.1242/jeb.00272

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