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Muscle designed for maximum short-term power output: quail flight muscle

Graham N. Askew^1,* and Richard L. Marsh²

¹ School of Biology, University of Leeds, Leeds LS2 9JT, UK
² Department of Biology, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA

View larger version (14K): [in a new window]   Fig. 1. Effects of take-off angle on power output calculated for blue-breasted quail for a range of flight velocities using the techniques presented in Askew et al. (2001). For all conditions, the bird was assumed to travel at a constant velocity.

View larger version (22K): [in a new window]   Fig. 2. Illustration of the technique for estimating the mechanical power output of the muscles of blue-breasted quail during take-off. To provide a vertical force, the wings must impart a downward momentum to the air to balance the weight and any vertical acceleration of the bird. The actuator disc represents the area over which the wings interact with the air to give it a downward impulse, and it generates a vertical force equal to the weight and acceleration force Mb(g+) by imparting an induced velocity kw to air entering the disc with vertical velocity . The rate at which work is done by the disc is the induced power, Pind, and is equal to this force multiplied by the total air velocity. The second and third terms of the induced power equation are the rate of increase of potential energy of the body (Mbg) and the rate of increase of kinetic energy (Mb). Both these quantities were calculated from the movement of the centre of mass. The profile power (Ppro) required to overcome the pressure and friction drag acting on the wing was calculated from the resultant velocity and area of the wing using blade-element analysis. The parasite power (Ppar) required to overcome drag on the body was calculated from the velocity and frontal area of the bird. The total aerodynamic power (Paero) is equal to the sum of the induced power, profile power and parasite power. See Askew et al. (2001) for further details of the calculations. Mb is body mass, g is acceleration due to gravity, is vertical velocity, . is vertical acceleration, k is the induced power factor, w is induced velocity, v is the velocity of the bird, is air density, Si is wing strip area, VR,i is the velocity of the wing strip, Sb is body frontal area, CD,pro is the profile drag coefficient and CD,par is the parasite drag coefficient.

View larger version (15K): [in a new window]   Fig. 3. In vivo sonomicrometry (A) and corresponding electromyography (B) recordings from the pectoralis muscle during take-off in blue-breasted quail (Coturnix chinensis).

View larger version (10K): [in a new window]   Fig. 4. (A) Length/force relationship based on data for rat muscle (Woledge et al., 1985). The bold line shows the relative length range over which a muscle should operate to maximise work and power output for a strain of 0.23. (B) Optimum relative length (Lm/L0) about which a muscle should operate to maximise work and power for a range of strains. An optimum Lm/L0 of 0.95 should be used for a strain of 0.23. Note that the divergence of the line at strains shorter than 0.06 represents the range of relative optimum lengths (Lm; see text) over which the muscle can still operate on the plateau of the length/force relationship.

View larger version (14K): [in a new window]   Fig. 5. Comparison of the strain (A) and shortening velocity (B) for the simulation and sawtooth cycles and the stress (C) and instantaneous power output (D) developed. The stress was higher during the simulation (in vivo length cycle) cycle than during the sawtooth cycle throughout most of shortening; however, the instantaneous power output was only greater during the later part of shortening. L is muscle length, and saw 70% cycle refers to a sawtooth length cycle in which 70% of the total cycle duration is spent shortening.

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