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Mechanical power output during running accelerations in wild turkeys

Thomas J. Roberts^* and Jeffrey A. Scales

Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331-2914, USA
Present address: Department of Biology, Baylor University, Waco, TX 76798-7388, USA

View larger version (15K): [in a new window]   Fig. 1. The mean velocities (symbols) and the velocity change from toe-down to toe-off (vertical lines) for each acceleration analyzed. The highest accelerations corresponded to a change of nearly 1 m s-1 in the speed of the center of mass during a single footfall. Mean running speed was independent of acceleration for the runs analyzed.

View larger version (19K): [in a new window]   Fig. 2. Representative force traces and kinematic diagrams for a steady-speed run (A) and a high acceleration (B). High accelerations involved high peak propulsive horizontal forces (dotted lines), no braking horizontal forces and a delay in vertical force (solid line) development. The insets in the force graphs represent the mean orientation of the ground reaction force vector during stance. The turkey diagrams illustrate the changes in limb angle during acceleration that help to maintain the alignment between the forward-oriented ground reaction force (bold arrow) and the center of mass. These diagrams were traced from video frames at the times indicated by arrows on the force graphs. The gray shaded circle represents the approximate location of the animal's center of mass.

View larger version (16K): [in a new window]   Fig. 3. Peak horizontal propulsive ground reaction force Fh+ and peak vertical force Fv increased with acceleration, while negative horizontal forces Fh- decreased (A). Force is given as a proportion of body weight. The peak propulsive horizontal force, as a fraction of peak vertical force, increased with acceleration (B). The mean angle of the resultant ground reaction force (GFR) from vertical, averaged over the stride, increased with acceleration (C). Linear regressions (N=49) are shown for Fv (y=0.13x+1.69, r2=0.52, P<0.01), Fh+ (y=0.18x+0.207, r2=0.91, P<0.01), Fh- (y=0.06x-0.27, r2=0.61, P<0.01), Fh/Fv (y=0.071x+0.132, r2=0.81, P<0.01) and ground reaction force angle (y=4.28x+1.03, r2=0.91, P<0.01).

View larger version (17K): [in a new window]   Fig. 4. Increasing acceleration correlated with a decrease in hindlimb protraction angle (A) and an increase in retraction angle (B). The net pitch of the body (the angle at toe-off minus the angle at toe-down) did not change significantly (y=0.84x+1.15, r2=0.06, P=0.10) with increasing acceleration (C). Linear regressions (N=49) are shown for protraction angle (y=2.64x+34.36, r2=0.55, P<0.01) and retraction angle (y=2.92x+18.74, r2=0.49, P<0.01).

View larger version (11K): [in a new window]   Fig. 5. Body-mass-specific power output profiles for a steady-speed run (thin solid line), a moderate acceleration (dotted line) and a high acceleration (thick solid line). The mean accelerations for these runs were 0.1 m s-2 (steady), 2.4 m s-2 (moderate) and 4.7 m s-2 (high).

View larger version (20K): [in a new window]   Fig. 6. Body-mass-specific work done per step as a function of acceleration. Net work (filled circles) increased as a result of both an increase in positive work (dotted circles) and a decrease in negative work (open circles). For the fastest accelerations, only positive work was performed. All birds are represented by the same symbol for clarity. The linear regression (N=49) is shown for net work (y=0.402x+0.034, r2=0.89, P<0.01).

View larger version (17K): [in a new window]   Fig. 7. Peak instantaneous muscle power produced per unit muscle mass increased with increasing acceleration (A). The power averaged over the entire stance period, the mean stroke power, increased with increasing acceleration (B). Muscle-mass-specific powers were calculated by assuming that all hindlimb extensors contributed to the power applied to the center of mass. Linear regressions (N=49) are shown for peak power output (y=64.7x+87.5, r2=0.84, P<0.01) and mean stroke power (y=30.8x-1.1, r2=0.93, P<0.01).