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First published online October 21, 2004
Journal of Experimental Biology 207, 4165-4174 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01253

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Adjusting muscle function to demand: joint work during acceleration in wild turkeys

Thomas J. Roberts^* and Jeffrey A. Scales

Oregon State University, Department of Zoology, 3029 Cordley Hall, Corvallis, OR 97331-2914, USA

View larger version (15K): [in a new window]   Fig. 1. The method for calculating joint moments, M, and powers, Pm. Joint moment (A) is the product of the magnitude of the ground reaction force, GRF, and the out-moment arm, R. R is the orthogonal distance from the GRF to the center of rotation of the joint. Arrows indicate the direction of positive net moments produced by muscles at a joint. Joint angles were measured as indicated in B. Joint velocities were calculated from the time derivative of the angle () change. The arrows denote the direction of positive velocities. Definitions: a, ankle; h, hip; k, knee; t, toe.

View larger version (14K): [in a new window]   Fig. 2. The sum of the measured joint work values, calculated from inverse dynamics, versus the work required to move the center of mass (COM) of the body calculated from force-plate ergometry. The line indicates unity. Different symbols identify individual birds.

View larger version (16K): [in a new window]   Fig. 3. As acceleration increased, the net work per kg body mass performed during a single step increased at the hip and the ankle. Net work was independent of acceleration at the knee and the tarsometatarsal–phalangeal (TMP) joint. Linear regressions (N=49) are shown for hip, y=0.154x+0.177, r2=0.64, P<0.01; ankle, y=0.192x–0.129, r2=0.84, P<0.01.

View larger version (16K): [in a new window]   Fig. 4. The mean moment produced at each joint during stance showed a small but significant increase with acceleration at all joints except the ankle. Linear regressions (N=49) are: hip, y=0.16x+2.55, r2=0.20, P<0.01; knee, y=–0.09x+0.24, r2=0.14, P<0.01; TMP, y=0.06x+1.14; r2=0.05, P<0.05.

View larger version (21K): [in a new window]   Fig. 5. The moment arm of the ground reaction force (R; Fig. 1) averaged over the step did not change with acceleration at the ankle or tarsometatarsal–phalangeal (TMP) joint. Hip moment arm increased with acceleration (y=0.0061x+0.069, r2=0.23, P<0.001). Mean moment arms during knee extension (green) and knee flexion (blue) were measured separately. Only the mean flexion moment arm at the knee changed significantly with acceleration. Linear regression for the knee flexion moment arm was y=–0.0070x–0.029, r2=0.27, P<0.05.

View larger version (17K): [in a new window]   Fig. 6. The net excursion at a joint during a single step, measured from the difference between angle at toe-down and angle at toe-off, as a function of acceleration. Positive values indicate net joint extension and negative numbers indicate joint flexion. The hip and the ankle extended more as acceleration increased, while the knee underwent less flexion. The tarsometatarsal–phalangeal (TMP) joint extended less as acceleration increased. Linear regressions (N=49) are shown for hip extension, y=5.13x+12.3, r2=0.51, P<0.01; knee, y=11.52x–69.64, r2=0.76, P<0.01; ankle, y=11.93x–18.98, r2=0.80, P<0.01; TMP, y=–2.98x+23.85, r2=0.19, P<0.01.

View larger version (15K): [in a new window]   Fig. 7. Hip joint moment, angle and power during single footfalls for three representative strides during a steady-speed run, a moderate acceleration and a high acceleration (mean acceleration during stance=0.13 m s-2, 2.31 m s-2, and 4.47 m s-2 respectively). Angles and moments are measured as indicated in Fig. 1. The area under the positive power curve represents work done at the joint, while the area above the negative region of the power curve is work done on the joint.

View larger version (15K): [in a new window]   Fig. 8. Knee joint moment, angle and power during foot contact for the three representative steps shown in Fig. 7.

View larger version (16K): [in a new window]   Fig. 9. Ankle joint moment, angle and power for the three representative steps shown in Fig. 7.

View larger version (15K): [in a new window]   Fig. 10. Tarsometatarsal–phalangeal joint moment, angle and power for the three representative steps shown in Fig. 7.

View larger version (16K): [in a new window]   Fig. 11. Ankle out-moment arm, R, versus time of stance, averaged for the five highest accelerations (thick line) and the five lowest (thin line, approximately steady speed). Mean values for individual trials were calculated after normalizing for stride time.