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

We measured the net work performed at hind limb joints in running turkeys to determine the source of mechanical power for acceleration. We tested the hypothesis that net mechanical work per step increases in proportion to acceleration at all four major hind limb joints (hip, knee, ankle and tarsometatarsal-phalangeal joint). This hypothesis was based on the idea that all hind limb muscles should contribute mechanical work to maximize performance during accelerations, and a previous study that indicated the mechanical power output of the entire turkey hind limb musculature was remarkably high. We used high-speed video and force-plate measurements to measure joint moment, velocity and power output during single foot-contacts of running accelerations. By measuring steps in which the animals were relatively more or less motivated to accelerate, we obtained data for a range of accelerations, all at approximately the same running speed. Net joint work per step increased at the hip and ankle as a function of acceleration. Hip net work per unit body mass was 0.12+/-0.09 J kg(-1) averaged over the five lowest accelerations (-0.22+/-0.08 m s(-2)), and 0.87+/-0.20 J kg(-1) for the five highest accelerations (4.86+/-0.27 m s(-2)). Ankle work was -0.21+/-0.11 J kg(-1) for the lowest accelerations and 0.71+/-0.28 J kg(-1) for the highest. The high work output at the ankle is consistent with the idea that elastic mechanisms function to increase muscle work during acceleration. The work performed at the knee and tarsometatarsal-phalangeal joint was independent of acceleration in a step. These results support the idea that hip and ankle extensors contribute significantly to the work necessary to accelerate the body. We also measured the change in joint moment and angular excursion with acceleration to determine whether the mechanism for increasing work output at a joint involved an increase in muscle force or muscle shortening. The increase in joint work at the hip and ankle resulted almost entirely from an increase in joint angular excursion during stance. Hip extension increased by more than threefold from the lowest to the highest accelerations, and the angular excursion of the ankle increased from -24.8+/-4.7 degrees (net flexion) at the lowest accelerations to 33.0+/-12.8 degrees (net extension) at the highest accelerations. Mean stance joint moment was unchanged with acceleration at the ankle and increased by approximately 35% at the hip across the range of accelerations. These patterns of joint moment and excursion indicate that turkeys increase mechanical work for acceleration primarily by increasing muscle shortening, rather than muscle force.