Roberts, T.J.*; Scales, J.A.: The Source of Power for Acceleration in Turkeys.

Many of the features that make some muscle-tendon units effective as springs during running – long tendons, short muscle fibers, articulations across more than one joint – may make them ill-suited to performing mechanical work. It has been suggested that the work required for activities such as uphill running or acceleration is produced by long-fibered muscles of distal limb segments. We measured the work done at individual joints in wild turkeys during acceleration to determine which muscles provide the work of acceleration. Force-plate and high-speed video measurements were used to measure joint work during single footfalls as turkeys accelerated along a 10 meter trackway. The net work per step at the hip and the ankle increased in direct proportion to the rate of acceleration. At the ankle, a decrease in joint flexion and an increase in joint extension contributed to the total increase in net work output. Hip work was increased by increasing the joint extension from an average of 12 degrees during steady speed running to above 40 degrees at the strongest accelerations. The knee and tarsometatarsal-phalangeal joint did not contribute to the work of acceleration, as net work output at these joints was independent of acceleration rate. Net work produced for the most rapid accelerations averaged 0.9 J/kg body mass at the hip and 0.8 J/kg body mass at the ankle. The large muscle masses at the ankle and the hip appear to be equally important for providing the work necessary for acceleration. The high work output of ankle extensors indicates that while long tendons and short fibers may improve a muscle’s ability to develop force economically, they do not necessarily limit its ability to perform mechanical work. Supported by the Medical Research Foundation of Oregon and NIH AR46499 to TJR.