Roberts Thomas J, Scales Jeffrey A
Department of Zoology, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331-2914, USA.
J Exp Biol. 2002 May;205(Pt 10):1485-94. doi: 10.1242/jeb.205.10.1485.
We tested the hypothesis that the hindlimb muscles of wild turkeys (Meleagris gallopavo) can produce maximal power during running accelerations. The mechanical power developed during single running steps was calculated from force-plate and high-speed video measurements as turkeys accelerated over a trackway. Steady-speed running steps and accelerations were compared to determine how turkeys alter their running mechanics from a low-power to a high-power gait. During maximal accelerations, turkeys eliminated two features of running mechanics that are characteristic of steady-speed running: (i) they produced purely propulsive horizontal ground reaction forces, with no braking forces, and (ii) they produced purely positive work during stance, with no decrease in the mechanical energy of the body during the step. The braking and propulsive forces ordinarily developed during steady-speed running are important for balance because they align the ground reaction force vector with the center of mass. Increases in acceleration in turkeys correlated with decreases in the angle of limb protraction at toe-down and increases in the angle of limb retraction at toe-off. These kinematic changes allow turkeys to maintain the alignment of the center of mass and ground reaction force vector during accelerations when large propulsive forces result in a forward-directed ground reaction force. During the highest accelerations, turkeys produced exclusively positive mechanical power. The measured power output during acceleration divided by the total hindlimb muscle mass yielded estimates of peak instantaneous power output in excess of 400 W kg(-1) hindlimb muscle mass. This value exceeds estimates of peak instantaneous power output of turkey muscle fibers. The mean power developed during the entire stance phase increased from approximately zero during steady-speed runs to more than 150 W kg(-1) muscle during the highest accelerations. The high power outputs observed during accelerations suggest that elastic energy storage and recovery may redistribute muscle power during acceleration. Elastic mechanisms may expand the functional range of muscle contractile elements in running animals by allowing muscles to vary their mechanical function from force-producing struts during steady-speed running to power-producing motors during acceleration.
野生火鸡（吐绶鸡）的后肢肌肉在跑步加速过程中能够产生最大功率。通过力板和高速视频测量，计算出火鸡在跑道上加速时单个跑步步骤中产生的机械功率。将稳定速度跑步步骤和加速过程进行比较，以确定火鸡如何从低功率步态转变为高功率步态来改变其跑步力学。在最大加速过程中，火鸡消除了稳定速度跑步所特有的两个跑步力学特征：（i）它们产生纯粹的推进性水平地面反作用力，没有制动力；（ii）它们在站立阶段产生纯粹的正功，在该步骤中身体机械能没有减少。稳定速度跑步时通常产生的制动和推进力对平衡很重要，因为它们使地面反作用力矢量与质心对齐。火鸡加速度的增加与触地时肢体伸展角度的减小以及离地时肢体回缩角度的增加相关。这些运动学变化使火鸡在加速过程中能够保持质心与地面反作用力矢量的对齐，此时大的推进力会产生向前的地面反作用力。在最高加速过程中，火鸡仅产生正机械功率。加速过程中测得的功率输出除以总后肢肌肉质量，得出峰值瞬时功率输出估计值超过400 W·kg⁻¹后肢肌肉质量。该值超过了火鸡肌肉纤维峰值瞬时功率输出的估计值。整个站立阶段产生的平均功率从稳定速度跑步时的约零增加到最高加速时的超过150 W·kg⁻¹肌肉。加速过程中观察到的高功率输出表明，弹性能量储存和恢复可能在加速过程中重新分配肌肉功率。弹性机制可能通过允许肌肉在稳定速度跑步时从产生力的支柱转变为加速时产生功率的发动机，来扩大跑步动物肌肉收缩元件的功能范围。
