Lee David V, Stakebake Eric F, Walter Rebecca M, Carrier David R
Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112-0840, USA.
J Exp Biol. 2004 Apr;207(Pt 10):1715-28. doi: 10.1242/jeb.00947.
The antero-posterior mass distribution of quadrupeds varies substantially amongst species, yet the functional implications of this design characteristic remain poorly understood. During trotting, the forelimb exerts a net braking force while the hindlimb exerts a net propulsive force. Steady speed locomotion requires that braking and propulsion of the stance limbs be equal in magnitude. We predicted that changes in body mass distribution would alter individual limb braking-propulsive force patterns and we tested this hypothesis by adding 10% body mass near the center of mass, at the pectoral girdle, or at the pelvic girdle of trotting dogs. Two force platforms in series recorded fore- and hindlimb ground reaction forces independently. Vertical and fore-aft impulses were calculated by integrating individual force-time curves and Fourier analysis was used to quantify the braking-propulsive (b-p) bias of the fore-aft force curve. We predicted that experimental manipulation of antero-posterior mass distribution would (1) change the fore-hind distribution of vertical impulse when the limb girdles are loaded, (2) decrease the b-p bias of the experimentally loaded limb and (3) increase relative contact time of the experimentally loaded limb, while (4) the individual limb mean fore-aft forces (normalized to body weight + added weight) would be unaffected. All four of these results were observed when mass was added at the pelvic girdle, but only 1, 3 and 4 were observed when mass was added at the pectoral girdle. We propose that the observed relationship between antero-posterior mass distribution and individual limb function may be broadly applicable to quadrupeds with different body types. In addition to the predicted results, our data show that the mechanical effects of adding mass to the trunk are much more complex than would be predicted from mass distribution alone. Effects of trunk moments due to loading were evident when mass was added at the center of mass or at the pelvic girdle. These results suggest a functional link between appendicular and axial mechanics via action of the limbs as levers.
四足动物的前后质量分布在不同物种之间有很大差异，然而这种设计特征的功能影响仍知之甚少。在小跑过程中，前肢施加净制动力，而后肢施加净推进力。稳定速度的运动要求支撑肢体的制动和推进力大小相等。我们预测体重分布的变化会改变单个肢体的制动 - 推进力模式，并通过在小跑的狗的质心、胸带或骨盆带附近增加10%的体重来检验这一假设。两个串联的力平台独立记录前肢和后肢的地面反作用力。通过对单个力 - 时间曲线进行积分来计算垂直和前后冲量，并使用傅里叶分析来量化前后力曲线的制动 - 推进（b - p）偏差。我们预测，对前后质量分布进行实验性操作将（1）在肢体带加载时改变垂直冲量的前后分布，（2）降低实验加载肢体的b - p偏差，（3）增加实验加载肢体的相对接触时间，而（4）单个肢体的平均前后力（归一化到体重 + 增加的重量）将不受影响。当在骨盆带增加质量时，观察到了所有这四个结果，但当在胸带增加质量时，只观察到了1、3和4。我们提出，观察到的前后质量分布与单个肢体功能之间的关系可能广泛适用于不同体型的四足动物。除了预测的结果外，我们的数据表明，向躯干添加质量的机械效应比仅根据质量分布预测的要复杂得多。当在质心或骨盆带添加质量时，由于加载引起的躯干力矩的影响很明显。这些结果表明，通过肢体作为杠杆的作用，附肢力学和轴向力学之间存在功能联系。
