Cavagna G A
Istituto di Fisiologia Umana, Università degli Studi di Milano, Via Mangiagalli, 32, 20133 Milan, Italy.
J Exp Biol. 2006 Oct;209(Pt 20):4051-60. doi: 10.1242/jeb.02344.
In the elastic-like bounce of the body at each running step the muscle-tendon units are stretched after landing and recoil before take-off. For convenience, both the velocity of the centre of mass of the body at landing and take-off, and the characteristics of the muscle-tendon units during stretching and recoil, are usually assumed to be the same. The deviation from this symmetrical model has been determined here by measuring the mechanical energy changes of the centre of mass of the body within the running step using a force platform. During the aerial phase the fall is greater than the lift, and also in the absence of an aerial phase the transduction between gravitational potential energy and kinetic energy is greater during the downward displacement than during the lift. The peak of kinetic energy in the sagittal plane is attained thanks to gravity just prior to when the body starts to decelerate downwards during the negative work phase. In contrast, a lower peak of kinetic energy is attained, during the positive work phase, due to the muscular push continuing to accelerate the body forwards after the end of the acceleration upwards. Up to a speed of 14 km h(-1) the positive external work duration is greater than the negative external work duration, suggesting a contribution of muscle fibres to the length change of the muscle-tendon units. Above this speed, the two durations (<0.1 s) are similar, suggesting that the length change is almost totally due to stretch-recoil of the tendons with nearly isometrically contracting fibres.
在每一步跑步中身体类似弹性的弹跳过程中，肌肉 - 肌腱单元在着地后被拉伸，并在离地前回弹。为方便起见，通常假定身体质心在着地和离地时的速度，以及肌肉 - 肌腱单元在拉伸和回弹过程中的特性是相同的。这里通过使用测力平台测量跑步过程中身体质心的机械能变化，来确定与这种对称模型的偏差。在腾空阶段，下落大于上升，而且在没有腾空阶段时，重力势能和动能之间的转换在向下位移过程中比在上升过程中更大。在矢状面内，动能峰值是在身体在负功阶段开始向下减速之前借助重力达到的。相反，在正功阶段，由于在向上加速结束后肌肉的推动继续使身体向前加速，所以达到的动能峰值较低。在速度达到14 km h(-1)之前，正的外部功持续时间大于负的外部功持续时间，这表明肌纤维对肌肉 - 肌腱单元长度变化有贡献。高于这个速度时，两个持续时间（<0.1 s）相似，这表明长度变化几乎完全是由于肌腱的拉伸 - 回弹以及几乎等长收缩的纤维所致。
