Jackson D K, Newman D J
Lockheed Martin Missiles and Space, Sunnyvale, CA, USA.
Aviat Space Environ Med. 2000 Sep;71(9 Suppl):A151-60.
Human performance and adaptation to altered loading levels is investigated. A previous astronaut jumping study demonstrated significantly altered landing performance following spaceflight, complementing reports of postflight postural and gait instabilities. A dynamic model indicated that leg stiffness changes accounted for the kinematic differences due to adaptation in open-loop modulation of leg impedance. Muscular atrophy or altered stretch reflexes and vestibular sensing could not be discounted.
We hypothesize that partial weight unloading can cause modulation of leg impedance and altered jump landing performance similar to longer-term microgravity exposure, while controlling for muscle atrophy and altered graviceptor inputs. Lower-body impedance changes after partial weight unloading support the hypothesis that postflight differences result primarily from modulation of leg impedance due to reduced postural control demands in microgravity.
Six subjects performed six baseline 30-cm downward jumps from a platform, followed by 10 jumps under 60% body weight unloading (the adaptation sequence), and then six additional jumps under full-body loading (termed "adapted jumps"). Joint and mass center kinematics were compared for the baseline and adapted jump landings. A second order model evaluated changes in vertical leg impedance.
Three subjects exhibited significant increased joint angles and rates. Vertical ground reaction forces showed more heavily damped responses after adaptation. Model fits to mass center motion indicated reduced leg stiffness.
Post-adaptation performance is similar to that of four astronaut subjects who demonstrated reduced postflight leg stiffness. The new study strongly suggests adaptive control of lower limb impedance to accommodate altered gravity levels which can be induced by minimal unloading exposure. Partial weight unloading provides a simple, inexpensive analog to spaceflight for certain postural and movement studies.
对人类的表现以及对负荷水平改变的适应情况展开了研究。此前一项宇航员跳跃研究表明，太空飞行后着陆表现发生了显著变化，这与飞行后姿势和步态不稳定的报告相呼应。一个动态模型表明，腿部刚度的变化解释了由于腿部阻抗开环调制中的适应而导致的运动学差异。肌肉萎缩或改变的牵张反射以及前庭感觉也不能被忽视。
我们假设部分体重卸载会导致腿部阻抗的调制以及跳跃着陆表现的改变，类似于长期微重力暴露，同时控制肌肉萎缩和重力感受器输入的改变。部分体重卸载后下体阻抗的变化支持了这样一种假设，即飞行后差异主要是由于微重力环境下姿势控制需求减少导致腿部阻抗调制所致。
六名受试者从平台上进行了六次基线30厘米的向下跳跃，随后在60%体重卸载条件下进行10次跳跃（适应序列），然后在全身负荷条件下再进行六次跳跃（称为“适应后跳跃”）。比较了基线跳跃和适应后跳跃着陆时的关节和质心运动学。一个二阶模型评估了垂直腿部阻抗的变化。
三名受试者的关节角度和速率显著增加。适应后垂直地面反作用力显示出阻尼更大的反应。对质心运动的模型拟合表明腿部刚度降低。
适应后的表现与四名宇航员受试者相似，他们在飞行后表现出腿部刚度降低。这项新研究有力地表明，下肢阻抗存在适应性控制，以适应重力水平的改变，这种改变可由最小程度的卸载暴露诱发。部分体重卸载为某些姿势和运动研究提供了一种简单、廉价的太空飞行模拟方式。
