Fung J, Macpherson J M
Neurological Sciences Institute, Oregon Health Sciences University, Portland, Oregon 97209, USA.
J Neurophysiol. 1999 Dec;82(6):3056-65. doi: 10.1152/jn.1999.82.6.3056.
Standing is a dynamic task that requires antigravity support of the body mass and active regulation of the position of the body center of mass. This study examined the extent to which the chronic spinal cat can maintain postural orientation during stance and adapt to changes in stance distance (fore-hindpaw separation). Intact cats adapt to changes in stance distance by maintaining a constant horizontal orientation of the trunk and changing orientation of the limbs, while keeping intralimb geometry constant and aligning the ground reaction forces closely with the limb axes. Postural adaptation was compared in four cats before and after spinalization at the T(6) level, in terms of the forces exerted by each paw against the support, body geometry (kinematics) and electromyographic (EMG) activity recorded from chronic, indwelling electrodes, as well as the computed net torques in the fore and hindlimbs. Five fore-hindpaw distances spanning the preferred distance were tested before spinalization, with a total range of 20 cm from the shortest to the longest stance. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. Three of the four cats could adapt to changes in stance distance, but the range was smaller and biased toward the shorter distances. The fourth cat could stand only at one stance distance, which was 8 cm shorter than the preferred distance before spinalization. All cats shifted their center of pressure closer to the forelimbs after spinalization, but the amount of shift could largely be accounted for by the weight loss in the hindquarters. The three cats that could adapt to changes in stance distance used a similar strategy as the intact cat by constraining the trunk and changing orientation of the limb axes in close relation with the forces exerted by each limb. However, different postures in the fore- and hindlimbs were adopted, particularly at the scapula (more extended) and pelvis (tipped more anteriorly). Other changes from control included a redistribution of net extensor torque across the joints of the forelimb and of the hindlimb. We concluded that the general form of body axis orientation is relatively conserved in the spinal cat, suggesting that the lumbosacral spinal circuitry includes rudimentary set points for hindlimb geometry. Both mechanical and neural elements can contribute toward maintaining body geometry through stiffness regulation and spinal reflexes.
站立是一项动态任务，需要对抗重力来支撑体重，并积极调节身体重心的位置。本研究考察了慢性脊髓损伤猫在站立过程中维持姿势定向以及适应站立距离（前后爪间距）变化的程度。完整的猫通过保持躯干水平方向恒定并改变四肢方向来适应站立距离的变化，同时保持肢体内几何形状不变，并使地面反作用力与肢体轴线紧密对齐。在T(6)水平脊髓横断前后，对四只猫的姿势适应性进行了比较，比较内容包括每只爪子对支撑物施加的力、身体几何形状（运动学）以及从慢性植入电极记录的肌电图（EMG）活动，还有前肢和后肢计算得出的净扭矩。在脊髓横断前，测试了跨越优选距离的五种前后爪距离，最短到最长站立距离的总范围为20厘米。脊髓横断后，每天训练猫站在力平台上，所有四只猫都能够支撑其全部体重。四只猫中有三只能够适应站立距离的变化，但范围较小且偏向较短距离。第四只猫只能在一个站立距离站立，该距离比脊髓横断前的优选距离短8厘米。所有猫在脊髓横断后都将压力中心向前肢移动，但移动量在很大程度上可归因于后躯体重减轻。三只能够适应站立距离变化的猫采用了与完整猫类似的策略，即通过限制躯干并根据每个肢体施加的力密切改变肢体轴线方向。然而，前肢和后肢采用了不同的姿势，特别是在肩胛骨（更伸展）和骨盆（更向前倾斜）处。与对照组的其他变化包括前肢和后肢关节净伸肌扭矩的重新分布。我们得出结论，脊髓猫身体轴线定向的一般形式相对保守，这表明腰骶部脊髓回路包括后肢几何形状的基本设定点。机械和神经因素都可以通过刚度调节和脊髓反射来维持身体几何形状。
