Winstein C J, Abbs J H, Petashnick D
Department of Biokinesiology and Physical Therapy, CSA 208, University of Southern California, Los Angeles 90033.
Exp Brain Res. 1991;87(2):465-9. doi: 10.1007/BF00231864.
This study investigated the influence of object weight and instructions on grip force responses in humans. Using a precision grip, subjects lifted a small instrumented test object to a predetermined height. Prior to each set of 40 trials, subjects were verbally instructed to either "hold" or "let go" of the object in response to any change in weight. Unpredictably on some trials (less than 20%), a sudden sustained increase (load) or decrease (unload) in vertical load was applied to the object. Grip responses to these induced weight changes were evaluated by measuring grip force, object position, and associated electromyographic (EMG) activity. Grip force changes for a load were over three times greater than those for an unload. Such asymmetry may reflect everyday grasp and manipulation in a gravity-influenced world. Grip force adjustments to loads following "hold" instructions were on the average somewhat larger than those following "let go" instructions, but there was no influence of instructions on responses to unloads. These findings contrast with more robust influences of verbal instruction on automatic postural and proximal upper limb responses and also may suggest that grip force adjustments are influenced to a greater extent by intrinsic task variables than by extrinsic volitional intent. Such organization appears tailored to functional task requirements in natural environmental contexts.
本研究调查了物体重量和指令对人类握力反应的影响。受试者采用精确握法，将一个装有仪器的小型测试物体举到预定高度。在每组40次试验之前，会口头指示受试者根据重量的任何变化“握住”或“放开”物体。在某些试验中（少于20%），会不可预测地对物体施加垂直负荷的突然持续增加（加载）或减少（卸载）。通过测量握力、物体位置和相关的肌电图（EMG）活动来评估对这些诱发的重量变化的握力反应。加载时的握力变化比卸载时大两倍多。这种不对称可能反映了在受重力影响的世界中的日常抓握和操作。按照“握住”指令对加载进行的握力调整平均比按照“放开”指令进行的调整稍大，但指令对卸载反应没有影响。这些发现与言语指令对自动姿势和近端上肢反应的更强影响形成对比，也可能表明握力调整在更大程度上受内在任务变量而非外在意志意图的影响。这种组织似乎是为自然环境中的功能任务需求量身定制的。
