Trunk use and co-contraction in cerebral palsy as regulatory mechanisms for accuracy control

In the present study, we examined whether individuals with cerebral palsy (CP) systematically vary motion of the trunk and co-contraction in the upper limb as a function of accuracy demands. Four participants with spastic tetraparesis, four with spastic hemiparesis, and four healthy controls were asked to repeatedly move a spoon back-and-forth between two target locations. The task was externally paced. In half the trials the accuracy demands were increased by filling the spoon with water. In addition, a condition in which the trunk was fixated was examined. When the movements were controlled for speed, trunk motion hardly varied as a function of accuracy. Co-contraction in the shoulder, however, was systematically higher under high-accuracy demands. Trunk fixation yielded differential group effects on the co-contraction of the shoulder muscles. It increased in control participants, tended to decrease in hemiparetic participants, and was unaffected in tetraparetic participants. Collectively, the present findings show that the increased trunk involvement and high co-contraction levels in CP should not exclusively be regarded as disorder-related phenomena. Regulation of co-contraction in the shoulder is a general strategy to cope with variations in movement-accuracy constraints, while increased trunk involvement proves a secondary reaction to these constraints.