We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze the structure of finger force variability in discrete (ramp) and oscillatory force production tasks performed by the index and middle fingers of the right hand acting in parallel. Subjects performed the tasks at fast and slow rates, with and without a visual template presented on the screen. The variance of finger forces was partitioned into two components, compensated variance (VCOMP), which did not affect total force, and uncompensated variance (VUN), which affected total force. Only minor effects of task (discrete or oscillatory) and of template (with or without) were seen on the variance profiles, leading us to conclude that the basic principles of synergy organization are common across discrete and oscillatory tasks. In contrast, the rate of force production had major effects on the structure of force variance. A modification of Goodman's model of motor variability was used to analyze the dependences VUN and VCOMP on the magnitude of force and on the rate of force production. VUN showed a strong relation to the rate of force production and only weak dependence on the magnitude of force. In contrast, VCOMP showed minimal effects of the rate of force production and strong effects of the force magnitude. The findings are interpreted as demonstrations of a limitation in the ability of the central nervous system to organize a two-finger synergy such that errors in the timing of individual finger force profiles are canceling each other's effects on the total force. In contrast, the synergy is efficiently intercompensating errors related to imprecise setting of force magnitudes of the two fingers.
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