In this article, the authors examined the hypothesis that the direction of the change (increase or decrease) in the dynamical degrees of freedom (dimension) regulated as a function of motor learning is task-dependent. Adult participants learned 1 of 2 isometric force-production tasks (Experiment 1: constant force output; Experiment 2: sinusoidal force output) over 5 days of practice and a 6th day with augmented information withdrawal. The results showed that over practice, the task goal induced either an increase (Experiment 1) or a decrease (Experiment 2) in the dimension of force output as performance error was reduced. These findings support the proposition that the observed increase or decrease in dimension with learning is dependent on both the intrinsic dynamics of the system and the short-term change required to realize the task goal.

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This study examined the role of sensorimotor system noise in the organization of the force output of the thumb and index finger and the coordination between the two digits in an isometric pinch grip force task as a function of age (6, 8, 10, 18-22 years), feedback condition (with and without visual feedback information), and force level (5, 15, 25, and 35% of maximal voluntary force. With increases in age under the visual feedback conditions, the signal-to-noise ratio increased, the sequential structure of the force output signals became more irregular, the degree of coherence between the digits at higher force levels was enhanced, and the digits exhibited a greater degree of coherence across the higher frequencies of the power spectrum at all force levels. However, these age differences were either minimized or eliminated under conditions without feedback. These findings show that the age-related performance differences in grip force variability are primarily due to more effective use of visual information rather than age differences in intrinsic sensorimotor noise.