Practicing elements versus practicing coordination: changes in the structure of variance

The authors explored effects of practice of a 2-finger accurate force production task on components of finger force variance quantified within the uncontrolled manifold (UCM) hypothesis, V(UCM), that had no effect on total force and V(ORT) that affected total force. A variable task with graded instability was designed to encourage use of variable solutions. Two groups of subjects (n = 9 each) were tested prior to a 1.5-hr practice session, after the session, and 2 weeks later (retention test). Group 1 practiced 1 finger at a time, while Group 2 practiced the task with 2 fingers (index and middle) pressing together. Both groups showed comparable improvements in the performance indices. Both groups showed a decrease in V(ORT), while only Group 2 showed an increase in V(UCM). These effects persisted during the retention test. The results show that practicing elements and practicing redundant groups of elements may lead to similar changes in performance (i.e., in the variability of the total force produced by the set of fingers), accompanied by dramatically different changes in the structure of variance: A drop in V(UCM) after the single-finger practice and an increase following the 2-finger practice. The strong retention effects promise applications of the method to rehabilitation.