MERGING DIFFERENT MOTOR PATTERNS - COORDINATION BETWEEN RHYTHMICAL AND DISCRETE SINGLE-JOINT MOVEMENTS

Subjects made fast, discrete elbow flexion movements while simultaneou sly producing rhythmical oscillations about initial and final visual t argets embedded ori a horizontal surface. Based on kinematic and elect romyographic (EMG) analysis, we found that the discrete movement could start at any phase of the cyclical movement. The most likely onset ti me occurred when the first agonist burst started at the same moment as a rhythmical burst would have appeared. This resulted in a smooth con jugation between discrete and rhythmical movements. The initiation of the discrete movement was associated with the resetting of the phase o f the rhythmical movements. Thus, the time characteristics of the two motor tasks were interdependent. A subset of trials with a uniform dis tribution of discrete movement onset phases could be selected in most subjects and was averaged to eliminate the cyclical component from the combined movement. Mean kinematic and EMG traces up until the peak ve locity were practically identical to those of the discrete movement ma de alone. The averaging procedure was ineffective in eliminating the r hythmical component following the discrete movement because of the res etting of the phase of oscillation. Using the same procedure it has be en shown that initiating the rhythmical movement at the same time as b eginning the discrete movement did not affect the initial part of disc rete movement. The whole discrete movement was not affected when subje cts simultaneously terminated the ongoing rhythmical movements. Our fi ndings are consistent with the hypothesis that although the rhythmical movement constrains the onset time of discrete movement, the latter, once initiated, proceeds independently of the ongoing rhythmical movem ent. We also subtracted the discrete component from the combined movem ent to see how the former affected the rhythmical movement. The residu al pattern showed that the rhythmical movements rapidly attenuated whe n the discrete movement started and then apparently resumed after the peak velocity of the discrete movement. The results corroborate the hy pothesis that the control signals underlying the two motor tasks canno t be applied simultaneously, since they may be associated with conflic ting stability requirements. Instead, these control signals may be gen erated sequentially, but the resulting kinematic responses may outlast them and be superposed.