Recordings were made of the firing of pairs of intrinsic hand muscle motor units active under different task conditions in man. The different tasks were defined as isometric contractions producing force in one of three different directions: finger abduction, finger extension, or finger flexion. The degree of motor-unit synchronization associated with each of these task conditions was compared with the use of cross-correlation analysis. 2. The average amount of synchronization between the firing of motor units recorded from within first dorsal interosseous muscle (IDI) was greater during index finger extension than during index finger abduction (n = 8 motor-unit pairings, 3 subjects). In addition, for another sample population of motor units, the average amount of synchronization was greater during index finger abduction than during index finger flexion (n = 11 motor-unit pairings, 4 subjects). 3. In a further series of experiments, one motor unit of each pair was recorded from second dorsal interosseous muscle (2DI), whereas the other motor unit of each pair was recorded from 1DI. The average amount of synchronization for these motor-unit pairings was greater during extension of the index and middle fingers than during abduction of the index and middle fingers (n = 8, 4 subjects). For another sample population of such motor-unit pairings, the average amount of synchronization was found to be greater during abduction of the index and middle fingers than during flexion of the index and middle fingers (n = 11, 4 subjects). 4. In approximately one-third of cases, it was not found possible to maintain the same firing rates from two motor units in 1DI when active under different task conditions. For instance, the "reference" motor unit might consistently fire at a faster rate than the "response" motor unit when active during index finger extension but consistently fire at a slower rate than the response motor unit when active during index finger abduction. Where such motor-unit pairs have been studied in detail, the pattern of task dependence in their synchronization was found to be similar to that described above for motor-unit pairs in which the firing rates remained constant under the different task conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
