Motor unit synchronization measured by cross-correlation is not increased with strength training of a hand muscle

It is commonly believed that motor unit synchronization in a hand muscle increases as a result of strength training, although this has never been assessed directly. The purpose of the study was to use cross-correlation to directly quantify the strength of motor unit synchronization before and after 4 weeks of strength training the first dorsal interosseous muscle. Four young subjects performed a training protocol 3 times/week consisting of 6 sets of 10 maximal isometric index finger abductions. Motor unit activity was recorded with pairs of intramuscular electrodes in the first dorsal interosseous muscle before (n=42 pairs) and after (n=41 pairs) the 4-week training protocol. The training intervention resulted in a 27% (51.0 ± 10.0 N to 64.9 ± 17.0 N, P = 0.007) increase in maximal index finger abduction force, whereas there was a 20% (common input strength index; 0.77 ± 0.41 pulses/s to 0.61 ± 0.38 pulses/s, P = 0.03) reduction in motor unit synchronization following 4 weeks of strength training. Furthermore, there was no association between the change in strength and the change in synchronization in individual subjects after training. These cross-correlation data suggest that increases in strength following 4 weeks of training a hand muscle are not accompanied by increases in motor unit synchronization.