Cross‐correlation analysis of motor unit activity recorded from two separate thumb muscles during development in man.

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Abstract
1. Multi‐unit surface EMG signals were recorded from the short and long thumb abductor muscles of seventy‐five children aged from 4 to 15 years and from nine adults during simultaneous abduction and extension of the left and right thumb. Ability to perform independent finger movements was investigated by timing a series of sequential finger‐to‐thumb oppositions. 2. Cross‐correlograms were constructed from the discharges of motor units recorded from the long and short abductor muscles acting on the same thumb. In the majority of subjects, short duration central peaks were present indicating the presence of a common drive to the motoneurone pools innervating these two muscles. Except for those subjects aged 4 and 5 years, the size of these central correlogram peaks did not differ significantly between the dominant and non‐dominant hands. 3. The prevalence of central cross‐correlogram peaks in different subjects increased from the age of 4 years to 15 years. The size of the central cross‐correlogram peak increased with age up to 10 years but did not alter significantly after this age. The duration of the central peak steadily decreased over the age range of 4 to 15 years. 4. Multilinear regression analysis of data recorded from children revealed that there was a positive, but weak, correlation between the size of the cross‐correlogram peak and the rate of performance of sequential finger movements after having controlled for age.