Acquisition of a new motor skill is accompanied by changes in cutaneomuscular reflex responses recorded from finger muscles in man

Cutaneomuscular reflex (CMR) responses and motor unit synchronisation have been recorded to investigate possible reorganisation of central nervous pathways in six healthy adults learning a novel skill with the non-dominant hand. Multi-unit surface EMG signals were recorded from first dorsal interosseous (1DI) and abductor digiti minimi (ADM) muscles during sustained flexion of the index finger and abduction of the little finger (flex/ab) and during bilateral controlled finger abduction (ab/ab). CMR responses were elicited by concomitant stimulation of the digital nerves of the index finger at 2.5 times threshold for perception. For training purposes the subject practised the novel flex/ab task phasically at a rate of 1 Hz for 10 min daily for 12 days. Cortically mediated components of the CMR responses recorded from 1DI and ADM were significantly larger in both muscles during training. Taking the data for all subjects together, when subjects performed sustained flex/ab (novel task), the size of the I1 and E2 components were increased by 25 and 55% in 1DI and 192 and 167% in ADM (Mann-Whitney U-test, P<0.05). Corresponding values for sustained ab/ab were more modest: 31 and 16% in 1DI and 88 and 54% in ADM. Changes in the size of the spinal E1 component were not the same in each muscle: values for sustained flex/ab and ab/ab were 33 and 31% smaller in 1DI and 89 and 59% larger in ADM. No significant changes were found in the amount of synchrony of motor unit firing between 1DI and ADM when subjects performed either task during training. These results suggest that learning a new motor skill produces changes which take place predominantly in the cortical pathways of the CMR and these may be due to changed connectivity within motor and/or sensory cortex which has previously been shown in the monkey.