Abstract
Failure to suppress antagonist muscles can lead to movement dysfunction, such as the abnormal muscle synergies often seen in the upper limb after stroke. A neurophysiological surrogate of upper limb synergies, the selectivity ratio (SR), can be determined from the ratio of biceps brachii (BB) motor evoked potentials to transcranial magnetic stimulation prior to forearm pronation versus elbow flexion. Surprisingly, cathodal transcranial direct current stimulation (c-TDCS) over ipsilateral primary motor cortex (M1) reduces (i.e. improves) the SR in healthy adults, and chronic stroke patients. The ability to suppress antagonist muscles may be exacerbated at high movement rates. The aim of the present study was to investigate whether the selective muscle activation of the biceps brachii (BB) is dependent on altering frequency demands, and whether the c-tDCS improvement of SR is dependent on task frequency. Seventeen healthy participants performed repetitive isometric elbow flexion and forearm pronation at three rates, before and after c-tDCS or sham delivered to ipsilateral left M1. Ipsilateral c-tDCS improved the SR in a frequency dependent manner by selectively suppressing BB antagonist excitability. Our findings confirm that c-tDCS is an effective tool for improving selective muscle activation, and provide novel evidence for its efficacy at rates of movement where it is most likely to benefit task performance.
Highlights
Selective activation of agonist muscles is necessary for producing skilled and coordinated upper limb movements in daily life
Data from 3 participants had to be discarded from the final analysis because they were not able to relax enough between contractions to maintain an average pretrigger root mean squared EMG (rmsEMG) below 10μV, and there Motor evoked potentials (MEPs) data could not be interpreted
The previous findings along with the current results suggest that improvements of selective muscle activation after cathodal tDCS (c-tDCS) are due to effects on uncrossed ipsilateral pathways, as opposed to interhemispheric cortical mechanisms
Summary
Selective activation of agonist muscles is necessary for producing skilled and coordinated upper limb movements in daily life. For many reaching and grasping movements, activation of the prime mover (agonist) is associated with simultaneous relaxation of the antagonist [1,2,3]. Reciprocal inhibition is the neurophysiological mechanism that underlies this functional.
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