Abstract
Cooperative hand movements (e.g., opening a bottle) require a close coordination of the hands. This is reflected in a neural coupling between the two sides. The aim of this study was to investigate in how far neural coupling is present not only during bilateral hand but also during bilateral finger movements. For this purpose unilateral mechanical and electrical nerve stimuli were delivered during bilateral sequentially and synchronously performed finger movements on a keyboard and, for comparison, during bilateral hand flexion movements. Electromyographic (EMG) activity and reflex responses in forearm flexor and extensor muscles of both sides were recorded and analyzed. Confounding EMG activity related to hand movements during the finger task was limited by wrist fixating braces. During the hand flexion task, complex reflex responses appeared in the forearm muscles of both sides to unilateral stimulation of the ulnar nerve (mean latency 57 ms), reflecting neural coupling between the two hands. In contrast, during the bilateral finger movement task, unilateral electrical nerve or mechanical stimulation of the right index finger was followed by dominant ipsilateral reflex responses (latency 45 and 58 ms, respectively). The results indicate that in contrast to the coupled hand movements, finger movements may not be coupled but can move independently on each side. Functionally this makes sense because during most activities of daily living, a close cooperation of the hands but not of individual fingers is needed. This independence of individual finger movements may rely on strong, specific, contralateral cortico‐motoneuronal control.
Highlights
Research on neural hand motor control is focused mainly on unilateral reach and grasp movements
Both mechanical and electrical stimuli were followed by reflex responses which were restricted to the ipsilateral side, and which appeared in the forearm flexor muscles with a latency of 57 ms (Figure 3a, mechanical) and in forearm extensor muscles with a latency of 45 ms (Figure 3b, electrical)
Electrical stimuli delivered during the hand task were followed by complex reflex responses which appeared in the activated forearm flexor muscles of both sides with a latency of approximately 65 ms
Summary
Research on neural hand motor control is focused mainly on unilateral reach and grasp movements. Previous research on cooperative hand movements indicated that neural coupling between the two hands provides a fast and accurate automatic coordination of bilateral hand movements in ADL (Dietz et al, 2015). This neural coupling is reflected in the appearance of Physiological Reports. | wileyonlinelibrary.com/journal/phy of 11 bilateral reflex EMG responses to unilateral arm nerve stimulation. The size of reflex responses was shown to depend on the level of forearm muscle activity, that is, an increase in movement velocity or resistance results in greater background muscle activation and, in greater reflex amplitudes ipsi- and contralateral to the stimulation site (Thomas, Dietz, Scharfenberger, & Schrafl-Altermatt, 2018)
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