Correlation between primary motor cortex neural activity and fingertip force following transcranial magnetic stimulation

A better understanding of the neural mechanisms of finger-force regulation can help to explain the relationship between the central nervous system and nerve-muscle force, as well as assist in motor functional rehabilitation and the development robot hand designs. In the present study, 11 healthy volunteers performed a different target force-tracking task, which involved the index finger alone, index and middle finger together, and the combination of four fingers (i.e., index, middle, ring, and little). The target force trace corresponded to 3 levels of 20% maximal voluntary changes (MVC), 30% MVC, and 40% MVC in 20 seconds. In the test, an unexpected single 120% motor threshold transcranial magnetic stimulation was applied to the primary motor cortex (M1) during force tracking. Results revealed that peak force changes increased with increasing background force and the number of involved task fingers. These results demonstrate that M1 neural activities correlate with finger-force production, and M1 plays a role in finger-force control. Moreover, different neuronal networks were required for different finger patterns; a complicated task required multi-finger combinations and a complicated neuronal network comprised a large number of neurons.