Effect of short-term training on fine motor control in trigeminally innervated versus spinally innervated muscles

We hypothesized that the trigeminally innervated jaw muscles and spinally innervated hand muscles would differ in the force control and muscle activity when similar fine motor training tasks are performed. Sixteen healthy volunteers performed six series (with ten trials each) of an oral fine motor task (OFMT) and a hand fine motor task (HFMT), in random order. The task was to hold-and-break a test material (5 cm spaghetti pasta) placed on the force transducer between either their anterior teeth (OFMT) or the thumb and the index finger (HFMT). The hold and the break forces along with the electromyographic (EMG) activity of the left and right masseter (MAL and MAR), left anterior temporalis (TAL) and digastric (DIG) muscles during OFMT, and first dorsal interosseous (FDI) and abductor pollicis brevis (APB) during HFMT, were recorded. There was no significant difference in the relative change of holding force during the six subsequent series, neither for the OFMT (P = 0.39) nor for the HFMT (P = 0.10). The relative change of EMG activity of MAL (P = 0.01) and MAR (P = 0.02) during the hold phase decreased significantly during the six series of OFMT. Also the relative change of break force (P = 0.001) and the relative change of EMG activity of APB during the hold (P = 0.003) and break phases (P = 0.002) decreased significantly during the six series of HFMT. The results indicate functional differences between the jaw and hand muscles during a similar hold-and-break task, with the most pronounced changes for the spinally innervated hand muscles. Overall, these findings indicate that training-related neuroplasticity cannot be extrapolated directly from the spinal to the trigeminal system and vice versa.