Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics
The present study was designed to investigate corticospinal excitability modulation during maximum isometric and eccentric muscle actions with two different velocities. Moreover, the study aimed to clarify the effect of muscle action type on muscle-tendon mechanical behavior in order to shed light into the possible role of sensory information in modulating corticospinal excitability during different muscle actions. We compared motor evoked potentials (MEPs) to transcranial magnetic stimulation and Hoffman reflexes (H-reflex) in soleus muscle during isometric, slow eccentric (25 deg/s) and fast eccentric (100 deg/s) muscle actions. Concomitantly, ultrasonography was utilized to access soleus and medial gastrocnemius fascicle behavior, characterized by fascicle length, velocity and pennation angle. The main finding was that soleus H-reflex was depressed (P < 0.001) during both fast and slow eccentric protocols as compared to isometric, while no differences in fascicle length (P = 0.569) and pennation angle (P = 0.293) were found among the three protocols. Furthermore, although the fast eccentric protocol had a greater fascicle velocity than slow eccentric (P < 0.05), there were no differences in H-reflex (P > 0.05). No differences in MEP sizes were found among the three protocols (P = 0.750), while absolute silent period was significantly shorter (P = 0.009) for both eccentric protocols as compared to isometric. Taken together, the present results corroborates with the idea that the central nervous system has an unique activation strategy during eccentric muscle actions (Duchateau & Enoka 2008, Enoka 1996), and further refutes the hypothesis that sensory information plays an important role in modulating these actions. ...
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