During isometric contractions (Gandevia, Allen et al. 1996; Taylor et al. 1996) and high-intensitycycling (Amann et al. 2008b; Amann, Blain et al. 2011) it has been shownthat the activation of group III/IV afferents, pain receptors (relays changesin metabolic environment from the muscle to the central nervous system (Almeida et al. 2004), and acts at both a supraspinal and spinallevel to influence motor unit firing (Martin et al.
2008b)), will increase to inhibit the cortex andin turn reduce the central motor drive. Furthermore, studies that partiallyblock muscle afferents in an exercising human have implied that these afferentgroups will restrain central motor drive to ensure critical threshold is notmet (Amann and Dempsey 2008a; Amann et al. 2009; Gagnon et al. 2012). If these muscleafferent groups are completely blocked it has shown that the potentiated twitchforce in the quadriceps would continue to decrease, meaning excessive increaseof peripheral fatigue at exercise termination (Amann, Proctor et al.
2009; Gagnon, Bussieres et al. 2012). PROBABLYTALK ABOUT PEARCEY AND ZEHR PAPER HEREIn the upper body, researchers have shownthat by firing the type III/IV group afferents in the triceps brachii(antagonist) and in the first dorsal interosseous muscle, resulted in a lowerVA of the elbow flexors (Kennedy et al. 2013; Kennedy et al. 2014). A decrease insupraspinal and an increase in spinal excitability can be induced by increasinggroup III/IV afferent feedback to the supraspinal and spinal regions during aweak isometric elbow flexor contraction (Martin, Weerakkody et al.
2008b). Pearcey,Bradbury-Squires et al. (2016) found an increasein CMEP amplitude after the sprinting protocol. It is possible that theexcitability of the motoneurone pool may be enhanced as motor output intensityincreased. Leading to increased activity of serotonergic neurons (~3-5 fold)during a central pattern generator driven motor output (Fornal et al. 1996), which upper bodycycling has been shown to be involved in the operation of a spinal centralpattern generator (Zehr et al. 2004).
However, it could also be possible thatvarious other factors can change motoneuron excitability such as a loweredvoltage threshold and decreased spike time for action potential initiation (Beaumont et al. 2003; Power et al. 2010), persistent inwardcurrents activating (Lee et al. 1998; Button et al.
2006; Heckman et al. 2008) (shown in humanbicep brachii motor units (Wilson et al. 2015)), and triggeringof monoaminergic system throughout exercise, which possibly alters thepersistent currents (Heckman 2003; Gardiner et al. 2006). Evidently, allthese factors could play a role in altering spinal excitability. Researchershave shown in both leg-cycling (Girard, Bishop et al. 2013b) and running sprints (Goodall, Charlton et al. 2015) no changes in MEPamplitudes following sprints, with one researcher showing a TMS reduced VAexhibiting that central fatigue did in fact occur.
Additionally, it has beenquestioned whether sprinting had any influence on the responsiveness of theneurons involved in motor cortical output. However, these previous studies didnot involve testing of the spinal excitability, so it may be possible that anincrease in spinal excitability had masked the decrease in the supraspinalexcitability. Pearcey,Bradbury-Squires et al.
(2016) showed thisoccurrence by creating a ratio of MEP amplitude over the CMEP amplitude, in orderto isolate any changes that occurred in supraspinal excitability (Gandevia, Petersen et al. 1999). They showed a decrease in supraspinalexcitability after the 5th and 10th sprint compared tothe pre-sprint.
Like speculated, the spinal excitability seemed to increase toadjust for the decrease in supraspinal excitability, unfortunately themechanism for this change is unknown and more research needs to be completed inthis area. Interestingly, it has been shown that during an elbow extensorfatiguing task group III/IV afferents will inhibit triceps brachii spinalmotoneuron excitability, while simultaneously enhancing the biceps brachiispinal motoneuron excitability (Martin et al. 2006). During an upperbody sprint, the triceps may fatigue; hence the possibility of the spinalexcitability of the biceps brachii being increased through afferent orreflexive feedback.