Effects of post-exercise blood flow occlusion on quadriceps responses to transcranial magnetic stimulation

For a fatigued hand muscle, group III/IV afferent firing maintains intracortical facilitation (ICF) without influencing corticospinal excitability. Exercise of larger muscles produces greater afferent firing. Thus, this study investigated if fatigue-related firing of group III/IV afferents from a large muscle group (quadriceps) modulates intracortical and corticospinal networks. In two sessions, participants (n=18) completed a 2-minute maximal voluntary isometric contraction (MVIC) of knee extensors with (OCC) or without (CON) post-exercise blood flow occlusion to maintain afferent firing. Pre- and post-exercise, single- and paired-pulse transcranial magnetic stimulation (TMS) elicited motor evoked potentials (MEPs) from vastus lateralis (VL), vastus medialis and rectus femoris. Test pulse intensities evoked VL MEPs of ~0.5 mV and were adjusted post-exercise. The conditioning stimulus for ICF and short interval intracortical inhibition (SICI) was constant and set to evoke ~50% of maximum ICF. Muscle pain was also assessed (0-10 scale). Post-exercise, muscle pain was greater for OCC than CON (Median = 8.6 vs. 1.0; P<0.001). MEPs were depressed for CON (all muscles: ∆ -24.3 to -34.1%; P≤0.018) despite increased stimulus intensity (~10%, P<0.001), but both MEPs and intensity remained unchanged for OCC. ICF was depressed post-exercise in OCC (VL and RF: ∆ -59.8% and -28.8%, respectively P=0.016-0.018) but not CON (all muscles: ∆ -3.8 to -44.3%, P=0.726-1.0), but was not different between conditions (interactions: P=0.143-0.252). No interactions were observed for SICI (all muscles: P≥0.266). Group III/IV afferent firing counteracts the post-contraction depression of MEPs in quadriceps. However, intracortical inhibitory and facilitatory networks are not implicated in this response.