Figure 2. Reductions in motoneuronal excitability induced by maximal and submaximal isometric exercise.

Raw traces of biceps brachii CMEPs recorded from a single participant during a sustained maximal (McNeil et al., 2009) or submaximal (McNeil et al., 2011) isometric contraction of the elbow flexors. CMEPs are recorded during the SP following TMS (100 ms ISI between TMS and CMS), and the reduction in CMEP size reflects a decrease in motoneuronal excitability because biceps brachii M_max increases during these tasks. Beneath each set of traces is a schematic representation of the biceps brachii motoneurone pool. Circles represent motoneurones of different size, whereas the colour indicates the presumed activation during the fatiguing contraction (black = active throughout, grey = active part of the time, white = not active at any point). The horizontal line above them indicates the motoneurones that would likely contribute to the CMEP considered in each set of traces (i.e., a small CMEP would involve only small, low-threshold motoneurones). Left traces: During a sustained 2-min maximal voluntary contraction (MVC), the reduction in motoneuronal excitability was so rapid that the CMEP was virtually abolished after 16 s. Middle and right traces: After six minutes of a sustained 10-min contraction at the level of integrated EMG activity produced at 25% MVC torque (25% iEMG), there was a marked decrease of the small CMEP (~15% M_max) but a modest decrease of the large CMEP (~50% M_max). This indicates that impairment of motoneuronal excitability is limited to the parts of the pool that have been repetitively activated. For comparable findings in the lower limb see (Finn et al., 2018).