Intensity-dependent alterations in the excitability of cortical and spinal projections to the knee extensors during isometric and locomotor exercise

Abstract

We investigated the role of exercise intensity and associated central motor drive in determining corticomotoneuronal excitability. Ten participants performed a series of nonfatiguing (3 s) isometric single-leg knee extensions (ISO; 10-100% of maximal voluntary contractions, MVC) and cycling bouts (30-160% peak aerobic capacity, W peak). At various exercise intensities, electrical potentials were evoked in the vastus lateralis (VL) and rectus femoris (RF) via transcranial magnetic stimulation (motor-evoked potentials, MEP), and electrical stimulation of both the cervicomedullary junction (cervicomedullary evoked potentials, CMEP) and the femoral nerve (maximal M-waves, M max). Whereas M max remained unchanged in both muscles (P > 0.40), voluntary electromyographic activity (EMG) increased in an exercise intensity-dependent manner for ISO and cycling exercise in VL and RF (both P < 0.001). During ISO exercise, MEPs and CMEPs progressively increased in VL and RF until a plateau was reached at ∼ 75% MVC; further increases in contraction intensity did not cause additional changes (P > 0.35). During cycling exercise, VL-MEPs and CMEPs progressively increased by ∼ 65% until a plateau was reached at W peak. In contrast, RF MEPs and CMEPs progressively increased by ∼ 110% throughout the tested cycling intensities without the occurrence of a plateau. Furthermore, alterations in EMG below the plateau influenced corticomotoneuronal excitability similarly between exercise modalities. In both exercise modalities, the MEP-to-CMEP ratio did not change with exercise intensity (P > 0.22). In conclusion, increases in exercise intensity and EMG facilitates the corticomotoneuronal pathway similarly in isometric knee extension and locomotor exercise until a plateau occurs at a submaximal exercise intensity. This facilitation appears to be primarily mediated by increases in excitability of the motoneuron pool.

Group III/IV locomotor muscle afferents alter motor cortical and corticospinal excitability and promote central fatigue during cycling exercise

“…As a side note, since increases in EMG have been documented to facilitate motoneurons during cycling exercise, the unchanged CMEPs during CTRL (which was characterized by a substantial increase in EMG from start to exhaustion) indirectly suggest a depressing effect of fatigue on motoneuronal excitability (Weavil et al, 2015, 2016). As fentanyl blockade had no effect on CMEPs at exhaustion, this depression might not be accounted for by group III/IV muscle afferents.…”

“…This was achieved by passing a high voltage pulse (100 μs pulse width, D-185 mark IIa, Digitimer Ltd.) between a set of self-adhesive electrodes (3-cm, round) attached to the skin in the groove between the mastoid processes and the occiput (cathode on the left, contralateral to the right limb muscle). Based on the linear relationship between the increase in contraction strength and the increase in CMEP up to 50% MVC (Martin et al, 2008a; Weavil et al, 2015), the stimulation intensity was set to achieve a CMEP size corresponding to ~20% M max during 20% quadriceps MVC (418 ± 14 V; range 300–520 V).…”

“…~20% M max ) during a 20% MVC. This methodology has been used previously (Sidhu et al, 2012; Weavil et al, 2015). The established TMS (and CMS) intensities were used to measure cortical and motoneuronal excitability during cycling and the pre- and post-exercise assessment of quadriceps function.…”

Phase- and Workload-Dependent Changes in Corticospinal Excitability to the Biceps and Triceps Brachii during Arm Cycling

“…Considerably less information is currently available regarding corticospinal excitability (CSE) modulation during locomotor outputs, though we are beginning to understand CSE modulation during leg [7,12,13] and arm cycling [8,14,15,16]. CSE can be assessed by measuring the amplitude of motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid electrical stimulation (TMES) of corticospinal axons.…”

“…Together, these measures give an indication of supraspinal and spinal excitability [8,14,16,17,18,19]. Using these techniques, Sidhu and colleagues have recently shown that CSE is phase-, muscle- and intensity-dependent to the leg muscles during cycling [7,12,13]. Work from our lab has shown phase-, task- and cadence-dependent modulation of CSE to the biceps brachii during arm cycling [8,14,15,16].…”