Neuromuscular Factors Contributing to Reductions in Muscle Force After Repeated, High-Intensity Muscular Efforts

Abstract

Multiple neuromuscular processes contribute to the loss of force production following repeated, high-intensity muscular efforts; however, the relative contribution of each process is unclear. In Experiment 1, 16 resistance trained men performed six sets of unilateral isometric plantar flexor contractions of the right leg (3 s contraction/2 s rest; 85% maximal voluntary contraction torque; 90-s inter-set rest) until failure with and without caffeine ingestion (3 mg kg -1 ) on two separate days. Corticospinal excitability and cortical silent period (cSP) were assessed before and immediately, 10 and 20 min after the exercise. In Experiment 2, electrically evoked tetanic force and persistent inward current (PIC)-mediated facilitation of the motor neuron pool (estimated using neuromuscular electrical stimulation with tendon vibration) were assessed before and after the same exercise intervention in 17 resistance trained men. Results showed decreases in peak plantar flexion torque (Experiment 1: -12.2%, Experiment 2: -16.9%), electrically evoked torque (20 Hz -15.3%, 80 Hz -15.3%, variable-frequency train -17.9%), and cSP (-3.8%; i.e., reduced inhibition) post-exercise which did not recover by 20 min. Electromyographic activity (EMG; -6%), corticospinal excitability (-9%), and PIC facilitation (-24.8%) were also reduced post-exercise but recovered by 10 min. Caffeine ingestion increased torque and EMG but did not notably affect corticospinal excitability, PIC amplification, or electrically evoked torque. The data indicate that a decrease in muscle function largely underpins the loss of force after repeated, high-intensity muscular efforts, but that the loss is exacerbated immediately after the exercise by simultaneous decreases in corticospinal excitability and PIC amplitudes at the motor neurons.

Effects of caffeine on neuromuscular function in a non‐fatigued state and during fatiguing exercise

“…However, it is important to note that the exact time point of the start of the inhibitory mechanisms that explain SP and the beginning of the excitatory mechanisms reflected in the onset of MEP might not coincide. Nevertheless, a similar behaviour has been observed in previous studies (Girard, Bishop, & Racinais, 2013; Kirk, Trajano, Pulverenti, Rowe, & Blazevich, 2019; Latella, Hendy, Vanderwesthuizen, & Teo, 2018; Mira et al., 2017). We hypothesize that this suppression of inhibitory mechanisms might be the expression of a compensatory phenomenon to try to overcome a deficit of VA, which was observed throughout the fatiguing protocol and recovery.…”

Involuntary sustained firing of plantar flexor motor neurones: effect of electrical stimulation parameters during tendon vibration

“…A sustained involuntary torque after the cessation of vibration was not observed in all participants, and some responders showed low levels of involuntary torque. The observation of responders and non-responders has been previously reported in other human studies in which a combination of tendon vibration and NMES (Kirk et al 2019;Magalhães and Kohn 2010) or NMES only (e.g. Wegrzyk et al 2015) was imposed.…”

“…Self-sustained torque observed after the end of such trials (or after vibration alone; Gorassini et al 2002) could be explained by PIC-related bistable behavior in some spinal MNs (Lee and Heckman 1998). The progressive increase of torque reported during vibration plus NMES (Magalhães and Kohn 2010;Trajano et al 2014;Kirk et al 2019) is similar to the warm-up effect that is thought to reflect depolarization-induced facilitation of voltage-gated calcium channels (Svirskis and Hounsgaard 1997). Finally, the "extra torque" is joint angle (i.e.…”

“…This technique combines tendon vibration and percutaneous NMES to provoke involuntary muscle contractions (Kirk et al 2019;Magalhães and Kohn 2010;Trajano et al 2014). The present study was designed to systematically manipulate the NMES parameters used with this technique to determine their influence on involuntary torque variables.…”

“…Neuromuscular electrical stimulation (NMES) and tendon vibration have been used independently (e.g. Magalhães et al 2013;Bochkezanian et al 2018b) and simultaneously (Magalhães and Kohn 2010;Trajano et al 2014;Bochkezanian et al 2018a;Kirk et al 2019) to induce motor unit (MU) activation, and hence muscle contractions. This involuntary MU activation is induced via both peripheral and central pathways.…”