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.

Keywords: caffeine; corticospinal excitability; excitation–contraction coupling; neuromuscular fatigue; persistent inward currents.

FIGURE 1

(A) Experiment 1 design. Participants completed PREP before being tested before (PRE) and immediately (POST), 10 min (POST-10) and 20 min (POST-20) after the fatiguing contractions. The test protocol consisted of eight resting TMS pulses (white arrows), six active TMS pulses (black arrows) superimposed during MVC (active TMS), and three resting M_max stimuli (gray arrows). (B) Participants completed PREP, followed by three MVCs and M-waves, Control, and PRE. Upon completion of the fatiguing exercise protocol, the participants were tested at POST, POST-10, and POST-20. The test protocol at Control and PRE consisted of 20 Hz (blue arrow), VFT (red arrow), 80 Hz (green arrow) stimulations, and the VIB+STIM protocol (yellow arrow). The POST test protocol consisted of 20 Hz, VFT, 80 Hz stimulations, VIB+STIM, one MVC, and one M_max measurement.

FIGURE 2

Schematic representation of the tendon vibration and superimposed tibial nerve stimulation protocol used to elicit reflexive muscular contractions and the respective time points at which torque was recorded. T_vib, torque is measured after the fifth (last) bout of electrical stimulation; T_sust, torque is measured 500 ms after vibration cessation (self-sustained torque). EMG trace indicates pronounced SOL but minimal TA muscle activity throughout the VIB+STIM sequence.

FIGURE 3

Experiment 1, changes in torque, EMG, MEP/M, and cSP measured from PRE to immediately (POST), 10 min (POST-10), and 20 min (POST-20) after exercise. Changes in (A) MVC torque, (B) triceps surae EMG normalized to M-wave amplitude (EMG/M), (C) motor-evoked potential amplitude during MVCs normalized to M-wave amplitude (MEP/M), and (D) cortical silent period (cSP). ^∗Significantly different from PRE, p < 0.05.

FIGURE 4

Example data obtained from one participant in the non-caffeine session of Experiment 1 before (PRE) and immediately (POST), 10 min (POST-10) and 20 min (POST-20) after the exercise protocol. Decreases in MVC torque, SOL_RMS, and MEP amplitude during MVC (second row) and are noticeable immediately after the exercise protocol.

FIGURE 5

Experiment 2, changes in torque, EMG, reflexive torque, and tetanic torque measured from before to immediately (POST), 10 min (POST-10) and 20 min (POST-20) post-exercise. (A) MVC torque, (B) triceps surae EMG normalized to M-wave (TS/M_max), (C) mean torque after the fifth (last) stimulation during vibration (T_vib) and self-sustained torque (T_sust) measured 500 ms after vibration cessation, (D) peak torque during 20-Hz, 80-Hz, and VFT trains of electrical stimulation. ^∗Significantly different from PRE, p < 0.05.

FIGURE 6

Example data obtained from one participant in the non-caffeine session in Experiment 2 before (PRE) and immediately (POST), 10 min (POST-10) and 20 min (POST-20) after the exercise protocol. Decreases in MVC torque (first row), reflexive torque (second row), and tetanic stimulations are noticeable immediately after the exercise protocol.