Adaptations in antagonist co-activation: Role in the repeated-bout effect

Abstract

Eccentric exercise results in an adaptation which attenuates muscle damage from subsequent exercise—termed the “repeated-bout effect (RBE).” Purpose: Study examined antagonist co-activation and motor-unit recruitment strategy, assessed via dEMG, concomitant to the RBE. Methods: Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and its recruitment threshold (RT) was examined using linear regression. Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. Surface EMG of the biceps and triceps were collected during eccentric exercise. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated. Results: Changes in MVC (-32±14% vs -25±10%; p = 0.034), ROM (-11% vs 6%; p = 0.01), and DOMS (31.0±19mm vs 19±12mm; p = 0.015) were attenuated following the second bout of exercise. Triceps EMG was reduced (16.8±9.5% vs. 12.6±7.2%; p = 0.03) during the second bout of eccentric exercise. The slope (-0.60±0.13 vs -0.70±0.18; p = 0.029) and y-intercept (46.5±8.3 vs 53.3±8.8; p = 0.020) of the MFR vs. RT relationship was altered during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% of MVC. Conclusion: A reduction in antagonist co-activation during the second bout of eccentric exercise suggests less total force was required to move an identical external load. This finding is supported by the increased negative slope coefficient and an increased y-intercept of the linear relationship between RT and MFR.

Neuromuscular and Metabolic Responses during Repeated Bouts of Loaded Downhill Walking

“…This greater efficiency could translate into a lesser cardiometabolic demand and perceived exertion during Ex2. Changes in motor recruitment pattern could be explained by different mechanisms such as (I) a shift in motor unit recruitment toward low-threshold motor units during the second bout (6); (II) an enhanced synchronization of motor units (6); (III) a lower antagonist muscle activation (36), although no significant difference in BF EMG activity during the stance phase in the present study; and (IV) an attenuation of supraspinal fatigue (18). Moreover, Hyldahl et al (6) reported that muscle stiffness might increase by skeletal muscle extracellular matrix remodeling after eccentric exercise, which may protect muscle from damage during a subsequent eccentric exercise.…”

Modulation of intracortical inhibition and excitation in agonist and antagonist muscles following acute strength training

“…These responses may be incongruent with the longer-term supressed responses of the antagonist muscle which may be expected with the downregulation of antagonist activity accompanying improvements in muscular strength. There is early evidence to suggest that shifts in co-activation during training are observable following just a single session of training (Hight et al 2017); however, this is yet to be thoroughly examined.…”