Motor unit activity after eccentric exercise and muscle damage in humans

Abstract

It is well known that unaccustomed eccentric exercise leads to muscle damage and soreness, which can produce long-lasting effects on muscle function. How this muscle damage influences muscle activation is poorly understood. The purpose of this brief review is to highlight the effect of eccentric exercise on the activation of muscle by the nervous system, by examining the change in motor unit activity obtained from surface electromyography (EMG) and intramuscular recordings. Previous research shows that eccentric exercise produces unusual changes in the EMG–force relation that influences motor performance during isometric, shortening and lengthening muscle contractions and during fatiguing tasks. When examining the effect of eccentric exercise at the single motor unit level, there are substantial changes in recruitment thresholds, discharge rates, motor unit conduction velocities and synchronization, which can last for up to 1 week after eccentric exercise. Examining the time course of these changes suggests that the increased submaximal EMG after eccentric exercise most likely occurs through a decrease in motor unit conduction velocity and an increase in motor unit activity related to antagonist muscle coactivation and low-frequency fatigue. Furthermore, there is a commonly held view that eccentric exercise produces preferential damage to high-threshold motor units, but the evidence for this in humans is limited. Further research is needed to establish whether there is preferential damage to high-threshold motor units after eccentric exercise in humans, preferably by linking changes in motor unit activity with estimates of motor unit size using selective intramuscular recording techniques.

Neural Contributions to Muscle Fatigue

“…Similar to isometric studies, MU firing rates show variable or no changes following a submaximal fatiguing shortening task (35, 37). Following muscle-damaging eccentric contractions, when strength was reduced immediately and for the subsequent 24h, the very few studies indicate that MU firing rates are slightly increased in relation to substantially lower recruitment thresholds compared with before the eccentric tasks (82 for review). With continuing improvements and validation of high-density surface electrodes (28) and miniaturization of intramuscular multi-channel electrode arrays (64), recording of MU properties may be forthcoming from more complicated dynamic fatiguing tasks involving multiple joint actions such as walking or cycling.…”

Prolonged Reduction in Shoulder Strength after Transcutaneous Electrical Nerve Stimulation Treatment of Exercise‐Induced Acute Muscle Pain

“…We used an eccentric fatigue protocol to induce delayed‐onset muscle soreness (DOMS) in this study. DOMS models include repetitive performance of eccentric contractions, which result in high force production at a reduced metabolic cost . This experimental pain model results in endogenous pain symptoms that last several days (peaking around 24 to 48 hours after exercise) .…”

“…The mechanism of DOMS‐related acute pain symptoms are thought to be due to significant neuromuscular changes that occur following bouts of repetitive eccentric contractions . Exercise‐induced pain models are especially useful for examining the impact of acute pain as there are short‐term functional consequences for individuals who have little experience with DOMS …”

[18F]-FDG positron emission tomography—an established clinical tool opening a new window into exercise physiology

“…The only limitation in this regard with FDG-PET is that the movements should ideally be monotonous, repetitious movements for longer periods of time, such as in endurance disciplines, although some successful attempts have been made to use FDG-PET while running sprints (47). Additionally, EMG is much more insightful when considering issues related to single motor unit favoring the use of EMG in those studies (57,58).…”